Does birth order shape personality? Firstborns, middle children, the youngest and only children in large studies

“Firstborns are responsible, middle children read the room, the youngest are charmers”, “only children are selfish”: you have probably heard lines like these. Explaining someone by being the eldest son or daughter, or the baby of the family, sounds quite plausible. So does the order in which you were born among your siblings really shape your personality? This guide answers with large studies.

The short answer: a study of three large national panels from the United States, Great Britain and Germany (5,240, 4,489 and 10,457 people) found no birth-order effect on extraversion, emotional stability, agreeableness, conscientiousness or imagination (Rohrer et al., 2015). Summarising their own study of 377,000 US high school students (Damian & Roberts, 2015a; sample size as cited in Lejarraga et al., 2019), the authors reported that the average association between birth order and personality was about a correlation of 0.02 (Damian & Roberts, 2015b). Intelligence test scores and self-rated intellect, however, have repeatedly come out very slightly higher for firstborns, mostly in Western data, and one large study, of Swedish conscription records, did report firstborn advantages in some personality-related traits (Black et al., 2018).

This guide covers where the idea came from, what the large studies found, the small difference in intelligence, what the studies that found differences claim, research on only children, and why the stereotypes feel so accurate. mind.io does not offer a test that judges personality by birth order. You can look at your own personality traits through your answers in the Big Five personality test, and that result is not linked to where you were born among your siblings.

Interest in birth order is old. In 1874 Francis Galton, the youngest of nine siblings, found firstborns overrepresented in a sample of English scientists and suspected that parents give eldest sons special treatment. About half a century later Alfred Adler, the second of six children, extended the idea to personality. In his account, firstborns were privileged but also burdened by excessive responsibility and a fear of “dethronement”, and so prone to neuroticism, while later-borns, overindulged by their parents, were expected to lack social empathy (Galton and Adler as cited in Rohrer et al., 2015). The description of the middle child as ambitious and competitive also comes from Adler (Adler, 1928, cited in Black et al., 2018).

Frank Sulloway brought the idea back to the centre of debate. In his 1996 book Born to Rebel he proposed a family niche theory: siblings compete for their parents’ attention and occupy different roles, or niches, within the family. In this theory, firstborns become conscientious by acting as surrogate parents to please their parents, and dominant, and so less agreeable, because they are bigger and stronger; later-borns, exploring for an unoccupied niche, become more sociable, more imaginative and more likely to rebel against the status quo (Sulloway, 1996, as cited in Rohrer et al., 2015, and Lejarraga et al., 2019). Sulloway first supported the theory by analysing the social attitudes and birth order of historical figures (Rohrer et al., 2015).

There was disagreement early on. Reviewing studies from 1946 to 1980, Ernst and Angst concluded that in studies accounting for family size and socio-economic differences the effect of birth order on personality was negligible, and that the reported differences were artefacts of poor research design. Sulloway contested this, arguing from a meta-analysis of the same studies that birth order influences the Big Five (Ernst & Angst, 1983, and Sulloway’s reply as cited in Black et al., 2018). Later studies kept splitting between those that supported Sulloway’s predictions and those that supported only part of them or found no effect at all (Rohrer et al., 2015). The studies in the next section tried to settle the debate with large samples.

A study published in PNAS in 2015 analysed representative panel data from Great Britain (a survey following everyone born there in one week of 1958), the United States and Germany together. Leaving out only children and twins and keeping families of up to four children, it ran a between-family analysis comparing people from different families (17,030 people) and a within-family analysis comparing siblings from the same families (3,156 people) on non-overlapping samples, adjusted personality scores for age and accounted for the number of siblings. Birth order had no significant effect on extraversion, emotional stability, agreeableness, conscientiousness or imagination. Comparing only firstborns with last-borns, or keeping only siblings born 18 to 60 months apart, led to the same conclusion. The authors reported that their between-family samples gave a 95% chance of detecting a difference as small as 5% of a standard deviation between firstborns and later-borns (Rohrer et al., 2015).

