birth order assignment

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How Does Birth Order Shape Your Personality?

Beware the stereotypes

Verywell Mind / Getty Images

What Is Adler’s Birth Order Theory?

First-born child, middle child.

• Impact on Relationships

Debunking Myths and Limitations

Birth order refers to the order a child is born in relation to their siblings, such as whether they are first-born, middle-born, or last-born. You’ve probably heard people joke about how the eldest child is the bossy one, the middle child is the peace-maker, and the youngest child is the irresponsible rebel—but is there any truth to these stereotypes?

Psychologists often look at how birth order can affect development, behavior patterns, and personality characteristics, and there is some evidence that birth order might play a role in certain aspects of personality .

At a Glance

Researchers often explore how birth order, including the differences in parental expectations and sibling dynamics, can affect development and character. According to some researchers, firstborns, middle children, youngest-children, and only child-children often exhibit distinctive characteristics that are strongly influenced by how birth order shapes parental and sibling behaviors.

Early in the 20th century, the Austrian psychiatrist Alfred Adler introduced the idea that birth order could impact development and personality. Adler, the founder of individual psychology, was heavily influenced by psychoanalyst Sigmund Freud .

Key points of Adler's birth order theory were that firstborns were more likely to develop a strong sense of responsibility, middleborns a desire for attention, and lastborns a sense of adventure and rebellion.

Adler also notably introduced the concept of the " family constellation ." This idea emphasizes the dynamics that form between family members and how these interactions play a part in shaping individual development.

Adler's birth order theory suggests that firstborns get more attention and time from their parents. New parents are still learning about child-rearing, which means that they may be more rule-oriented, strict, cautious, and sometimes even neurotic .

They are often described as responsible leaders with Type A personalities , a phenomenon sometimes referred to as " oldest-child syndrome ."

"Older siblings, regardless of gender, often feel more deprived or envious since they have experienced having another child divert attention away from them at some point in their lives. They tend to be more success-oriented,” explains San Francisco therapist Dr. Avigail Lev.

Firstborn children are often described as:

• High-achieving (or sometimes even over-achieving )
• Structured and organized
• Responsible

All this extra attention firstborns enjoy changes abruptly when younger siblings come along. When you become an older sibling, you suddenly have to share your parent's attention. You may feel that your parents have higher expectations for you and look to you to set an example for your younger siblings.

Consider the experiences of the oldest siblings, who are frequently tasked with caring for younger siblings. Because they are often expected to help fill the role of caregivers, they may be more nurturing, responsible, and motivated to excel.

Such traits are affected not only by birth order but also by how your position in the family affects your parent's expectations and your relationship with your younger siblings.

Research has found that firstborn kids tend to have more advanced cognitive development , which may also confer advantages when it comes to school readiness skills. However, it's important to remember that being the oldest child can also come with challenges, including carrying the weight of expectations and the burden of taking a caregiver role within the family.

Adler suggested that middle children tend to become the family’s peacemaker since they often have to mediate conflicts between older and younger siblings. Because they tend to be overshadowed by their eldest siblings, middle children may seek social attention outside of the family.

In families with three children, the youngest male sibling is likely to be more passive or easy-going.

Middleborns are often described as:

• Independent
• Peacemakers
• People pleasers
• Attention-seeking
• Competitive

While they tend to be adaptable and independent, they can also have a rebellious streak that tends to emerge when they want to stand apart from their siblings.

" Middle child syndrome " is a term often used to describe the negative effects of being a middle child. Because middle kids are sometimes overlooked, they may engage in people-pleasing behaviors as adults as a way to garner attention and favor in their lives.

While research is limited, some studies have shown that middle kids are less likely to feel close to their mothers and are more likely to have problems with delinquency.

Some research suggests that middle children may be more sensitive to rejection . As a middle child, you may feel like you didn't get as much attention and were constantly in competition with your siblings. You may struggle with feelings of insecurity, fear of rejection, and poor self-confidence .

Lastborns, often referred to as the "babies" of the family, are often seen as spoiled and pampered compared to their older siblings. Because parents are more experienced at this point (and much busier), they often take a more laissez-faire approach to parenting . 

Last-born children are sometimes described as:

• Free-spirited
• Manipulative
• Self-centered
• Risk-taking

Adler's theory suggests that the youngest children tend to be outgoing, sociable, and charming. While they often have more freedom to explore, they also often feel overshadowed by their elder siblings, referred to as " youngest child syndrome ."

Because parents are sometimes less strict and disciplined with last-borns, these kids may have fewer self-regulation skills.

"If the youngest of many children is female, she tends to be more coddled or cared for, leading to a greater reliance on others compared to her older siblings, especially in larger families," Lev suggests.

Only children are unique in that they never have to share their parents' attention and resources with a sibling. It can be very much like being a firstborn in many ways. These kids may be doted on by their caregivers, but never have younger siblings to interact with, which may have an impact on development.

Only children are often described as:

• Perfectionistic
• High-achieving
• Imaginative
• Self-reliant

Because they interact with adults so much, only children often seem very mature for their age. If you're an only child, you may feel more comfortable being alone and enjoy spending time in solitude pursuing you own creative ideas. You may like having control and, because of your parents' high expectations, have strong perfectionist tendencies .

How Birth Order Influences Relationships

Birth order may affect relationships in a wide variety of ways. For example, it may impact how you form connections with other people. It can also affect how you behave within these relationships.

Dr. Lev suggests that the effects of birth order can differ depending on gender. 

"For instance, in a family with two female siblings, the younger one often appears more confident and empowered, while the older one is more achievement-focused and insecure," she explains.

She also suggests that there is often a notable rivalry between same-sex siblings versus that of mixed-gender siblings. Again, this effect can vary depending on gender. Where an older sister might be less secure and the younger sister more secure, the opposite is often true when it comes to older and younger brothers.

"This could be because older sisters often assume a motherly role, while older brothers might take on more of a bully role. As a result, younger brothers are generally more insecure, whereas younger sisters tend to be more confident than their older siblings," she explains.

Some other potential effects include:

Communication

Birth order can affect how you communicate with others, which can have a powerful impact on relationship dynamics.

• Firstborns and only children are often seen as more direct, which others can sometimes interpret as bossy or controlling.
• Middle children may be less confrontational and more likely to look for solutions that will accommodate everyone.
• Lastborns, on the other hand, may rely more on their sense of humor and charm to guide their social interactions.

Relationship Roles

Birth order may also influence the roles that you take on in a relationship.

• Firstborns, for example, may be more likely to take on a caregiver role. This can be nurturing and supportive, but it can sometimes make partners feel like they are being "parented." 
• Middle children are more likely to be flexible and take a more easygoing approach.
• Lastborns may be more carefree and less rigid.

Expectations

What we expect from relationships can sometimes also be influenced by birth order.

• Firstborns often have high expectations of themselves and others, sometimes leading to criticism when people fall short.
• Middle children are more prone to seek balance in relationships and want to make sure that everyone is treated fairly and contributing equally.
• Lastborns may place the burden of responsibility on their partner's shoulders while they take a more laissez-faire approach.

"Generally, older siblings are more likely to be in the scapegoat role, while the youngest siblings often have a more idealized view of the family," Lev explains.

Other Factors Play a Role

How birth order influences interpersonal relationships can also be influenced by other factors. Some of these include personality differences, parenting styles , the parents' relationship with one another, and even the birth order of the parents themselves.

While birth order theory holds a popular position in culture, much of the available evidence suggests that it likely only has a minimal impact on developmental outcomes. In other words, birth order is only one of many factors that affect how we grow and learn. 

While some research suggests that there are some small personality differences between the oldest and youngest siblings, researchers have concluded that there are no significant differences in personality or cognitive abilities based on birth order.

Birth order doesn't exist in a vacuum. Genetics, socioeconomic status, family resources, health factors, parenting styles, and other environmental variables influence child development. Other family factors, such as age spacing between siblings, sibling gender, and the number of kids in a family, can also moderate the effects of birth order.

Adler’s birth order theory suggests that the order in which you are born into your family can have a lasting impact on your behavior, emotions, and relationships with other people. While there is some support indicating that birth order can affect people in small ways, keep in mind that it is just one part of the developmental puzzle.

Family dynamics are complex, which means that your relationships with both your parents and siblings are influenced by factors like genetics, environment, child temperament, and socioeconomic status.

In other words, there may be some truth to the idea that firstborns get more attention (and responsibility), that middleborns get less attention (and more independence), and that lastborns get more freedom (and less discipline). But the specific dynamics in your family might hinge more on things like resources and parenting styles than on whether you arrived first, middle, or last.

Individual aspects of your own personality are shaped by many things, but you may find it helpful to reflect on your own experiences in your family and consider the influence that birth order might have had.

Damian RI, Roberts BW. Settling the debate on birth order and personality . Proceedings of the National Academy of Sciences . 2015;112(46):14119-14120. doi:10.1073/pnas.1519064112

Luo R, Song L, Chiu I. A closer look at the birth order effect on early cognitive and school readiness development in diverse contexts . Frontiers in Psychology . 2022;13. doi:10.3389/fpsyg.2022.871837

Salmon CA, Daly M. Birth order and familial sentiment . Evolution and Human Behavior . 1998;19(5):299-312. doi:10.1016/S1090-5138(98)00022-1

Cundiff PR. Ordered delinquency: the "effects" of birth order on delinquency . Pers Soc Psychol Bull . 2013;39(8):1017-1029. doi:10.1177/0146167213488215

Çabuker ND, Batık HESBÇMV. Does psychological birth order predict identity perceptions of individuals in emerging adulthood? International Online Journal of Educational Sciences. 2020;12(5):164–176.

Damian RI, Roberts BW. The associations of birth order with personality and intelligence in a representative sample of U.S. high school students . Journal of Research in Personality . 2015;58:96-105. doi:10.1016/j.jrp.2015.05.005

By Kendra Cherry, MSEd Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

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What to Know About Birth Order

Birth order has been used to predict success, prosperity, jobs, and more. But does it really affect who you are and what you do in life? Here's a closer look at the truth and science behind birth order.

The Science of Birth Order

A researcher named Alfred Adler developed birth order theory in the 20th century. The theory claims that the order in which a child is born shapes their development and personality. Adler also claimed that family, community, and social aspects play a major role in shaping a child’s personality.

Today, many psychologists believe that where you fall in your family’s birth order has a major impact on your personality development.

Oldest child. If you're the firstborn child, you have your parents all to yourself for a period of time. Because it’s your parents’ first time being parents, they tend to dole out attention to you. They’re also extra careful when it comes to all aspects of child rearing — from bumps and bruises to early education. Eldest children benefit from all this attention. 

Your parents might expect a lot from you if you're the oldest, especially as other children are born into your family. Your parents might seem stricter with you, and often expect you to set an example for your younger siblings and show responsibility.

Studies show if you're the eldest child, you tend to demonstrate leadership skills. Maybe you're a leader in the workplace already, or rising to the top of your industry.‌

Middle child. It’s a stereotype that if you're a middle child you're a peacekeeper, but there seems to be a grain of truth behind the saying. Because you lack the title of being the oldest or the youngest, you seek to carve out a niche for yourself in the family dynamic . You tend to enjoy negotiation and compromise. You can often relate to people of different ages with ease.

You can be competitive with your older sibling — maybe by breaking their sports records, becoming more fluent in a language, or getting better grades. Or you might behave more rebelliously.

Youngest child. As the baby of the family, you tend to take one of two paths when developing your personality, according to Adler. The first path is a clear journey to success, where you try to excel in every way, often becoming the go-to person in the family. Youngest children can also become avoidant if they lack the confidence or drive to excel.

You might have freedoms your older siblings don't have. As your parents’ rules become more relaxed, your parents can be more hands-off with you. You're usually assured of your place in the family and can be very creative, rebellious, and outgoing.

Only child. If you're an only child, you've been surrounded by adults since birth. That’s not to say you weren’t socialized with other children through kindergarten, the playground, or school. But during your time at home you socialized with parents and other adults, mimicking their behaviors and becoming "small people".

Your parents might be overprotective of you, leading you to become dependent on your parents for support. You aren’t used to sharing clothes, space, or parental attention with siblings. This can make you intelligent and creative, but also stubborn and set in your ways.

Birth Order and Intelligence

Some theories claim that firstborns are more intelligent and have a higher IQ than later-borns. But the most recent studies show that firstborns only hold a 1-point IQ advantage on average over their siblings. Social factors could be to blame for this idea. 

Another study shows that firstborns might benefit from being surrounded by adults during their first years. They absorb the way adults talk, while later-borns are more exposed to the less-developed ways their siblings speak. 

The same study found that firstborns often take a leadership and teaching role to their younger siblings. Some theories say tutoring improves the intelligence of the tutor — in this case, the older sibling. 

Birth Order and Health

Some people think birth order affects health. Studies suggest if you’re the oldest, you're more likely to have a lower birth weight than your siblings. 

Later-born children have an increased risk of hospitalization due to avoidable accidents. This could relate to a lack of parental attention in a large family. Younger siblings may struggle with mental health during puberty and young adulthood, and are more likely to be admitted to the hospital for reasons relating to alcohol. 

Overall, researchers found most birth order differences are due to where your parents choose to spend their time and resources. 

Other Factors That Influence Birth Order

Birth order is a complex subject. There isn’t a “one-size-fits-all” for every family. Different factors affect social family dynamics, and results differ from child to child and family to family.

Some other factors that influence birth order include:

• Parental attitudes and culture. In some cultures, even if a boy is born after four girls, he may still be treated as the oldest.
• Age difference. Adler notes that if children have an age difference of more than three years, subgroups with different dynamics may form. Most experts agree, but place the age gap at five years.
• Twins. Most twins get special attention from parents, so experts say these rules don’t necessarily apply.
• Confidence. A child’s opinion of themselves determines their outlook and attitude. While birth order helps shape their personality, it’s not the only factor.

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• v.112(46); 2015 Nov 17

Settling the debate on birth order and personality

Rodica ioana damian.

a Department of Psychology, University of Houston, Houston, TX, 77204;

Brent W. Roberts

b Department of Psychology, University of Illinois at Urbana–Champaign, Champaign, IL, 61820

Author contributions: R.I.D. and B.W.R. wrote the paper.

Birth order is one of the most pervasive human experiences, which is universally thought to determine how intelligent, nice, responsible, sociable, emotionally stable, and open to new experiences we are ( 1 ). The debate over the effects of birth order on personality has spawned continuous interest for more than 100 y, both from the general public and from scientists. And yet, despite a consistent stream of research, results remained inconclusive and controversial. In the last year, two definitive papers have emerged to show that birth order has little or no substantive effect on personality. In the first paper, a huge sample was used to test the relation between birth order and personality in a between-family design, and the average effect was equal to a correlation of 0.02 ( 2 ). Now, in PNAS, Rohrer, Egloff, and Schmukle ( 3 ) investigate the link between birth order and personality in three large samples from Great Britain, the United States, and Germany, using both between- and within-family designs. The results show that birth order has null effects on personality across the board, with the exception of intelligence and self-reported intellect, where firstborns have slightly higher scores. When combined, the two studies provide definitive evidence that birth order has little or no substantive relation to personality trait development and a minuscule relation to the development of intelligence.

In the wake of these findings, one may ask why previous findings were inconclusive. To address this question, it is essential to understand the current state of research on birth order and personality, as well as the vital methodological contributions of the Rohrer et al. report ( 3 ).

Why Were Previous Studies of Birth Order Inconclusive?

Over the past two decades, hundreds of studies have produced widely ranging estimates of the effects of birth order on personality traits, falling anywhere between a correlation of 0.40 ( 1 ) and 0 ( 4 ). One possible explanation for these inconsistent findings is the pervasive use of underpowered study designs using nonrepresentative population samples. Regarding the link between birth order and intelligence, the results are much more consistent, possibly because of the large representative samples used ( 5 , 6 ). The Rohrer et al. ( 3 ) study addresses the power issues by using three large representative samples from three different countries. This is notable, because only one previous study ( 2 ) had tested the effect of birth order on

Scientific evidence strongly suggests that birth order has little or no substantive relation to personality trait development and a minuscule relation to the development of intelligence.

personality in a large representative sample (in a between-family context). The Rohrer et al. ( 3 ) study replicates the latter findings, and extends them significantly by investigating cross-national patterns and by being the first study to ever explore within-family effects simultaneously with between-family effects in large representative samples. This study also replicates results on birth order and intelligence that have been previously found in large samples in both between- and within-family designs.

A second reason for the lack of consensus has to do with changing standards on what would be deemed the optimal method for testing birth-order effects on personality. Recently, some have argued that between-family designs were inadequate and that only within-family comparisons were up to the task of testing and revealing the role of birth order on personality. A between-family study design compares the personality traits and intelligence of a cross-section of unrelated people who have different birth ranks. In contrast, a within-family design compares the personality traits and intelligence of first- and laterborn siblings from the same family. Between-family designs have been criticized primarily for not being able to adequately control for between-family differences in sibship size, genetic differences, and specific family practices ( 7 ). Ignoring these sources of variance is likely to produce biased estimates of birth-order effects. For example, sibship size, which represents the total number of siblings present in the family, is an important confound because firstborns (vs. laterborns) are more likely to be “found” in low sibships. Because wealthier more educated parents tend to have fewer children, firstborns tend to be overrepresented among families of a high socioeconomic status, the latter being related to personality and intelligence ( 8 ). Thus, any serious attempt at testing the effects of birth order on personality in a between-family design should statistically control for sibship size, which the study by Rohrer et al. ( 3 ) does.

The second criticism brought to between-family designs is that they do not reflect the within-family dynamics put forward by the evolutionary niche-finding model, whereby each child is trying to find a niche that has not yet been filled, to receive maximum investment from the parents ( 9 ). The study by Rohrer et al. ( 3 ) also addresses this issue by supplementing their between-family design with a within-family design (using a subsample of siblings from the same datasets).

Although within-family designs of birth order may be considered superior to between-family designs because they can adequately control for some confounding factors and because they reflect the within-family dynamics put forward by the evolutionary model ( 7 ), they also pose some problems. First, within-family designs, as they are currently used, tend to introduce a perfect age confound ( 10 ). Specifically, studies so far have tested all siblings at the same time, which means the firstborn was always older than the laterborns at the time of assessment. Given what we know about personality development and maturation ( 11 ), it is very possible that the firstborn only appears to be more conscientious, for example, because of being older. The study by Rohrer et al. ( 3 ) is the first study to date to ever address this issue when employing a within-family design by using age-adjusted t -scores.

The second criticism brought to within-family studies of birth order and personality is that they may suffer from demand effects or social stereotypes that may inflate the correlations ( 12 ). This problem is enhanced by the fact that the existing within-family research on birth order and personality has been limited by its use of a single rater from each family ( 4 ). Specifically, the single rater compares oneself against one’s siblings, thus increasing the likelihood of perceiving a contrast. The study by Rohrer et al. ( 3 ) addresses this issue by using independent self-reports collected from each sibling. This is only the second study to ever use independent ratings in the within-family context, and the first to do so while using large representative samples.

Finally, Rohrer et al. ( 3 ) tested the robustness of their findings by conducting additional analyses. One important finding was that the results did not differ by gender, which is relevant because previous theories proposed that stronger effects may emerge among pairs of male siblings ( 13 ). Another important finding is that limiting the data to an age gap between siblings no larger than 5 y also did not change the results. This is important because previous theory ( 1 ) suggested that large age gaps make the effects disappear because there is no sibling competition within the family, but that strong effects should appear for age gaps smaller than 5 y. Rohrer et al. ( 3 ) did not find support for this idea, and their study is unique in its ability to test this hypothesis in a large sample.

In sum, by using large representative samples from three different countries, by assessing personality traits and intelligence in the same study, by using both between- and within-family designs, by using independent self-reports of personality in the within-family context, by taking into account important confounds (such as sibship size in the between-family context and age in the within-family context), and by testing the robustness of the findings in multiple additional analyses, this is the most methodologically sound birth order study to date ( 3 ). When combined with the prior study by Damian and Roberts ( 2 ), which was the largest test of birth order and personality relations, the conclusion is inescapable. Birth order is not an important factor for personality development.

Why Has Birth Order Persisted and Why Might it Still Persist as a Zombie Theory?

If science is truly self-correcting, we feel that the Rohrer et al. ( 3 ) study, when combined with the Damian and Roberts ( 2 ) study, should be the standard against which any new studies on birth order and personality are considered. The largest, most methodologically sophisticated studies in existence show little or no functional relation between birth order and personality. Newer data will have to provide evidence for much larger effects in equally large samples to counter the weight of the evidence.

We are not optimistic that opinions on the effect of birth order will change quickly for a variety of reasons. First, change in science happens slowly. It may take a few years for researchers to digest these findings. Second, some researchers will point out that some of the effects, though quite small in size, were still statistically significant. Although technically correct, this position fails theoretically because the idea of a birth-order effect on personality has always been proposed under the assumption that it could be seen within any given family. We know from past research that it is difficult for observers to detect personality differences that are smaller than one standard deviation in size ( 14 ). The largest birth-order effects we could find were on the order of a 10th of a standard deviation, with the average effect being equivalent to a 25th of a standard deviation. Even if the difference turns out to be statistically significant, it fails to reach a level that parents, relatives, siblings, or friends could notice. In that way, birth-order theory fails despite the statistically significant effects demonstrated in these large studies.

Third, and possibly most interestingly, birth order is an idea that will probably never go away entirely because of its perfect confounding with age. This means that almost everyone has direct experience in which they see older children, who are firstborn, acting and behaving differently than younger children, who are laterborn. Because people are susceptible to weighing anecdotal information more heavily than data-driven findings ( 15 ), there will always be a tendency to think that birth-order effects exist because they will be confused with age differences. The interesting aspect of this perfect confound is that this is one circumstance where personal experience will be wrong and the truth can only be discovered through good scientific reasoning and investigation. The problem in this case is that data-driven findings are seldom as compelling as personal experience.

In conclusion, scientific evidence strongly suggests that birth order has little or no substantive relation to personality trait development and a minuscule relation to the development of intelligence. We commend Rohrer et al. ( 3 ) for conducting the most thorough and methodologically sophisticated examination of the relation between birth order and personality to date. We hope, that the cumulative evidence on birth order and personality is now compelling enough that the idea does not simply become undead ( 16 ), but is clearly laid to rest as a viable explanation for the fascinating differences we see across people and siblings in the typical ways in which they feel, think, and behave.

The authors declare no conflict of interest.

See companion article on page 14224 .

Birth Order and Its Influence on Personality Essay

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Birth order is defined as a person’s rank by the sequence of birth among his or her siblings. Birth order is commonly believed to have a profound and lasting effect on psychological development.

Sulloway (1995, 1996), states that first-born siblings are expected to be rated higher on the characteristic trait of dominance whereas siblings who are younger or born later can be rated higher on the sociability aspect.

The concept of ‘birth-order’ considerably influencing the personality of a being is not a novel phenomenon. Way back in 1874, Sir Francis Galton (Galton, 1874) proposed that renowned male scientists had a greater possibility to be ‘first-borns’ in their family units rather than ‘later-born’ (Forer, 1969). Investigation and study have revealed that individuals who are born first are found in larger numbers in political office (Hudson, 1990). Zajonc, (2001) reveals that there are likely to be birth-order variation in the levels of intellect along with the ‘Big Five’ facets of character including extraversion, anxiety, meticulousness, amicability, and naivety to experience (Paulhus, Sulloway 1996, Trapnell & Chen 1999).

Sulloway (1996, 2001) has suggested a representation of family unit dynamics relating to the results of birth-order in the character and conduct, which has varied ‘‘causal mechanisms’’. In support of this, Sulloway proposes that whilst parental income is inadequate, the parents tend to focus the available resources more profoundly on a single child, usually the one who is born first. Furthermore, Sulloway infers that first-borns are better built and tougher than those born subsequently and thereby employ these competitive physical features tremendously to their benefit.

Further ‘causal mechanisms’ incorporate ‘de-identification’, where children seek to discriminate themselves from each other another with respect to significance, societal approach, individuality, and role demarcation when sometimes, brothers and sisters implement special characteristics in the families for example, “the rebel of the family”, or “the lamb of the family”, so as to lessen rivalry (Sulloway, 1996, 2001).

Then experimental verification to sustain this model of family dynamics has primarily originated from Sulloway’s (1996) ‘meta-analysis’ of one hundred and ninety six studies involving birth-order. Maintaining the hypothesis, first-born position affirmatively allied to meticulousness, anxiety, and assertiveness, whereas the category of later-born absolutely connected with amicability and ingenuousness to experience.

Sulloway further elucidated these results by proposing that first-borns have a greater propensity to accepted by parents and have great enthusiasm to fulfill parental hopes. Moreover, he states that, first-borns tend to be additionally forceful and envious so as to safeguard cherished parental assets. Sulloway further reveals that later-born children, in contrast, have a greater probability to be pleasant to facilitate the decrease of any potential hostile conflicts with their elder siblings.

Jefferson et al., (1998) too found outcomes corresponding to Sulloway’s calculations in a study conducted by him, permitting the friends, neighbors or co-workers to rate the participants instead of the participants evaluating themselves. Keeping in line with the earlier results, later-borns were recognized by their peers as displaying characteristics which were more friendly, pioneering and reliant than their first-born siblings.

Alfred Adler (1870-1937) who was an Austrian psychiatrist was among the foremost theorists proposing that birth order impacts the individuality of a person. He debated that the order in which a child is born has the potential to significantly alter the lifestyle of a person which involves several facets of the personality such as companionship, love, occupation and many other aspects of life. Adler proposes that when a second child is born, the firstborns are ‘dethroned’, thereby having the potential of influencing them permanently as younger siblings and single children are sometimes pampered.

Michael E. Lamb and Brian Sutton-Smith, elucidate the fact that relationships between siblings habitually remain for an entire life span in their book ‘Sibling Relationships: Their Nature and Significance across the Lifespan’ . They highlight the ‘lifespan’ term proposing that progress within relationships is an incessant process and birth-order affects the lifelong process of development of the personality of an individual (Jefferson et al.1998) and as such research has indicated that unification of persons with dissimilar birth orders tend to be steadier than those of persons of the same birth order (Leman 1998, Toman 1976).

A Greek study affirmed that among individuals at the low birth order there is an increased risk for divorce, and debated that this consequence was predominantly apparent among single children (Skalkidou 2000).

The reason why partners’ birth order is likely to impact matrimonial strength is due to the effect of birth position of either spouse which consequently affects the personality and its development process, thereby producing specific traits in individuals bringing variations in their compatibility to others.

First-borns are likely to be dominating individuals and the subsequent unification of two individuals of the same dominating order, presents a greater likelihood of a divide. The same rule would apply to the union of two last born individuals who are likely to have comparatively unsteady relationships (Leman 1998, Toman 1976).

The reasons why birth order of individuals has the potential of affecting divorce rates is primarily due to the fact that individuals may have specific traits in their personalities such as a poorer levels of risk avoidance which may perhaps influence marriage termination pace. Personalities are known to be affected by birth order because the relations of parents with children is diverse and is to a great extent impacted by the birth order of the siblings (Ernst and Angst 1983), as a result of which dissimilar birth orders can stimulate parents to socialize in dissimilar ways with their children.

Influence of birth order in understanding of the different scientists

According to Eisenman (1992), first-borns are extra apprehensive, superior accomplishers and more innovative, resulting their lives to be extremely dissimilar merely because of them being born as a first child in the family. This may attribute to the fact that a majority of parents tend to be excessively concerned for their first children, and also may possibly be more restraining with him or her than with children born later. It is this theory that elucidates the higher risk of divorce among marriages between first-borns, because restriction is certainly interrelated with an annulment threat (Jockin et al. 1996).