Summarising their own study the same year, which compared 377,000 US high school students between families (Damian & Roberts, 2015a; sample size as cited in Lejarraga et al., 2019), the authors reported that the average effect of birth order on personality was about a correlation of 0.02. Looking across their study and other large ones such as Rohrer et al. (2015), they summarised that the largest effects they could find were around a tenth of a standard deviation, and the average effect about a twenty-fifth. Citing earlier work showing that observers find it hard to detect personality differences smaller than one standard deviation, they concluded that such differences, even when statistically significant, are not of a size parents, siblings or friends could notice (Damian & Roberts, 2015b).

Studies of narrower traits reached similar conclusions. In the German panel (6,500 to 10,500 people in between-family analyses, 900 to 1,200 within families), life satisfaction, locus of control, interpersonal trust, reciprocity, risk taking, patience, impulsivity and political orientation showed no meaningful birth-order effect; only self-rated intellect did (Rohrer et al., 2017; data as described in Lejarraga et al., 2019). The idea that younger siblings are more adventurous was not borne out either, whether in self-reports from the German panel, in 39 risk-taking measures including tasks with real money at stake, or in the life choice of becoming an explorer or a revolutionary (Lejarraga et al., 2019). A meta-analysis of siblings’ choice of sports, however, found later-borns more likely to take part in high-risk sports, with a mean odds ratio of 1.5 to 1, and the effect was larger in children than in adolescents and adults (Sulloway & Zweigenhaft, 2010, cited in Lejarraga et al., 2019). Besides differences in study methods, Lejarraga and colleagues suggested that this may reflect risk taking that pays off in childhood, when siblings compete for parental attention, without necessarily carrying over into an adult disposition.

Data from outside the West look similar. A within-family study of up to 11,188 participants in the Indonesian Family Life Survey reported effect sizes consistent with birth order having relatively little impact on intelligence, the Big Five and risk aversion (Botzet et al., 2021). In a representative Chinese sample of 14,148 people analysed both between and within families, the links between birth order and the Big Five were not statistically significant and were negligible (Mu et al., 2024). We did not find a large study testing the same question in a representative sample of Korean adults.

Unlike personality, intelligence test scores in large Western datasets repeatedly come out slightly higher for firstborns. In the three-country study above, IQ was about 1.5 points lower (10% of a standard deviation) with each step down in birth order. To give a sense of the size: in two-child families, a randomly picked firstborn had a 52% chance of a higher IQ than a randomly picked second-born, and the second-born a 48% chance. Comparing two siblings from the same family, the older one scored higher in 6 out of 10 pairs; put the other way, the younger one scored higher in 4 out of 10 (Rohrer et al., 2015).

A study using the military entrance test records of Norwegian men estimated the gap between first- and second-borns at about one fifth of a standard deviation, or about 3 IQ points, yet birth order explained only about 3% of the IQ differences within families (Black et al., 2011). In a study by Norwegian researchers of nearly 250,000 conscripts, IQ depended on social rank in the family rather than on birth order as such, which the authors took as support for an explanation based on family interaction (Kristensen & Bjerkedal, 2007). Building on this work, Swedish researchers analysed families in which an older sibling’s death or adoption made birth order and rank in the family differ (Black et al., 2018). These results come mostly from Western data, though, and the authors of the Indonesian study concluded that the small birth-order effects reported elsewhere appear to be culturally specific (Botzet et al., 2021).

As for where the gap comes from, some studies support explanations based on parenting. In an analysis of US longitudinal data (NLSY79), later-born children already scored lower on cognitive tests than their older siblings as early as age one, and much of the gap before school entry was explained by differences in parenting, such as cognitive stimulation by mothers (Lehmann et al., 2018). In a Swedish survey at age 13, which compared children from families with the same composition rather than siblings from the same family, later-borns also spent less time on homework and more time watching TV, and their parents were less likely to discuss school work with them; the authors saw this as a possible driver of the gaps in non-cognitive abilities (Black et al., 2018). Self-rated intellect, with items such as “I am quick to understand things”, was likewise about 10% of a standard deviation lower with each step down in birth order, and the gap remained after taking measured IQ into account. The authors suggested that teaching younger siblings, or comparing oneself with them, may leave a trace on how people rate their own intellect (Rohrer et al., 2015). What an IQ score means, and how far to trust one, is covered in the IQ test guide.