Moore et al. (1997), who researched thirty nine mothers and their first and second born children of two months of age, found that mothers tend to positively influence their second-born children as compared to the first-borns, consequently resulting in the second rank children to develop a more positive personality than the first born rank.

Obviously, this behavioral difference can affect the risk of divorce given the fact that positivism and negativity in emotions are directly correlated to potential divorce risks (Jockin et al. 1996).

We can thus safely conclude that birth-order does have effects on the individual personality development and the primary aspects of extraversion, friendliness and domination, vary in relation to birth-order (Jefferson et al.,1998).

Beer and Horn (2000), implemented an innovative approach by researching a sample of adopted children, and found a similarity in proved results emphasizing that first-reared children do have an elevated intensity of meticulousness. Behavioral patterns in individuals are firmly fashioned in childhood with the profound influence of parents and siblings and therefore are likely to affect marriage stability in the life of individuals. Children who are born first along with single children have a greater likelihood of developing particular characteristic qualities, including anxiety and aspiration, resulting a child who is a first born to develop a steadier relationship with a potential later-born having dominating aspects in the persona.

However, in the case of both the marital partners being first-borns, potentially implies a higher level of conflict given that both may reflect qualities of being stubborn or less compromising behaviors. For instance, if either partner is a first-born individual, and the other is a later-born, the resultant relationship is likely to be steadier in view of the “leader-follower” kind of union, where one partner is less dominating than the other, thereby reducing the chances of disagreement between them. In the dissimilar case of both the marital partners being later-born individuals, there may be an absence of a leader in the relationship, resulting in an unsatisfying relationship between the two.

Thus we see how there is an effect of birth order on divorce risk. It is evident that single children run a lower risk of dissolving their marriages. Marriages between two first-borns are the most unstable relationships, whereas marriages between two only children are the most stable.

Ever since the 1970s, the ‘confluence model’ of Robert Zajonc has been significant in elucidating one of the most important hypothesis to explicate why firstborn children or individuals habitually achieve better scores for their intellect and accomplishment tests than other children. According to the model, firstborn children early on in their lives, primarily have greater adult authority surrounding them and as a result, they spend their preliminary childhood years intermingling in a scholarly environment. This theory also proposes that firstborn children tend to be more intelligent than ‘only children’, because ‘single children’ cannot benefit from the ‘tutor effect’ of elder siblings teaching the younger ones.

According to recent researchers conducted and reported in the journal Science (June 2007), “the eldest children in families tend to develop slightly higher IQs than their younger siblings” (Harris, Judith Rich, 2006). These results could possibly be an outcome of the valuable quality time that parents tend to spend with their children who are born first rather than those who are born later. (Price, Joseph, 2008)

The discussion as to whether birth-order affects the personality development of an individual is in no way over and expectantly new researchers will explore fresh perspectives and means in the birth-order research.

Adler, A. (1964). Problems of neurosis . New York: Harper and Row.

E. Beck et al. / Personality and Individual Differences.

Eisenman, R. 1992. Birth order, development and personality. Acta Paedopsychiatr. 1992;55(1):25-7.

Jockin V, M. McGue and D. Lykken (1996): Personality and divorce: a genetic analysis.

Ernst, C., & Angst, J. 1983. Birth order: Its influence on personality. Berlin: Springer-Verlag.

Forer, L. K. (1969). Birth-order and life roles. Illinois: Charles C. Thomas.

Galton, F. (1874). English men of science: Their nature and nurture. London: Macmillan.

Harris, Judith Rich (2006), No Two Alike: Human Nature and Human Individuality (pp. 107-112)

Hudson, V. M. (1990). Birth-order of world leaders: an exploratory analysis of effects on personality and behaviour. Political Psychology, 11, 583–601.

Jefferson, T. J., Herbst, J. H., & McCrae, R. R. (1998). Associations of birth-order and personality traits: evidence from self-reports and observer ratings. Journal of Research in Personality, 32, 498–509.

Lamb, M. E., Sutton-Smith, B. (1982). Sibling Relationships: Their Nature and Significance of the Lifespan. Lawrence Erlbaum Associates.

Leman, K. 1998. The New Birth Order Book: Why You Are the Way You Are”.

Moore, G.A., Cohn, J.F., & Campbell, S.B. 1997. Mothers’ affective behavior with infant siblings: Stability and change. Developmental Psychology.

Paulhus, D. L., Trapnell, P. D., & Chen, D. (1999). Birth-order effects on personality and achievement within families. Psychological Science.

Price, Joseph (2008). “Parent-Child Quality Time: Does Birth Order Matter?”

Skalkidou, A. 2000. “Parental Family Variables and Likelihood of Divorce”.

Sulloway, F. J. (1995). Birth-order and evolutionary psychology: a meta-analytic overview. Psychological Inquiry, 6, 75–80.

Sulloway, F. J. (1996). Born to rebel: Birth-order, family dynamics, and creative lives. New York: Pantheon Books.

Sulloway, F. J. (2001). Birth-order, sibling competition, and human behavior. In H. R. Holcomb, III (Ed.), Conceptual challenges in evolutionary psychology: Innovative research strategies. Dordrecht, the Netherlands: Kluwer Academic.

Toman, W. Family Constellation 1976, New York: Springer.

Zajonc, R. B. (2001). The family dynamics of intellectual development. American Psychologist, 56, 490–496.

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IvyPanda. (2021, August 17). Birth Order and Its Influence on Personality. https://ivypanda.com/essays/birth-order-and-its-influence-on-personality/

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1. IvyPanda . "Birth Order and Its Influence on Personality." August 17, 2021. https://ivypanda.com/essays/birth-order-and-its-influence-on-personality/.

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• Family Dynamics

Does Your Birth Order Actually Matter?

New research tries to separate the myths from the facts..

Posted November 17, 2015 | Reviewed by Ekua Hagan

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• One study reported firstborn children have a higher IQ of 1.5 points; another study cited the IQ difference as "almost imperceptible."
• New studies find that personality is not determined by one's birth order.
• The differences parents see in their children as relates to a child’s position in the family may be more an effect of age than anything else.

Forget the labels you assign to yourself or your children based on your or their position in a sibling hierarchy. Personality traits we have long applied to firstborn, middle, or youngest children have been challenged and the previous evidence found questionable, if not just incorrect.

In 1928, Alfred Adler looked at children’s position in the family and claimed that birth order had a significant effect on personality. Decades later, proponent Frank Sulloway advanced that position in his notable book, Born to Rebel: Birth Order, Family Dynamics, and Creative Lives . Sulloway argued that firstborns are more assertive , ambitious, and conforming, while later-born siblings are more adventurous and rebellious, to cite just a few examples.

Psychological theory and cultural archetypes about birth order remain embedded in current thinking. Who hasn’t heard comments like, “I can always count on my oldest, he’s so dependable," or, about one's youngest, “She’s so outgoing and social.”

Views like Sulloway’s have been accepted as fact and held fast—until now.

Birth Order Thinking Debunked

Contrary to popular belief, birth order is not an important factor in personality development, although it is frequently cited to explain that development, as well as differences in intelligence within families.

New studies, however, find that personality is not determined by one's birth order. Two large studies scrutinized what is known as “The Big Five ” personality traits— extraversion , emotional stability , agreeableness , conscientiousness , and openness to experience —to determine the effect of birth order on personality development.

In one study, Julia Rohrer of the University of Leipzig, and her colleagues Boris Egloff and Stefan Schmukle, scrutinized data from 20,000 adults in the United States, Germany, and England. In addition to comparing siblings in the same family (within-family design), they also looked at differences between families (between-family design). (These study models, when used independently of each other, result in findings that are often questioned by researchers.)

Rohrer determined that over a life’s course, “birth order does not have a lasting effect on broad personality traits outside of the intellectual domain.” She found "no birth-order effects on extraversion, emotional stability, agreeableness, conscientiousness, or imagination . This finding contradicts lay beliefs," she wrote, as well as prominent scientific theories, and indicates that the development of personality is less determined by one's role within the family of origin than previously thought.

According to the study, the differences parents see in their children as relates to a child’s position in the family may be more an effect of age than anything else. A younger sibling may become more conscientious with time, or as conscientious and reliable as an older brother or sister who is the firstborn. In other words, we might be calling a behavior a birth order trait when we should be wondering if it’s an age effect .

The second study drew similar conclusions. Researchers Rodica Damian and Brent Roberts of the University of Houston and the University of Illinois, respectively, note that after looking at data for 337,000 students, “We found very small associations between birth order and personality."

In a commentary for The Proceedings of the National Academy of Sciences entitled, “Settling the debate on birth order and personality,” they wrote:

“Scientific evidence strongly suggests that birth order has little or no substantive relation to personality trait development and a minuscule relation to the development of intelligence.”

Are Firstborns Really Smarter?

Both studies looked at the claim that firstborns are more intelligent and did find a small edge for the oldest child. Rohrer’s study reports a 1.5-point difference in IQ; Damian and Roberts found “an almost imperceptible 1 point on an IQ test in the largest case," and added:

“Although, again, the effect sizes were quite small, we did find larger partial correlations for verbal ability (as opposed to math or spatial ability), which is in line with the predictions of the model, whereby the parental attention advantage that firstborns have mostly consists of verbal stimulation.”

Birth Order Labeling Is Stereotyping

The birth order debate has gone on in the scientific community for decades. We now realize that often the weight we put on birth order is simply a convenient way to explain our children’s (or our own) behavior: “What do you expect? I was the youngest child.” “Of course, Kelly will have everything ready. She’s my responsible firstborn.”

However, labeling children feels unproductive for their development, besides probably being inaccurate. In reality, birth order is just one more type of stereotyping. We know that people cling to longstanding notions whether or not they are valid: The stereotypes assigned to only children have lingered in spite of reams of evidence disproving them.

Copyright @2015, 2019 by Susan Newman

Adler, A. (1928) Characteristics of the first, second and third child. Children 3:14–52.

Damian, R.I., Roberts, B.W. “The Associations of Birth Order with Personality and Intelligence in a Representative Sample of U.S. High School Students.” Journal of Research in Personality : Volume 58, October 2015, Pages 96–105.

doi: http://dx.doi.org/10.1016/j.jrp.2015.05.005

Damian, R.I. and Roberts, B.W. “Settling the debate on birth order and personality.” The Proceedings of the National Academy of Sciences. October, 2015. doi:10.1073/pnas.151906411

Rohrer, Julia, Boris Egloff and Stefan C Schmukle, “ Examining the effects of birth order on personality. ” October, 2015.

Sulloway FJ (2007) Birth order and intelligence. Science 316:1711–1712.

Sulloway, Frank J. Born to Rebel: Birth Order, Family Dynamics, and Creative Lives . New York: Pantheon,1996 First edition. http://amzn.to/1MpUPiu

Susan Newman, Ph.D. , is a social psychologist and author. Her latest book is The Book of No: 365 Ways to Say it and Mean it—and Stop People-Pleasing Forever.

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Exploring “Birth Order Theory” & Attachment Styles

Birth order theory and attachment theory are linked in that they can influence our personalities and how we think and act in relationships. This article explores the connection between both phenomena and how they can impact our development across the lifespan.

Have you ever felt like your siblings have a different relationship with your caregivers than you? While your parents may shrug this off as untrue, there may be some accuracy to it, and these differences could be partially due to your birth order.

Birth order is a person’s rank among their siblings in order of age. According to birth order theory , our birth order can significantly affect many aspects of life, including the attachment style we develop and our thoughts and behaviors in later relationships.

To answer all of your questions on birth order theory and attachment styles, this article will cover what birth order theory is and the influence of birth order theory on attachment.

What Is Birth Order Theory?

Birth order theory is the idea that the order in which we’re born plays a significant role in shaping our personality as children. This idea was first developed by researcher Alfred Adler in the 20th century. However, since then, many psychologists have studied the phenomenon and claim that birth order can also influence the attachment style we develop and how we think and behave in later relationships.

How Might Birth Order Influence Personality?

When birth order theory was first developed, Adler claimed that firstborns were “power-hungry,” the middle child stereotype was that they were highly competitive, and lastborns were lazy and spoiled. However, you’ll be pleased to know that much has changed as we’ve learned more about the effects birth order has on us.

Recent research into birth order theory appears to show that firstborns tend to be more conscientious than their siblings. Their conscientiousness is often a combination of ambitiousness, responsibility, organization, and academic success. Firstborns also tend to be more extroverted in a dominant and assertive way and often score higher on neuroticism—the personality trait associated with anxiety, irritability, and self-doubt. While this could be due to a number of environmental factors relating to being a firstborn, experts suggest that this personality difference may be due to their anxiety around their status in the family.

Laterborns also often show extroversion, but in a more friendly and fun-loving way. Youngest child traits also include being generally more agreeable than firstborns, too—they tend to be more selfless, compassionate, and eager to please. Furthermore, they’re typically more open to new experiences, so they may be more adventurous and unconventional, frequently engaging in more risk-taking activities like extreme sports.

Influence of Birth Order on Attachment Styles

Adler proposed that the caregiver-child relationship and the care and protection the caregiver provides are dependent on birth order. So, based on this, it’s likely that birth order plays a role in the attachment style we develop. However, it’s important to note that our attachment bonds are multifaceted. Birth order is by no means the only influential factor—instead, it’s one of many.

So, let’s take a look at the pattern for firstborns, middle children, and lastborns, as well as only children, and how their birth order influences their attachment style.

Firstborns & Stereotypes

Birth order personality traits are not “set in stone” – such children are not born with particular personality traits ingrained into their persona. Nevertheless, there are some stereotypes around firstborn personality traits. For example, Adler believed that firstborn children are influenced by their caregivers to be more responsible and ambitious. Other firstborn stereotypes include being more independent, assertive, and responsible. Furthermore, people believe that firstborns attempt to impress their parents by being more conscientious and acting as caretakers to younger children.

Firstborn Traits

Studies concerning birth order theory suggest that there are a number of factors outside of familal which contribute to certain personality traits in firstborn children. These factors include:

• Physical environment
• And situational factors

Regardless, there are still certain personality traits associated with firstborns according to birth order theory. These personality traits include:

• Dependability and reliability
• High achievement and leadership
• Feelings of superiority
• Feelings of rejection when a second child is born
• Authoritarian behaviors around rules and regulations
• People-pleasing actions
• Attention-seeking behaviors to capture caregivers’ attention

However, bear in mind that these are traits in general, and that other factors outside of the family environment can go a long way towards shaping a child’s personality.

Firstborns & Attachment Styles

Firstborn children have their caregivers’ undivided attention until their first sibling comes along. Our first two years of life are fundamental in forming our attachment style: So, if a firstborn child had their caregivers’ undivided attention for the first two years of their life, they may be more likely to develop a secure attachment style .

However, this isn’t always the case. For example, it’s normal for caregivers to have new-parent nerves when caring for their first child. As the experience is novel and unknown, caregivers are often uneasy, worried, or over-protective, sometimes resulting in a stricter or more anxiety-led parenting style. Firstborns may be less self-sufficient and independent as a result and might develop an insecure attachment style .

Furthermore, a firstborn’s “dethronement” (as many experts call it) when their first sibling is born may cause an attachment rupture , potentially leading to an insecure attachment style.

YouGov—a global public opinion and data company—conducted a survey on birth order in 2023 and found that 32% of firstborns considered themselves securely attached, whereas 16% of them said they had a disorganized attachment . 20% identified as avoidantly attached , and 18% as anxiously attached .

However, we need to take these findings with a pinch of salt. As this survey was based on self-reported data, it may not be 100% accurate, as, sometimes, self-reporting can be prone to bias. Nevertheless, these general trends can help us build an understanding of how birth order may influence our attachment styles.

If you’re unsure of your attachment style and want help identifying it, check out our free Attachment Style Quiz .

Middle Children & Stereotypes

As we mentioned for first borns, birth order personality traits are not rigid – children are not born with particular personality traits ingrained into their persona. Nevertheless, there are some stereotypes around middle child personality traits. For example, “middle child syndrome” is a widely known phenomenon whereby people understand that middle children are attention seekers, feel excluded or even neglected, and have low self esteem as a result. However, studies concerning birth order theory suggest that there are a number of factors outside of familal which contribute to certain personality traits in middle children. These factors include:

Regardless, there are still certain personality traits associated with firstborns according to birth order theory. We’ll discuss these next.

Middle Children Traits

According to studies concerning birth order theory and personality traits, middle born children may be prone to characteristics such as:

• High independence
• Competitiveness with other children in the family
• Good social skills with friends and adults
• Risk-taking behaviors
• Feelings of isolation
• Easy going nature so as to not “rock the boat”
• Low self-esteem
• Good sharing abilities
• Peacemaking between family members

Having said as much about these traits of middle children, remember, this is just a guideline based on birth order. The other environmental factors discussed above will influence a child’s development.

Middle Children & Attachment Styles

The middle child tends to be in the most competition for their caregivers’ attention because they have to rival both their older and younger siblings. They aren’t given the undivided attention of the firstborn and aren’t the prime focus of their caregivers’ attention for long, either, as their younger sibling comes along.

This continual competition for attention may cause middle children to feel uncertain about their caregivers’ feelings towards them and their value as a whole, leading to an insecure attachment style. Research also suggests that middle children often feel less invested in their families but tend to turn to friends as important attachment relationships.

The YouGov Survey found that 33% of middle children were securely attached, 20% had a disorganized attachment style, 20% were avoidantly attached, and 18% were anxiously attached.

Lastborns & Stereotypes

Again, the personality traits according to birth order theory and last borns are not inflexible; childrens’ environmental dynamics also influence the traits the develop. However, there are certain stereotypes around the characteristics of last born children, which include acting jovial and clownish, yet also manipulative and charming in attempts to get what they want from family members.

However, studies around the environmental dynamics which influence a child’s personality traits include:

Regardless, there are still certain personality traits associated with lastborns according to birth order theory. We’ll discuss these next.

Lastborn Traits

According to studies concerning birth order theory and personality traits, lastborn children may be prone to characteristics such as:

• A fun-loving nature
• Charming and charismatic personality
• Manipulative tendencies
• Extraversion
• Attention seeking actions
• Self-centredness
• Good social skills

Although these traits of lastborn children are in line with birth order theory, it’s still important to be mindful of the environmental factors which contribute to a child’s personality traits.

Lastborns & Attachment Styles

Being the last born has unique benefits—they receive attention from their caregivers and their older siblings. What’s more, as the youngest is the last child caregivers will ever have and are the most vulnerable to illness because they are the smallest (as children, anyway), caregivers tend to offer them more support than middle children. As a result, lastborns tend to have stronger attachments than middle children and feel like a more integral part of the family as they get older.

The YouGov Survey found that 37% of lastborns say they have a secure attachment style. 18% report having a disorganized attachment, 16% an avoidant attachment style, and 16% an anxious attachment style.

Only Children & Stereotypes

The stereotypes of an only child’s personality include those of these children being spoiled, narcissistic, and self-centred in comparison to children with siblings. However, these are out of date stereotypes and the traits of only children according to birth order theory are quite different. We’ll discuss these traits momentarily. First, as a recap, the environmental factors associated with a child’s personality development include:

Only Child Traits

The research concerning birth order theory and only children account for the following personality traits:

• Independence
• Sensitivity
• Cautiousness
• Ambitiousness
• Lack of competitiveness

However, as previously mentioned, these traits don’t account for the environmental factors which we discussed.

Only Children & Attachment Styles

Only children are never “dethroned,” like lastborns. However, they differ from lastborns in that they don’t ever have to compete for their caregivers’ attention, as they have no siblings to compete with.

Yet, despite the lack of sibling rivalry, being an only child still appears to influence attachment style. For example, only children often spend most of their time with adults as they have no one else their age within their direct family unit. As a result, they become “adult-oriented”—they behave in more adult ways and have worries more commonly associated with adults.

Only children also tend to develop higher conversational abilities but less age-appropriate social skills, which means they generally feel more comfortable around adults than people their age. So, while approximately 27% of only children consider themselves securely attached (as per the YouGov Survey), they may struggle to form healthy attachment relationships later in life because of these social difficulties.

Disorganized attachment appears particularly common in only children, with around 25% of only children claiming they have this attachment style. A smaller 20% of only children identify as having an avoidant attachment, and 17% as anxiously attached.

A Small Caveat of Birth Order Theory & Attachment Styles

Before we conclude this article, it’s important to point out that these are the general trends we see based on the current research. Any child, regardless of their birth order, is able to develop a secure attachment style as long as their caregiver attunes to their needs and responds to them with empathy, warmth, and understanding.

Furthermore, our unique childhood experiences influence us in adulthood in different ways. So, for example, you can be a firstborn and still be adventurous, or you can be a lastborn and be highly independent. Your birth order doesn’t define you, but it may make you more likely to encounter certain experiences.

Final Word on Birth Order Theory & Attachment Styles

Birth order theory suggests that we’re more likely to be secure or insecurely attached based on whether we’re born first, sandwiched between two or more siblings, or last. While firstborns get their caregivers’ undivided attention, this may not be for long. When their siblings come along, they’re “dethroned,” which can shake the very foundation of their attachment bonds.

Middle children often pull the short straw as they never get their caregivers’ undivided attention, but they can find solace in the comfort of friendships. Yet, the opposite can be said for lastborns—they tend to develop strong family bonds and receive undivided attention due to being the youngest and, therefore, most vulnerable.

Only children, on the other hand, never face sibling rivalry. However, they face different challenges, developing adult tendencies in childhood, and making relationships harder in later life. Yet, as we previously mentioned, your birth order doesn’t define you–your experiences, personality, and relationships to date help make you the unique person you are.

Adams, B. N. (1972). Birth order: a critical review. Sociometry, 35(3), 411–439.

Ballard, J. (2023, November 9). Most only children wish they had at least one sibling. YouGov.

Bayraktar, S., & Yahşi, G. (2016). The effect of birth order on attachment style according to attachment theory. International Journal of Social Sciences and Education Research, 2(1), 209–218.

Buunk, B. P. (1997). Personality, birth order and attachment styles as related to various types of jealousy. Personality and Individual Differences, 23(6), 997–1006.

McGuirk, E. M., & Pettijohn, T. F. II. (2008). Birth order and romantic relationship styles and attitudes in college students. North American Journal of Psychology, 10(1), 37–52.

Reebye P. (2007). Understanding Attachment & Attachment Disorders: Theory, Evidence, and Practice. Journal of the Canadian Academy of Child and Adolescent Psychiatry, 16(4), 184–185.

Salmon, C. (2003). Birth order and relationships. Human Nature, 14(1), 73–88.

Suitor, J. J., & Pillemer, K. (2007). Mothers’ Favoritism in Later Life: The Role of Children’s Birth Order. Research on Aging, 29(1), 32–55.

Sulloway, F. J. (1996). Born to rebel: Birth order, family dynamics, and creative lives. Pantheon Books.

WebMD Editorial Contributors. (2023, July 12). What to Know About Birth Order. WebMD.

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July 2, 2014 by Christie Hunter

Birth Order as an Adult

July 2, 2014 04:55 by Christie Hunter    [About the Author]

Although parents love every child equally, they cannot pay equal attention to every child. Therefore, factors like the number of siblings, age gap between siblings and birth order decide the personality of a child (Leman, 2009). 1

This article will highlight the ways in which birth order affects the adult life of a person and what are the factors or personality traits which are directly influenced by the birth order. Moreover parents will also be guided to adopt the right parenting strategy for every child in accordance with the birth order:

Personality Analysis According to Birth Order

Varma (2013) 2 in a research study conducted an in-depth analysis of how birth order can influence various personality traits of a person. Here is a quick overview of personality analysis according to the birth order:

1. First-Born

First-borns are usually considered as experimental children. Mostly they are overly pampered kids which make them dominant and a little overpowering. Parents’ adequate attention and time during early childhood years help them in developing their cognitive and learning abilities.

As Adults :   As adults, first-borns are keen observers and confident. They like to make decisions on their own and avoid depending on others. Their ability to differentiate between right and wrong and responsibility make them reliable leaders. However, first-borns are usually attention seekers and don’t like others overpowering them.

2. Middle Children

Griffin (2012) 3 conducted a detailed research on the influence of birth order on human behavior and learning abilities in which the author mentioned that middle children are the trickiest ones to deal with. They usually feel deprived of attention and care because of younger and older siblings, therefore needing constant attention from parents.  However, most parents try to keep middle children engaged in different activities so they don’t feel attention deprived, which makes them creative and innovative.

As Adults :   As adults, middle children are stubborn and sometimes unreasonable. Although they are not perfectionists, they are extremely creative and have a different vision from others. Another quality of middle children which makes them stand out of the crowd is their sprit to strive for their right. They are usually quite straightforward and outspoken.

3. Last-Born

Youngest children not only receive attention and affection from their parents, but also from older siblings, which makes them quite demanding. Irrespective of their age, parents and other siblings always treat them as kids and this can impact their decision-making abilities.

As Adults :  As adults, they are usually careless and unorganized. Attention and care makes them self-centered and sometimes manipulative. However, they are fun-loving, innovative and have good learning abilities.

Aspects Influenced by Birth Order

The study Barnett (2013) 4 highlighted a number of aspects of adult life that are influenced by the birth order. Some of these aspects are:

First-borns usually have good reading habits and therefore have higher intelligence levels than other siblings. As adults, their ability to analyze various aspects of a problem makes them reliable decision makers.

2. Career Success

As youngest children are usually self-centered and ambitious they strive a lot for a successful career. However, a number of studies mentioned that career success and choice also depend on a number of factors, like competence, leadership abilities etc.

3. Decision-Making Abilities

As middle children usually don’t get adequate time from parents and learn to make decisions on their own from a very young age, they are usually brilliant decision makers. However, unlike first-borns they are not dominant and consult a number of people before making a move.

4. Marital Relationship

Usually middle children and first-borns are more understanding than the youngest ones. This is the reason that youngest children usually don’t lead content married lives, as they are usually rigid and have unrealistic expectations for their partner. 

Parenting Strategies

Pollet (2009) 5 mentioned in the study that it is very important for parents to understand the psychology of their children and adopt the right parenting strategy in accordance with their needs. It is also very important for parents to maintain a good age gap between children, so every child can receive adequate attention and affection from parents.

_________________________________________________________________________________________________________________________________

1. Leman K. (2009). The Birth Order Book: Why you are the way you are?

2. Varma S. (2013). What Birth Order Can Predict About You?

3. Griffin L. (2012). Born In Between the secret power of middle children

4. Barnett H. (2013). How your birth order affects your adult life?

5. Pollet TV. (2009). Birth order and adult family relationships: Firstborns have better sibling relationships than later-born?

About the Author

Christie Hunter is registered clinical counselor in British Columbia and co-founder of Theravive. She is a certified management accountant. She has a masters of arts in counseling psychology from Liberty University with specialty in marriage and family and a post-graduate specialty in trauma resolution. In 2007 she started Theravive with her husband in order to help make mental health care easily attainable and nonthreatening. She has a passion for gifted children and their education. You can reach Christie at 360-350-8627 or write her at christie - at - theravive.com.

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Birth Order Theory: Insights Into Your Personality

Personality can develop from various sources and influences in a child's life, including birth order. Studying personality and its formation has interested researchers, psychologists, and scientists for centuries. Enduring characteristics, traits, and behavior can shape each person's unique adjustment to life.

Birth order theory suggests that while personality is mainly unpredictable, specific general characteristics can be linked to a person's birth order in their family. Birth order refers to the rank of siblings in relation to age. It's thought that parents intentionally or unintentionally assign roles based on birth order, which may impact a child’s personality development.

In this article, we'll explore the theories and studies behind personality development, focusing on birth order theory.

Birth order theory: Why it matters

Theories on personality formation, adaptation, and environmental influences across cultures vary. The concept of birth order is often credited to Austrian psychoanalyst Alfred Adler in the early 1900s. He was one of the first to explore the idea that a person's place in their family tree could predict personality traits. 

Adler believed that firstborn children typically have higher expectations placed upon them by parents and thus develop a greater sense of responsibility and ambition. He proposed that later-born siblings, on the other hand, were often treated more leniently by their parents compared to firstborns, leading them to become more rebellious and independent.

However, it is important to note that Adler's theories are not universally accepted, and a person's place in their family tree does not always dictate their personality traits. Ultimately, each person is unique and should be treated as such. Each person has strengths and weaknesses independent of their birth order.

What birth order theory is not

Birth-order personality traits are not necessarily present when a child is born into a family. For example, firstborn children are not necessarily born with niche or particular personality traits ingrained in their psyche. Instead, in birth order theory, Adler illustrates how family environments and dynamics can shape individual psychology during a child's formative years. Although every family is different, there are similarities in the interactions between parents and children and siblings.

The family's role in birth order personality traits

Birth order research and studies show several influences shaping personality in addition to birth order. Common factors include:

• Biological: Children tend to inherit many traits and features from their parents. These can include intelligence, courage, and physical characteristics.
• Social: By interacting with others in an individual's social circle, children learn behaviors and thought patterns from their experiences, like those in the education system and beyond.
• Cultural: A child growing up within a culture consciously or unconsciously can adopt traits consistent with the culture's beliefs, ideas, and norms.
• Physical Environment: An individual's surroundings often impact the development of personality. For example, the personalities of those growing up in a rural area may differ from those living in an urban environment.
• Situational: As a child grows up, they face different situations, which may help them adapt and change their personality. These situations could include meeting new friends, experiencing trauma, or welcoming a new sibling.

When looking at these factors, we see family life can incorporate all of these. Since most children's lives are, at first, shaped by everything going on in the family, it makes sense that some psychologists have remained interested in birth order theory throughout the years since Adler first proposed his idea.

How birth order may affect personality

The following traits are general examples of how birth order differences and personality may be related. Of course, many other factors could impact the development of a child's personality; some of these reasons will be discussed further below.

These children tend to get much more attention from adults than a child with siblings does. This means many of their early interactions involve individuals significantly older than them. These interactions can make them feel like "tiny adults," and they can seem more mature than their peers with siblings. Traits may include:

• Maturity for their age
• Sensitivity
• Use of adult language
• Self-centeredness
• A tendency to enjoy being the center of attention
• Refusal to cooperate with others
• A tendency to feel unfairly treated when not getting their way
• A desire to be more like adults, so may not relate well with peers

First child

Since the firstborn child is used to being an only child until siblings come along, they may exhibit some of the characteristics of an only child. Also, the firstborn may have these birth order personality traits:

• Achiever and leader
• Feelings of superiority over other children
• Difficulty when the second child is born, such as feeling unloved or neglected
• A tendency to be controlling and focused on being correct about results
• Use of good (or bad) behavior to regain parents' attention
• A tendency to be bossy or authoritarian about rules
• A desire to please others
• Reliability

Second child

Second-born and middle children begin their lives with their parents' attention on the firstborn. Having an older sibling as a role model makes second-born and middle children try to catch up with older children. Adler believes the second child can be better adjusted. A second child may:

• Be more competitive
• Lack of the undivided attention of parents
• Be a people pleaser
• Be a peacemaker
• Develop abilities the first child doesn't exhibit to gain attention
• Be rebellious
• Be independent and not need the support of others

Middle child

Many have heard of the "middle child syndrome" and the difficulties these children can present. They may become frustrated or resentful of the significant changes they experience early in life. Not only do they lose their "youngest child" status, but they also must compete for attention with older and later-born children. 

Middle-born children in larger families are typically less competitive than single middle children. Their parents' attention can be spread thinner due to the dynamics of a bigger family. Middle children in bigger families may be more prone to use cooperation to get what they want. Middle children may demonstrate the following tendencies:

• Can feel life is unfair
• Can be even-tempered
• May feel unloved or left out
• May not have the rights and responsibilities of the oldest sibling or the privileges of the youngest
• May be adaptable
• Can be impatient
• May be outgoing and rambunctious
• May treat younger siblings more roughly
• Can feel "squeezed" in the family environment

Youngest child

The "baby" of the family tends to get more attention from parents since the older siblings are developing and becoming more independent. Traits of the youngest child may include the following:

• May be charming and outgoing
• Can be an attention seeker
• Behaves like the only child
• Feels inferior, like everyone is bigger or more capable
• Expects others to make decisions and take responsibility
• May not be taken seriously
• Can become "speedier" in development to catch up to other siblings

Other factors that may influence birth order personality

Each family is different and has unique dynamics. The subject of birth order positions alone will not determine the complexities of a person’s personality. Certain circumstances or measures may impact a child's personality as children and families develop and evolve. Across different families, children of the same birth order can show diverse personality differences, especially across a large representative sample.

Blended or step-families

When two parents remarry, especially when children are in their formative years, the family of origin often goes through a period of disorientation and competition. For example, two firstborns in the new family may search for their "place" and may compete to keep their "firstborn status."

Differences in ages

When there is an age gap of three or more years between siblings, it is common for the birth orders to restart. In a family with many children, this could create birth order subgroups with varying birth order effects.

Health and mental issues

A child born with significant physical or neurodevelopmental disabilities can seem to remain in the "youngest" position regardless of birth order. It may impact the psychological birth order position of the other children.

Gender of siblings

Most psychological competition tends to occur between children of the same gender and similar ages. The competition, partly for parental attention , can start in childhood and move into young adulthood and beyond.

Death of a sibling

The impact of a child's death can be devastating for families. Some children may adapt by developing overindulgent tendencies. Also, a glorification of the deceased child can occur, whereby other siblings may never live up to the image of the deceased sibling. It can profoundly alter the birth order effect.

An adopted child often has special circumstances in the family dynamic. Having an adopted child may be seen as a special gift for parents with difficulties conceiving. These parents may have a greater tendency to spoil or overindulge the child. When an adopted child comes into an established family, they may find difficulties fitting into the dynamic.

Does a correlation between birth order and personality exist?

Multiple factors, including socioeconomic status, parental attitudes, gender roles, and social influences, can also shape an individual's personality. In a study of more than 20,000 participants, however, details revealed no significant effects of birth order of the Big Five personality traits: extraversion, emotional stability, agreeableness, conscientiousness, and openness to experience.

However, some research has linked higher intelligence to a family's older children. It could be because parents have more emotional and intellectual resources to give when fewer children are present in the home.

Gain additional insights into your personality in therapy

If you’d like to gain more insight into your personality or how your birth order shapes it, consider working with a mental health professional. With a therapist, you can explore how your early childhood experiences shape your current behavior and develop coping skills to help you navigate life’s challenges. Your therapist can also provide evidence-based strategies for managing challenging emotions and building healthy relationships.

If you are not interested in traditional in-person therapy, consider using online counseling through BetterHelp . Online therapy can be a convenient way to get mental health care. Research suggests that online treatment is  as effective as in-person therapy and can often fit into your schedule more easily. You can speak with a therapist from your home or anywhere you have internet connection.

How does birth order affect the development of our personality?

According to Alfred Adler’s psychological theories, birth order affects personality development through its impacts on relationship dynamics, including both sibling relationships and parent-child interactions. Since a person’s family is almost always their first experience of connections with other people, it may shape the assumptions, habits, and strategies that individuals carry throughout their lives. 

For example, Adler believed that eldest children were more prone to neuroticism. He thought that their experience of undivided parental attention in their early years, followed by the appearance of younger siblings who competed for attention, could lead to a sense of insecurity. On the other hand, he also believed firstborn children tended to form leadership skills early on due to the experience of teaching and protecting younger siblings. 

There might also be biological effects of birth order. Some researchers have suggested that changes to a woman’s immune system following repeated births might have developmental impacts on the personalities of children born later. However, the evidence for this theory is currently very limited, and some research seems to contradict it .

In twins, birth order might appear to affect personality because the larger and healthier twin is often born first . This can mean that younger twins are more likely to have health and developmental problems, which may have long-term psychological impacts. 

How does birth order affect socialization?

Birth order might affect socialization because of the different roles that siblings of different ages tend to assume within a family. The oldest child might feel a sense of responsibility for their younger siblings, causing them to develop a greater propensity for leadership. Youngest children, in contrast, might have to struggle for their independence when their older siblings get bossy. This could lead them to be more rebellious in later interactions. Middle children might find themselves taking on the role of mediators, causing them to develop strong interpersonal skills.

Is birth order an important factor in determining intelligence myth?

Some studies have found a possible birth order effect on intellectual achievement. A 2015 research paper reported that older children appeared to score slightly higher on measures of intelligence, as well as rating their own intelligence higher. Later borns tended to display slightly lower IQ along with lower intellectual confidence.

However, while the observed effect was statistically significant, it was quite small. There’s little evidence that birth order plays a major role in determining a person’s intelligence.

What are three arguments made to support that birth order does affect our personalities?

One common argument in favor of the idea that birth order can shape personality is that early childhood experiences are known to be important for many different long-term psychological outcomes, such as relationship attachment styles. Since the experiences of siblings may be different depending on their position in the family, it might make sense to expect them to be important in personality formation.

Another argument in favor of birth order effects is that parental attention may be important in forming cognitive abilities. Because children with fewer siblings receive more of their parents’ focus, they might have a slight advantage in building certain mental skills, which could in turn affect their personalities. 

The third main argument supporting birth order personality effects is that this idea is backed by research. Although the birth order effects found so far have been small, many important shifts in scientific theories begin with the observation of seemingly minor phenomena.

Does birth order affect one's personality impression?

There’s currently not much evidence that birth order affects the kinds of personality traits that are most important in determining the impression an individual makes on others. Even in the study offering the strongest support for the effect of birth order on intelligence, researchers found no relationship between birth order and characteristics such as:

• Agreeableness
• Willingness to try new experiences
• Extraversion
• Emotional stability
• Imagination
• Conscientiousness

How can birth order impact a child's attitude and behavior?

Birth order might affect a child’s behavior through the differences in parental expectations for different family members. Parents may consciously or unconsciously assign older siblings to a leadership role, expecting them to care for and instruct their younger brothers or sisters. Younger children might receive less discipline because their parents are older or have less ability to monitor an individual’s behavior within a large family.

Predicting the exact outcomes of these kinds of expectations may be difficult, though. An eldest child expected to act as a teacher and protector for their siblings might embrace this role. However, if they’re given poor support from their parents or punished for their siblings’ misbehavior, it could lead to resentment and avoidance of responsibility.

Birth order might also play a role in determining mental health, which could have important behavioral effects. A 2021 study reported that later-born children appeared to have lower rates of mental disorders and higher rates of happiness and prosocial attitudes.

Some theories of birth order suggest that “psychological birth order” might matter more than actual birth order. In other words, a child’s perception of their place within the family might be the most important factor.  It’s often said that if a child is born five years or more after their next-oldest sibling, their personality characteristics will be more similar to a firstborn.

Can birth order determine success or failure?

Despite the prevalence of stories in popular media about how many astronauts or Nobel Prize winners are firstborn children, birth order is unlikely to be a major factor in a person’s success or failure in life. 

While many psychologists used to consider it very important, more recent studies with better methodological design have found little evidence for strong effects. Other elements, such as genetics, socioeconomic standing, developmental health, and life experiences, are likely to be substantially more important.

Does birth order seem like a good way to describe personality?

Many people use birth order as a shorthand for personality makeup, such as describing someone as a “typical middle child”. Yet many people have very different ideas about what this means in practice. For instance, some sources may describe oldest children as outgoing and independent, while others say that these people tend to be shy and cautious. 

Because of these differences, and the lack of strong evidence in favor of birth order effects, these stereotypes may not be useful ways to describe someone’s personality.

Does birth order affect self-concept?

Some theories of birth order effects have suggested that a person’s family position might affect their self-concept. In these frameworks, older and only children were thought to have the highest self-esteem, while middle children were believed to have the lowest. Several studies in the 1980s and 1990s tested these theories in middle and high school students with mixed results. While some researchers reported significant birth order effects, others found that the evidence was weak and inconsistent. 

More recent investigations have found little support for the idea that self-esteem is affected by birth order. There may not be enough evidence to definitively state that oldest, middle, and youngest children have different self-concept strengths.

Does birth order affect the behavior of children?

Birth order may have some effects on childhood behavior. Research on children aged 9-10 found the highest rates of cooperative, prosocial behavior among those who were latest in birth order. Eldest children seemed to be more prone to conduct problems and disruptive behavior. That said, these findings may need to be replicated before it’s possible to say definitively that birth order impacts behavior.

• Defining Your Family Of Origin And How It Impacts You Medically reviewed by Audrey Kelly , LMFT
• Six Family Types And Their Unique Dynamics Medically reviewed by Andrea Brant , LMHC
• Relationships and Relations

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What Is Birth-Order Theory, and Is There Any Truth to It?

We’ve all heard it said that the order in which siblings were born can go a long way toward explaining their peronality traits. But where did the the birth-order theory actually come from, and is there any truth to it? With TikToks examining the concept regularly going viral, there couldn’t be a better time to dig in, and to find out just what your spot in the family line-up is supposed to mean. Read on for more on birth-order theory.

Where did birth-order theory come from?

The beginnings of birth-order theory can be traced back to the early 20th century, when Austrian psychologist Alfred Adler—who was, himself, heavily influenced by Freud—introduced the concept of the “family constellation,” writing of the specific personalities, strengths, and weaknesses that emerge in children from the same family. “Of course there is much which is the same for all in the same home,” Adler wrote, “but the psychic situation of each child is individual and differs from that of others, because of the order of their succession.”

What’s the birth-order theory conception of an “oldest child”?

According to Adler, firstborn children are likely to be neurotic (as a function of having to learn to share their parents once another child was born), dutiful, perfectionistic, and relatively conventional or conservative compared to their siblings. A 2015 study by psychologist Rodica Damian found that firstborn children tend to “enjoy a small IQ advantage over their younger siblings,” which might be some small consolation for having to deal with eldest-daughter syndrome.

What about a middle child?

Adler noted that middle children may have lower self-esteem than their older or younger siblings due to receiving less emotional or financial support from their parents—but, by that same token, they are more likely to achieve emotional stability and form strong bonds outside of the family unit. Adler also suggested that middle children “may have trouble finding a place or become a fighter of injustice” as a result of their placement in between older and younger siblings. (Fun celebrity fact: Bella Hadid is a middle child!)

And a youngest child?

Adler considered youngest children to be most likely to develop rebellious, outgoing, or extroverted personality traits in order to “catch up,” or, in some cases, differentiate themselves from older siblings. Adler also noted that in adulthood, youngest children might be more likely to “want to be bigger than the others,” or “have huge plans that never work out.” ( Amy March burning her older sister Jo’s manuscript in Little Women ? That was classic youngest-child behavior, according to a strict reading of birth-order theory, but then again, so was chicly running off to Europe to paint—so there are negative and positive traits associated with that particular birth placement.)

Why is birth-order theory suddenly everywhere?

You can thank TikTok for this one: the phrase “birth order dating theory” has found purchase among users trying to understand the role that birth order can play in romantic relationships. (The hashtag #birthorder is also a popular search term on the platform, having accrued over 500 million views.) However speculative, certain conclusions have proven fascinating: Just consider the example of relationship coach @iammichailatyson , an eldest daughter who realized that she had only been in relationships with men who had older sisters. “[It makes] so much sense that I have gravitated towards men like that—typically men that are looking for mothers, and the role I stepped into was mothering those men,” she said in a video posted in March. Elsewhere, users like @t0nit0ne have broken down the compatibility of different birth-order pairings. (Hint: It’s better to be a firstborn involved with a lastborn than a firstborn with another firstborn. You’ve been warned!)

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Birth Order Is Basically Meaningless

Any differences in personality are so small that they barely matter, new research says.

In the late 1920s, Alfred Adler, a colleague of Sigmund Freud’s, theorized that firstborns feel “dethroned” when a younger sibling comes along, making them neurotic and self-centered, but prone to leadership. The youngest would be spoiled, outgoing, and immature, while a middle child would be independent, rebellious, and relatively healthier. Of course, Adler was a middle child.

The idea that first-born children are smarter can be traced back to Francis Galton, a 19th-century scientist who worked in psychology, biology, and anthropology, among other fields. He noticed that a lot of his English scientist colleagues were first-born sons, and proposed that their intellectual success came from the resources and attention poured into them by their families. (First-born sons are the heirs, after all.)

The idea that birth order determines siblings' personality and intelligence remains entrenched in society, even as modern scientific findings on the matter have historically been inconsistent. But a couple of recent studies of large samples suggest that birth order does not matter when it comes to personality, and barely matters when it comes to intelligence.

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In a study published this week in Proceedings of the National Academy of Sciences, researchers from the University of Leipzig and Johannes Gutenberg University of Mainz, both in Germany, looked at more than 20,000 adults from the United States, United Kingdom, and Germany, comparing siblings both within the same family, and people with the same birth order across families.

“All in all, we did not find any effect of birth order on extraversion, emotional stability, agreeableness, conscientiousness, or imagination, a subdimension of openness,” the researchers write.

The evolutionary view of birth order effects is that siblings have to compete for their parents’ attention and favor, and they find different ways to do that. The oldest are physically stronger and mentally more developed than the younger siblings (at least while they’re all still children), so it behooves them to be aggressive (and bossy?). The later-born siblings need more help from others, and so become more extraverted.

None of this bore out in the PNAS study, but logically, if these effects existed, they should be more pronounced when the siblings are young and still living together, since the need to compete for parental attention wanes with age. (Hopefully.) But another study published this summer in the Journal of Research in Personality looked at 377,000 U.S. high-school students, and also found little evidence for personality differences based on birth order.

The researchers looked at the Big Five personality traits—openness, agreeableness, neuroticism, conscientiousness, and extraversion—and predicted that “firstborns (versus laterborns) should be higher in Conscientiousness, Neuroticism, and the dominance aspect of Extraversion, whereas laterborns should be higher in Agreeableness and the sociability aspect of Extraversion.”

Over this entire huge population, they found that firstborns did tend to be a little more conscientious and dominant, and less sociable, but also that they tended to be more agreeable and less neurotic (which goes against the stereotype). But the effect size of this finding was tiny—so tiny that if they hadn’t looked at hundreds of thousands of people, it wouldn’t have been significant at all. When it came to intelligence, firstborns did have an advantage—of one IQ point. As a comparison, another study found that some adolescents’ IQs changed by as much as 20 points within four years.

“Birth order is often invoked as an important variable to explain the development of personality and intelligence within and across families,” the researchers wrote in the high-school birth-order study. “We would have to say that, to the extent that these effect sizes are accurate estimates of the true effect, birth order does not seem to be an important consideration for understanding either the development of personality traits or the development of intelligence in the between-family context.”

Birth order is a bit like a horoscope, in that it presents you with vague enough traits that it’s easy to project them onto yourself.  A firstborn might think, “I am a natural leader;” the youngest, “Everyone does love me;” the middle child, “I am a bit of a rebel, aren’t I?” For example, in a graduate thesis written at the University of Wisconsin, Stout, the counseling student Stacey Armitage found that the college students she interviewed generally saw themselves as fitting the Adlerian description of their birth-order role, though there were some qualities that everyone thought they had more than their siblings. Everyone thought they were the most rebellious, for example, and the most considerate.

People do love to sort themselves into categories, though, whether they’re scientifically sound or not. And any idea that’s permeated culture as much as birth order has is bound to have an effect on how people see themselves. Maybe the stereotypes about birth order have more to do with how people compare themselves to their siblings than how they actually are, on their own.

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Article Contents

Introduction, materials and methods, classification, acknowledgements, authors' contributions, data availability.

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Birth order differences in education originate in postnatal environments

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Martin Arstad Isungset, Jeremy Freese, Ole A Andreassen, Torkild Hovde Lyngstad, Birth order differences in education originate in postnatal environments, PNAS Nexus , Volume 1, Issue 2, May 2022, pgac051, https://doi.org/10.1093/pnasnexus/pgac051

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Siblings share many environments and much of their genetics. Yet, siblings turn out different. Intelligence and education are influenced by birth order, with earlier-born siblings outperforming later-borns. We investigate whether birth order differences in education are caused by biological differences present at birth, that is, genetic differences or in utero differences. Using family data that spans two generations, combining registry, survey, and genotype information, this study is based on the Norwegian Mother, Father, and Child Cohort Study (MoBa). We show that there are no genetic differences by birth order as captured by polygenic scores (PGSs) for educational attainment. Earlier-born have lower birth weight than later-born, indicating worse uterine environments. Educational outcomes are still higher for earlier-born children when we adjust for PGSs and in utero variables, indicating that birth order differences arise postnatally. Finally, we consider potential environmental influences, such as differences according to maternal age, parental educational attainment, and sibling genetic nurture. We show that birth order differences are not biological in origin, but pinning down their specific causes remains elusive.

Birth order is associated with educational outcomes, with earlier-born siblings outperforming later-borns. Using genetic data and data on education from a large Norwegian cohort, we find that these differences are not due to genetic differences or in utero differences. Later-born children even have higher birth weight, which is positively associated with education. When we take indicators of genetic and in utero differences into account, earlier-born siblings still do better in school. This establishes that birth order differences result from factors after birth, consistent with explanations that focus on home environments.

Sibling differences receive less attention than sibling similarities, although they make up an important part of the picture of social inequality. Even for socio-economic outcomes, inequality within families (i.e. among adult siblings) can be as large as inequality between families ( 1 ). While evidence indicates a large role for environmental influences in producing differences among siblings in the same family, documenting specific, systematic influences has been more elusive. Developmental scientists have talked about a “gloomy prospect” in which nearly all influence in and by social environments is idiosyncratic ( 2 , 3 ).

Birth order has long been offered as an example of a systematic source of environmental differentiation within families. Empirically, firstborn siblings have slightly higher intelligence ( 4–6 ), educational achievement ( 7 , 8 ), and income ( 9 , 10 ) than their siblings born later. These differences are routinely interpreted as reflecting causal mechanisms related to childhood experiences. For example, some work suggests that parental resources are more diluted for later-born siblings as a result of competing demands for parental attention ( 11 , 12 ), while other work considers whether the presence of older siblings adversely affects the cognitive environment in which younger siblings are raised ( 13 , 14 ).

Even though birth order differences in achievements are routinely presumed to reflect different social environments between siblings, there are several reasons to worry they may be caused biologically.

For one, biological differences may be induced by fertility decision-making. Genetic differences among siblings have been described as a “lottery” ( 15–17 ), and parents may be more likely to have an additional child if their already-born children evince desirable traits consistent with a favorable draw from this “lottery” ( 18 ). Later-born children may exhibit a ”regression to the mean" phenomenon of less propitious genetic endowments for these same traits. When deciding whether to have another child, parents have information on behavior and traits of their current child(ren) that are correlated with genotype.

Predictive information about a child's later cognitive ability is measurable at least as early as 10 months ( 19 ). Consistent with the findings of another study ( 20 ), in our data, we find that the polygenic score (PGS) for educational attainment is inversely associated with a number of behaviors at age 2 to 3 (e.g, . “Can't sit still,” “Gets in fights”) including some behaviors net of parent's PGSs ( Figure S1, Supplementary Material ). Thus, observable child traits exist at a quite early age that correlate with the genetic endowments associated with educational attainment, and in theory parents’ observation of these traits may influence some of their decisions on whether to have additional children.

If parents adjust their fertility-behavior dependent on the first child's behaviors in childhood and those behaviors are partly influenced by genetics, this could lead to differences in genetics between siblings according to birth order. Studies show that parents adjust their parenting according to child genotype too, engaging in more cognitively stimulating activities with children with higher PGS for education ( 21 , 22 ). Thus gene–environment correlations could exacerbate any genetic or other biological differences present at birth.

Biological differences could also result from involuntary processes. Miscarriage occurs in 8% to 30% of all pregnancies. The risk of miscarriage increases with maternal age, but also, net of mother's age, studies have found that number of previous pregnancies is associated with likelihood of miscarriage, albeit not in a consistent direction ( 23 , 24 ). Most miscarriages happen early in the pregnancy, with the main factors being genetic abnormalities (i.e. chromosomal aberrations like autosomal trisomy) and uterine malformation ( 25 ). Changing risk of miscarriage by either factor over pregnancies, or their combination, could result in survivorship-based biological differences by birth order in live births.

Maternal age also may influence in utero environments ( 26 , 27 ), such as the rate of antibody attacks ( 28 ). Beyond this, maternal nutrition, stress, medical professional visits, and other health-related behaviors may vary according to birth order ( 29 ), causing differences in birth weight and other outcomes, which influence intelligence and educational attainments later in life ( 30 , 31 ). Other conditions, such as preeclampsia, have been linked to birth order ( 32 ), health, and cognition, although often not in ways consistent with the expectation of firstborn advantage ( 33 , 34 ).

For that matter, mutations also rise with parental age, in particular with paternal age at conception ( 26 ). There is evidence that advanced paternal age increases the probability of offspring intelligence disorder, schizophrenia, and autism ( 35–37 ), as well as the rate of several other health-related traits ( 38 ). A consequence of these many unknowns may be that there are biological mechanisms that produce differences between siblings present at birth, which we are still learning about. These can result in biological differences by birth order, albeit not necessarily a causal effect of birth order as such.

Against this backdrop it is important to recognize the limited research designs on which a considerable portion of the existing literature on birth order and educational outcomes is based. Many studies do not compare siblings in the same family, leaving genetic differences across families as an uncontrolled confounder ( 6 ). Birth-order studies often define birth order by the rearing family or by siblings who share a mother ( 7 , 39 ), so that genetic differences by birth order may result from paternal genetic differences by siblings who do not share a father. Even when studies intend to include only full siblings, this is usually based only on maternal self-report. Sometimes mothers are unsure themselves who the father is ( 40 ) or misreport paternity for other reasons. For half-siblings with different fathers, paternal genetic differences could be associated with birth order, as mothers who have children with multiple partners typically have children with less educated fathers for their later-born children ( 41 , 42 ).

Our study assesses birth order differences in educational outcomes within families using genetically confirmed full siblings. We do so with data that combines registry, survey, and genotype information from families in Norway (see Methods). At stake is that, if birth order differences are not due to postbirth environments, this would imply that using birth order as an exemplar of a documented systematic effect of family environments on sibling differentiation is incorrect, furthering contentions that intrafamilial environments are far more inert than many suppose. Moreover, if differences were specifically genetic, it would also undermine various causal inference strategies that effectively assume genetic differences among full siblings are independent ( 18 , 43 ). We examine whether birth order effects on education are influenced by genetic differences as measured by PGSs for educational attainment. We also consider broader prebirth biological differences by looking at whether there are birth order differences in birth weight and birth length, as indicators of a combination of genetic and in utero factors. Furthermore, we look at nontransmitted alleles of parents, whose influence on development is sometimes referred to as “genetic nurture.”

In all, we address four research questions:

Is birth order associated with educational outcomes? Based on previous research, we anticipate birth order differences in which earlier-borns fare better than later-borns. We make use of the full population of Norway to answer this question. We have two outcomes for two different generations, educational achievement measured at age 30 available for the “adult generation,” and national test scores performed in fifth, eighth, and ninth grade available for the “child generation.” This allows us to investigate if birth order differences are robust over time and educational outcomes, as well as whether birth order differences are present from childhood or if they arise at later ages.

Is there a birth-order difference in genetics, as measured by PGSs for educational attainment? We again, here, examine data from both the adult and child generation of genotyped MoBa participants.

Are there differences in birth weight and birth length between birth orders? Our data here are the the child generation in the MoBa-sample.

Does the putatively socially based effect of birth order remain after accounting for genetic differences and in utero variables? Here, we use both the adult and child generation data in the MoBa, but while we can adjust for PGS and in utero variables in the child generation, we do not have the latter in the adult generation and so adjust for PGS only

Our analyses below of these four questions are consistent with the conclusion that birth order differences are not biological in origin. After presenting these, we also investigate whether our data permit any further illumination of the mechanisms behind birth order differences. We consider whether birth order interact with PGSs, vary by family background, and include both nontransmitted and sibling alleles.

We begin by analyzing the full population of Norway using administrative data to see whether there are birth order differences among children and adults. To better parallel subsequent analyses with genomic data, we exclude participants whose parents were not born in Norway (see Methods). For the population of children we study (b. 1994 to 2009), we examined performance on national tests by computing the child's mean score of tests in three subjects measured at three times (grades five, eight, and nine; standardized within subject, age, and cohort; N  = 301,795). For the adult population (b. 1945 to 1988), our outcome is completed years of education at age 30 (standardized within cohort, N  = 2,067,878). We use family-level fixed effects when estimating the bivariate association between birth order and these outcomes, meaning we are comparing siblings within the same family. We control for sex and maternal age ( 44 ), and run the models separately by sibship size.

Consistent with other studies, we find that firstborn siblings have better educational outcomes than their later-born siblings. Figure  1 shows the magnitude of these associations. The top panel shows lower test scores for each successive birth order for all family sizes from two to five siblings. Most of these differences are present in the first test scores we observe (fifth grade), but the gaps do grow modestly from the last scores in our data (ninth grade; see Figure S2, Supplementary Material ). The bottom panel shows similar patterns for educational attainment in the adult population. Figure  1 also shows that birth order differences increase when adjusted for maternal age.

Birth order and educational differences in the population. (A) and (B) Results from family-fixed effects linear regression models run separately by sibship size, with controls for sex and maternal age and cluster-robust standard errors. Firstborns serve as the reference category. All point estimates presented with 95% CI. In (A) children generation in the population ( N  = 301,795), where the outcome is the mean of national test score standardized within test, year of test, and birth cohort. (B) Adult generation in the population ( N  = 2,067,878), where the outcome is educational attainment at age 30, standardized within birth cohort.

Figure  2 shows the relationship between birth order and the PGS for educational attainment among families with two and three children, which are the vast majority of sibships in Norway. Sample sizes for this and subsequent analyses using genetic data are much smaller than those used in the previous figure as these data are only available for a portion of the population (see Methods). We provide separate results for the child generation (panels A and B; N  = 24,507; 2,705) and the adult generation (panels C and D, N  = 43,316; 3,897). Within each generation, we provide estimates for models comparing siblings both between families and within families. In addition to sex and cohort, between-family models adjust for the 10 principal components of the GWAS data to address potential confounding by ancestral differences (i.e. population stratification).

PGS and birth order. (A), (B), (C), and (D) Association between educational attainment PGS and birth order. Results from linear regression models run separately by sibship size, with controls for sex and mothers age. Firstborns serve as the reference category. All point estimates presented with 95% CI. In (A) and (C), between-family estimate, adjusted for 10 principal components. In (B) and (D), family-level fixed effects models with cluster-robust standard errors. In (A) and (B), children generation in the MoBa-sample ( N  = 24,507 (A); 2,705 (B)); (C) and (D) adult generation in the MoBa sample ( N  = 43,316 (C); 3,897 (D)).

In all panels in Figure  2 , CIs overlap zero for birth orders two and three, meaning that we find no differences in the PGS for educational attainment among different birth orders. Consequently, the genetic differences captured by that PGS cannot explain the observed relationship between birth order and educational outcomes.

In the introduction, we raised the possibility genetically influenced child behaviors could influence parents’ subsequent fertility decision-making. Considering this empirically, we looked specifically at whether a child's PGS for educational attainment predicted whether parents had another child. It did not ( Table S1, Supplementary Material ). While there are observable child behaviors in 2- to 3-y-old children that are correlated with PGSs for educational attainment, we do not find any evidence that a child's PGS influences their parents’ subsequent fertility.

Birth weight and birth length

In Figure  3 , we turn to birth order differences in birth weight and birth length, available in the child generation of the sample only ( N  = 2,705). We adjust for sex, gestational age, and the PGS for education.

Birth length, birth weight, and birth order. (A) and (B) Results from family-level fixed effects linear regressions run separately by sibship size, with dummies for birth order. Children generation of the MoBa-sample ( N  = 2,705). Cluster robust standard errors, 95% CI. Firstborns serve as the reference category. In (A), birth length, with different control variables, pink point: sex; orange triangle: sex, gestational age, and educational attainment PGS; blue rectangle: sex, gestational age, educational attainment PGS, and mothers age at birth. In (B), birth weight, with controls, pink point: sex; orange triangle: sex, gestational age, educational attainment PGS, and birth length; blue rectangle: sex, gestational age, educational attainment PGS, birth length, and mothers age at birth.

On average, later-born children are both longer and heavier than firstborn children. Differences in length account for roughly half of the difference in weight. Further analyses indicate that the differences in mean length or weight are not due to differences only in especially low- or high-weight births (see Figure S3, Supplementary Material ). We find nearly identical results in the between-family models (see Figure S4, Supplementary Material ). If higher birth weight indicates an advantaged prenatal environment, then it appears that later born children are actually advantaged in this regard relative to their first born siblings, despite their lower ultimate achievement.

Maternal age and birth spacing

We considered also the possibility that birth order differences were due to maternal age, as maternal age has been variously posited to influence cognitive development via both biological and social mechanisms. To confound an observed firstborn advantage, mothers age would need to be inversely associated with test scores. When we simultaneously model within- and between-family effects, we find maternal age to be positively associated with test scores between families and even more so within families (see Table S2, Supplementary Material ). Consequently, as noted, the birth order differences presented in Figure  1 actually increase after accounting for maternal age. Within families, sibling differences in maternal age are equivalent to the birth spacing intervals. Our finding is, thus that birth order differences get smaller as both maternal age and the spacing between births increases. While this pattern is consistent with explanations of birth order differences rooted in differential parental investment and overall intellectual climate in the family, it is not consistent with ideas of there being in utero, mutational, or other biological advantages to being born to a younger mother.

Educational achievement and attainment with controls

In Figure  4 , we show birth order differences within families after adjusting for all the aforementioned measures simultaneously: sex, gestational age, birth weight, birth length, and mothers age at birth, as well the PGS for education. We show results for birth order differences in educational achievement (children, panel A, N  = 2,705) and educational attainment at age 30 (adults, panel B, N  = 3,365). In all models, point estimates are negative for later-born children compared to firstborn. Although controlling for mothers' age at birth increases the uncertainty of estimates and CIs overlap zero (blue rectangle in Figure  4 ), all point estimates remain negative, and some again increase.

Educational achievement/attainment and birth order. (A) and (B) Results from family-level fixed effects linear regressions run separately by sibship size, with dummies for birth order where firstborns are the reference. Cluster robust standard errors, 95% CI. In (A), children generation in the MoBa-sample ( N  = 2,705) with mean of national test score as the outcome. Controls for sex (black circle); sex, gestational age, and educational attainment PGS (yellow triangle); sex, gestational age, birth weight, birth length, and educational attainment PGS (blue rectangle). In (B), adult generation in the MoBa-sample ( N  = 3,365) with educational attainment at age 30 as the outcome. Controls for sex (black circle); sex, educational attainment PGS (yellow triangle); and sex, educational attainment PGS, and mothers age (blue rectangle).

Taken together, these analyses rule out various scenarios by which birth order differences in educational outcomes are either confounded or the result of genetic or prenatal causes. The availability of multigenerational genotyped data with extensive phenotyping does not contradict an understanding of birth order differences as originating in postnatal environments.

Interrogating environmental origins

Given this, we next turned to see what additional clues the genomic data may offer for how these environmental influences operate. First, we examined whether birth order differences varied by PGS. Muslimova et al. ( 45 ) posit that birth order differences may be strongest among children with the highest genetic score for achievement, as these children may be able to capitalize best on increased parental investment. In the child sample, we observed no difference (see Table S3 and Figure S5, Supplementary Material ). In the adult sample, we find a borderline statistically significant pattern opposite the expected direction: a smaller firstborn advantage among participants with higher PGSs. If that is true, higher genetic potential could mitigate the processes that produce birth order differences. Either way, there is no indication in our data that firstborn advantage only or more strongly exists among those with higher PGSs.

Second, we also considered whether, net of PGS, birth order differences are moderated by family background. Contradictory hypotheses have been proposed to this end. One is that advantaged environments would have greater disparities in effective parental investment, implying larger birth order differences in those families. The other is that advantaged environments would have more compensatory investment in lower-performing children, yielding smaller birth order differences ( 46 ). Our results here were consistent with the former of these scenarios ( Table S4, Supplementary Material ): differences were larger in families in which parents had higher educational attainment, including within-families and net of PGS. But, these differences by family background were small compared to the overall birth order difference ( Figure S6, Supplementary Material )

Third, we examined the influence of other family member's PGSs on child achievement, net of the child's own score. Recent findings of “genetic nurture” have documented relationships between nontransmitted parental alleles and child attainment and achievement ( 47–49 ). Given that mothers are typically more involved in childrearing than fathers—not infrequently to a substantial degree—the finding of some past research that maternal nontransmitted alleles matter more for attainments than paternal nontransmitted alleles would not be surprising ( 50 ). We did not find this pattern in our data (see Table S6, Supplementary Material ). Also, when we include the PGS for the siblings, the sibling score accounts for more than half of the magnitude of the difference in achievement that had been attributed to parental PGSs. Net of one another, the relationship between sibling PGS and achievement is nearly a quarter as large as that of a child's own score ( Table S5, Supplementary Material ). These results point to the importance of considering that genetic nurture may reflect sibling influence to a greater extent than has been previously appreciated.

Our starting point for this investigation was the common presumption that birth order differences in educational outcomes reflect social environmental rather than genetic or prenatal mechanisms. Specifically regarding genetic differences, there are multiple potential reasons for genetic variation by birth order. Historically, the problem has been exacerbated by many studies using between-family samples and methods ( 51 ), but the possibility of genetic differences remains even in studies that compare siblings in the same family. The only way to assess decisively whether genetic differences may confound birth order studies is with data that contains genetic and comprehensive phenotypic information on many siblings. Using data that spans two generations of Norwegian siblings, we show no genetic differences by birth order as captured by PGSs for educational attainment. Similar preliminary findings have recently been reported using data from the United Kingdom ( 45 ).

We also examined indicators of possible in utero origins of birth order differences: birth weight, birth length, and maternal age. Our results suggest that none of these provide any leverage for explaining the robust advantage that earlier-born children have in test scores and ultimate educational attainment in our data. Indeed, later-born children are advantaged in birth weight and birth length, consistent with some earlier research ( 29 , 52 , 53 ), and so if anything they might have better in utero environments, and yet have worse educational outcomes.

These results strengthen the conclusion that birth order differences in educational outcomes mainly originate in postbirth environments ( 6 , 39 ). Another implication of our findings is that various methodologies that use siblings in causal inferences, i.e. studies using PGSs within-family to alleviate population stratification ( 54 , 55 ), and social science studies using siblings to capture the omnibus effect (of social background), are not confounded in ways they would be if birth order differences were due to genetic differences.

Given that birth order differences arise within families but seem unrelated to genetic factors and do not appear to be due to in utero environments, they constitute an interesting case of social inequality free from many sources of potential confounding. Understanding how they come about offers a window that may be more generally instructive for mechanisms that lead to inequalities among people with similar familial and sociodemographic backgrounds. To this end, we find that the birth order differences in test scores are already mostly present at our earliest time of observation (fifth grade, age 9/10) and grow only modestly from then to ninth grade. This clarifies that the environmental differences associated with birth order differences largely manifest in the first decade of life. The difference in educational attainment among adults is similar in magnitude, suggesting that these differences resulting from childhood environments have lifelong effects.

The two main environmental theories on birth order effects are linked to parental resources and sibling interactions. According to the resource dilution theory ( 12 , 56 , 57 ), economic and parental resources deplete as more household members arrive. In societies where resources are abundant like Norway, cultural resources are thought to be more important than economic ( 1 ). According to the theory, as earlier-born receive more of the parental cultural resources such as personal attention and help with homework than later-borns do, they excel in educational performance. While this pattern is consistent with the finding that birth order differences decline with increased spacing between births, it is not consistent with our results that maternal genetic endowments do not matter anymore than paternal endowments, given that mothers usually contribute more to child-rearing than fathers. Other studies have found stronger effects of maternal genetics ( 50 ), so more research will be needed to reach any decisive conclusion.

As for sibling interactions, these have been most prominently raised in variations of “confluence theory,” which proposes that siblings generally have negative effects on one another's cognitive development ( 13 , 14 , 58 ). Because firstborns have less exposure to being reared with siblings, and may also benefit from the opportunity to teach younger siblings, the negative influence of siblings is proposed to be least for firstborns. Our finding that the apparent influence of parental alleles shrinks markedly when sibling PGSs are included, and that sibling PGSs are significant associated with child outcomes net of a child's own score, indicates that some of what has hitherto been called “genetic nurture” could in fact be “sibling genetic nurture.” Recent enthusiasm for using sibling-based GWAS to purge GWAS studies of indirect genetic effects assumes that these indirect genetic effects are primarily due to parents ( 59 ). Because the approach does not account for indirect effects via siblings, the possibility that sibling genetic effects on achievement may be larger than commonly supposed ( 60 ) is important to resolve for evaluating the success of sibling-based GWAS.

Sibling effects have received less attention as compared to parental resources when it comes to explaining birth order effects. That said, Gibbs et al. ( 61 ) propose a conditional resource dilution (CRD) model, opening up for institutional- and family-level variation influencing birth order effects. In the CRD, brothers and sisters, and especially older siblings, may provide, rather than compete for resources. “Sibling genetic nurture” raises the possibility that these intersibling consequences for familial environments vary by sibling genotype, with genetic factors increasing achievement potential having positive spillovers for siblings.

Our study has some limitations. First, PGSs are based on common alleles, and we are not able to tell if there are differences in rarer variants. That said, If rarer variants in the form of de novo mutations were importantly associated with birth order, the most obvious explanation would have involved parental age, and this is contradicted by our findings regarding maternal age. Second, our proxies for in utero environments are partial, such that unmeasured consequential differences might still exist. Similarly, even though postnatal accounts of birth order differences focus on social environments, our design does rule out postnatal differences due to physiological causes or other, nonsocial phenomena. Third, the sample size for the within-family analyses is limited, as we have relatively few families with two or more full siblings. Statistical power curtails our present capacity to draw more decisive conclusions for some of our analyses, and we cannot rule out type II errors. Fourth, PGSs for educational attainment still only account for a limited portion of the overall heritability of educational attainment, and they will likely improve in the future. Last, we have no direct measures of parental and sibling behaviors, which obviously thwarts the ability to interrogate either as putative causes. Amidst the excitement for what incorporating genomic information may bring to the social sciences, we should not lose sight that the available phenotyping of population datasets often still leaves much to be desired.

In addition, even though birth order differences seem like a unary puzzle, the importance of different environmental mechanisms may shift in different societal and historical contexts. For the cohorts in our child sample, the vast majority of Norwegian children attend child care from age one, with the overall attendance being 90% for children aged one to five in 2010 ( 62 ). In such a context, mothers may be spending less time with their children than perhaps at any point in history, which might mean our data could be unpropitious for observing a maternal role in birth order effects. Scandinavian data has often been used in research on family dynamics research because of these countries’ population registers. Societies with exceptional data may also be exceptional in other respects. Moreover, Scandinavian population register data sources do not contain large-scale data on actual behaviors, resources, and practices within families, and there may simply be no way of getting to the bottom of birth order effects without such richer data. While our results show that birth order differences are largely environmental, the question of what specific environments factors are involved remains far from resolved.

Population data and variables from administrative registries

The data in this paper are from several sources. We begin with data from administrative registries covering the full population of Norway. The registries are of very high quality, and do not suffer from attrition, and have few registration errors. From the Central Population Registry we identify all family linkages and demographic variables, like birth cohort, sibship size, birth order, and mothers age at birth. We identify sibship size and birth order according to birth year within each mother. We also identify any sibships with multiple births, and exclude them as the assignment of birth order is less clear cut, and family dynamics may be different in multiple birth sibships. To better mirror our genomic sample we remove individuals born outside of Norway. Current genomic methods do not allow for us to include persons of non-European ancestry, and we restrict the data based on the population registry based on this related criteria, i.e. we only include Norwegian-born to Norwegian parents. For the adult part of the population, we restrict birth cohorts to 1945 to 1988.

We link the Central Population Registry to the National Educational database (NuDB) ( 63 ), also covering the full population. For the child generation in our analyses, we use standardized tests as the outcome. From NuDB, we use data from national tests conducted in fifth, eighth, and ninth grade in reading, mathematics, and English. We standardize each test to a z-score within test, year of test assessment, and birth cohort. Thereafter, we calculate a mean of all available test scores for each child, which serves as our outcome variable for the children in the sample. For the adult generation in our analyses, the outcome is educational attainment at age 30 following the International Standard Classification of Education (ISCED 2011) ( 63 , 64 ). The variable is continuous as we transform it to being measured in years, using how many years it takes to complete the level of education attained following normal progression, according to ISCED. We standardize this variable too to compare effect sizes between outcomes. After restricting our sample and removing people with missing information etc., our population-based data has an N of 301,795 for the child birth cohorts, and 2,067,878 for the adult birth cohorts.

The Norwegian Mother, Father, and Child Cohort Study (MoBa)

The prepared population data are linked to The Norwegian Mother, Father, and Child Cohort Study (MoBa) ( 65 , 66 ), which we use for the analysis with genomic and in utero variables. MoBa is a population-based pregnancy cohort study conducted by the Norwegian Institute of Public Health. Participants were recruited from all over Norway from 1999 to 2008, with the sample unit being pregnancy. The women consented to participation in 41% of the pregnancies. The cohort includes 114,500 children, 95,200 mothers, and 75,200 fathers. The current study is based on version 12 of the quality-assured data files. The establishment of MoBa and initial data collection was based on a license from the Norwegian Data Protection Agency and approval from The Regional Committees for Medical and Health Research Ethics. The MoBa cohort is now based on regulations related to the Norwegian Health Registry Act. The current study was approved by The Regional Committees for Medical and Health Research Ethics. Some 98,110 individuals in around 32,000 trios are currently genotyped. This interim release is known as “MoBa Genetics,” and is comprised of several separately genotyped and imputed batches due to partial funding for genotyping on a per-project basis. The current release is a merger of all subprojects after quality control and imputation. Details are available here: https://github.com/folkehelseinstituttet/mobagen .

MoBa-participants are sampled on pregnancy independent of previous pregnancies ( 67 ), meaning that children in MoBa could have siblings younger or older not included in the sample. However, as we identify family linkages and birth order from the population registry, we observe full sibship sizes and birth orders independently of the information in MoBa. The sampling of births within birth order is, therefore, random. As we construct sibship size variables from registries with observations from the birth of the parents and up until 2018, we are confident that we observe completed fertility histories for the vast majority of the sample, as most have reached ages where fertility rates are very low. For the adult generation in the MoBa-sample, we use the central population registry to link them to their parents (i.e. the grandparents of children in MoBa). As the pregnancies sampled for the MoBa are over the same cohort window (1999 to 2008), many of the coming parents sampled to the MoBa happens to be siblings too. Thus, we can establish sibship size, birth order, and other demographic information from the Central Population Registry, and use that to investigate birth order differences taking into account genomic information also for the adults in the MoBa-sample. Our observational window from the NuDB register is up until 2018. In a few cases the parents in MoBa have yet to have reached age 30 at our latest observation year 2018. Here, we take the latest observed age available, the lowest age being 27.

Variables from MoBa

MoBa contains genomic information for parents and children, which allows us to create PGSs for each individual based on a Genome-Wide-Association study (GWAS) for educational attainment of 1.1 million people ( 68 ). For the MoBa-sample, we conducted quality control using PLINK (v 1.90). Before we perform quality control in PLINK, we remove families with any individuals born in countries outside of Europe, The United States, Canada, New Zealand, or Australia. We also remove families with any multipartnered fertility parents in MoBa. Thresholds for genotyping call rate were set to 98%, minor allele frequency (MAF) 5%, and deviations from Hardy–Weinberg equilibrium was < 10 –4  with mid P -value adjustment. We remove individuals with poor genotype quality with a threshold missing rate of 5%, as well as those with heterozygosity rates which deviate ± 3 SDs from the sample mean. We remove ancestry outliers in plink with an identity-by-state binominal test (PPC) set to 0.05, MAF 0.01, and compare the 1 to 5 nearest neighbors in the data. We remove families with a z-score for any neighbor below 4 SDs. We use KING software ( 69 ) to confirm that siblings are genetic siblings based on identity-by descent.

We use PRSice ( 70 ) to make PGSs for parents and children. We use all available SNPs, genome-wide significant or not, and clump with a 250-kb window. Clump-r2 was set to 0.1, excluding SNPs with higher linkage disequilibrium. The PGS is standardized.

In addition to genomic information, MoBa has several survey waves. Birth length and birth weight were self-reported from the mother when the child was 6 months old. Questionnaires and instrument documentation for the MoBa is available here: https://www.fhi.no/en/studies/moba/for-forskere-artikler/questionnaires-from-moba/

After doing quality control on the genomic data, and removing people according to what we have just described, 79,057 individuals are left in the sample, 29,815 children, 24,032 fathers, and 25,210 mothers. However, while we use this sample for between-family analysis in some figures, we rely mostly on within-family analysis, i.e. family fixed-effects with clustering on mothers and robust standard errors. Here, our sample size is smaller, as at least two children born to the same mother are needed to estimate the models, and we need both siblings to be genotyped (children generation in MoBa N  = 2,705, adult generation in MoBa N  = 3,897).

This research is part of the OPENFLUX project that has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (ERC consolidator grant agreement no. 818420). The Norwegian Mother, Father, and Child Cohort Study is supported by the Norwegian Ministry of Health and Care Services and the Ministry of Education and Research. This research is part of the HARVEST collaboration, supported by the Research Council of Norway (#229624). We also thank the NORMENT Centre for providing the genotype data, funded by the Research Council of Norway (#223273), the South East Norway Health Authority and the KG Jebsen Stiftelsen.

Social sciences, genetics.

We are grateful to all the participating families in Norway who take part in this on-going cohort study. We thank the Norwegian Institute of Public Health (NIPH) for generating high-quality genomic data. We thank Per Minor Magnus, Pål Njølstad, and Ole Andreassen who headed the forementioned projects. We further thank the Center for Diabetes Research, the University of Bergen for providing the genotype data and performing quality control and imputation of the data funded by the ERC AdG project SELECTionPREDISPOSED, Stiftelsen Kristian Gerhard Jebsen, Trond Mohn Foundation, the Research Council of Norway, the Novo Nordisk Foundation, the University of Bergen, and the Western Norway health Authorities (Helse Vest). Isungset thank Robbee Wedow for making genomics less hard.

M.A.I. and J.F. designed and performed the research, M.A.I. analyzed the data; and M.A.I., J.F., O.A.A., and T.H.L. wrote the paper.

The consent given by the participants does not open for storage of data on an individual level in repositories or journals. Researchers who want access to data sets for replication should submit an application to [email protected]. Access to data sets requires approval from The Regional Committee for Medical Research Ethics in Norway and a formal contract with MoBa. Code is available at https://osf.io/fj3nb .

Competing Interest: The authors declare no competing interest.

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Angels designate Johnny Cueto for assignment after two starts

ANAHEIM, Calif. -- Pitcher Johnny Cueto was designated for assignment by the Los Angeles Angels on Friday after two rough outings.

Cueto, 38, gave up nine runs in 11 innings in his two starts for the Angels, both losses. He gave up six runs in five innings in Tuesday night's 6-2 loss at Detroit, with three of the six hits being home runs.

Even though Cueto's time with the big league club was short, manager Ron Washington said Cueto's experience benefitted the younger members of the pitching staff.

"He had a tremendous influence on them with his work ethic, how they do bullpens and that type of stuff. He was willing to give up his wisdom and knowledge," Washington said before Friday night's game against the Seattle Mariners . "It was hard, but we are at that point of the year where changes happen."

Cueto signed a minor league deal with the Angels on July 24 after being released earlier in the month by the Texas Rangers . He was 3-0 in four starts at Triple-A Salt Lake City before being called up on Aug. 21.

The Angels are the sixth team for Cueto, who is 144-113 with a 3.52 ERA in 370 major league games. His career began with Cincinnati, followed by stops in Kansas City, San Francisco, the Chicago White Sox and Miami.

Cueto will make way for some of the organization's younger arms after the team called up left-hander Sam Aldegheri and right-hander Caden Dana from Double-A Rocket City.

Aldegheri made his big league debut Friday and is the first player born and raised in Italy to pitch in the majors.

Dana, considered the organization's top pitching prospect, will start Sunday.

Los Angeles also put right-hander Carson Fulmer on the injured list retroactive to Tuesday because of right elbow inflammation and transferred right-hander José Marte to the 60-day injured list

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• LOS ANGELES-ANGELS
• CUETO JOHNNY

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What your birth order might say about who you date

• The birth order dating theory has been going viral on social media.
• Because birth order can influence your personality, it might also impact who you date.
• A psychologist explained why birth order compatibility feels so true for some people.

When it comes to dating, it's common to have a type. You might only date other runners or always fall for tech bros . Or, if you were a middle child, you might only date people who were the youngest child in their family, according to the birth order dating theory .

As summarized by TikTok , your birth order can predict who you'll be attracted to. Annie Wright , a licensed marriage and family therapist, told Business Insider that "birth order can influence personality traits and relationship dynamics."

It stems from the belief that birth order shapes who you are , a theory first introduced by Austrian psychiatrist Alfred Adler in the early 20th century. For example, parents might be more vigilant or strict with a firstborn than their other kids, Wright said.

While the significance of birth order continues to be debated , Wright said the dating theory rings true for a lot of people, including herself — an oldest daughter married to a youngest son.

Opposites attract when it comes to birth order

People seek out partners who have complementary birth orders to theirs, such as firstborns dating younger-borns , according to the theory.

"In my experience professionally, and personally as a firstborn, firstborns typically receive more attention and resources from parents," Wright said. "It can lead to higher academic achievement, greater leadership qualities, and a more conscientious nature." For example, the eldest daughter might be more goal-oriented or responsible if she had to care for her younger siblings .

In contrast, the youngest siblings "tend to develop a more sociable and accommodating personality," she said, due to parents having a more lax attitude around them. As a therapist, she's also noticed that younger-born children (including middle children ) are often more prone to seeking harmony and mediating conflict.

"The dynamics between a firstborn's tendency to take charge and a younger-born's inclination to cooperate can create more of a balanced, complementary relationship," she said.

Dating the same birth order can cause clashes

Just as it can work well to date someone of a different birth order, dating someone with the same birth order may lead to clashes.

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For instance, two firstborns dating may both want to take charge.

However, she said, "If you get two younger-borns together, maybe a bit more laid-back and easygoing in personality," the conflict might be more around finding structure and allocating responsibilities in the relationship.

Birth order matters less than the role you played

Not everyone may feel represented by the birth order theory, like only children , who Wright said might feel like a mix of firstborn and youngest-born kids. She said middle children could also have a mix of both characteristics and might not relate to the "mediator" traits assigned to their birth order.

"We can't speak in absolutes, whether it's talking about who you are because of your astrological sign or who you are because of your birth order," Wright said.

Whether you're a youngest-born child who still had a lot of responsibilities or an only child who relates to all the birth order traits, Wright said it's most important to ask yourself what your early experiences were like and how they might have shaped who you are today.

When it comes to future romantic relationships, your dynamic with your parents probably has more weight than being the second-oldest kid.

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