Not every large study ended with “no difference”. A within-family study of 564,789 Swedish men born between 1952 and 1982 who went through conscription (260,807 families) reported that firstborns scored higher than their younger brothers on emotional stability, persistence, social outgoingness, willingness to assume responsibility and ability to take initiative, as rated by a psychologist in a 20 to 30 minute interview at about age 18. On the combined non-cognitive score, the gap between first- and third-borns was about 0.20 standard deviations, shrinking to 0.11 once cognitive ability was taken into account. In occupational data on men born 1941 to 1974, firstborns were, in relative terms, almost 30% more likely than third-borns to become top managers (about 0.6% of the population), and later-borns were more likely to be self-employed (Black et al., 2018).

This study also measured something different. Instead of a self-report personality questionnaire, this was a psychologist’s interview assessing fitness for military service, centred on coping with stress and fitting into a group, and the authors judged it highly unlikely that the psychologist knew the conscript’s birth order. Analysing families in which an older sibling’s death or adoption made birth order and rank in the family differ, they concluded that the gap comes from the post-natal environment rather than biology. The sample was men only, and expressed as a correlation, the link between birth order and the overall score was 0.041. The authors held that sizable effects remain after accounting for cognitive ability, larger and more stable than those Damian and Roberts (2015a) found but smaller than the estimates in Sulloway’s meta-analysis (Black et al., 2018).

Studies that asked people to rate their siblings against each other found clearer differences. In four studies covering 1,022 families, where respondents compared themselves with their siblings, firstborns were nominated as most achieving and most conscientious, and later-borns as most rebellious, liberal and agreeable, whether or not birth order had been made salient, and the authors concluded that the results support the predictions of Sulloway’s niche model (Paulhus et al., 1999). Sulloway’s 2010 meta-analysis reported correlations in the range of 0.00 to 0.18 (Sulloway, 2010, cited in Black et al., 2018). But in such studies one person rates themselves and their siblings at the same time, and critics point out that stereotypes about birth order and contrast effects can inflate the results (Rohrer et al., 2015; Damian & Roberts, 2015b). In the large datasets where each sibling answered about their own personality, those differences did not appear (Rohrer et al., 2015).

In short, the same question gets different answers depending on what is measured and how. Self-report questionnaires answered separately by each sibling showed almost no difference; one person comparing their siblings produced larger differences; and some traits rated by outside assessors showed a firstborn edge. Either way these are differences in averages, so being a firstborn says little about any one person’s personality. What each trait means is explained on the conscientiousness, extraversion, agreeableness and openness pages.

Stereotypes about only children are old too, with “spoilt and selfish” the classic one. A 1986 review running six meta-analyses on 117 studies (achievement, adjustment, character, intelligence, mental illness and sociability) found that only children surpassed every other group except firstborns and people from two-child families in achievement and intelligence, and surpassed people from families of three or more children in adjustment and character (Falbo & Polit, 1986). A meta-analysis of 141 studies the following year looked at 16 personality domains and concluded that only children scored significantly better than other groups in achievement motivation and personal adjustment, but were comparable in most respects to people with siblings (Polit & Falbo, 1987).

The idea that only children are self-centred is still common. In a study where 556 laypeople rated a typical only child and a typical person with siblings, they ascribed more of both sides of narcissism, admiration and rivalry, to the only child. Yet in actual scores from a representative panel of 1,810 people, only children were not more narcissistic than people with siblings, and this held when major background variables were controlled for. The authors concluded that the stereotype is prevalent but inaccurate (Dufner et al., 2020). The narcissism guide explains what narcissism is.

One study did report disadvantages. In economics experiments with 421 people born just before and just after China introduced its One-Child Policy in 1979, those born after it were less trusting, less trustworthy, more risk-averse, less competitive, more pessimistic and less conscientious. The authors used the fact that the policy was imposed from outside to estimate the effect of being an only child separately from family background (Cameron et al., 2013). Still, it compares one Chinese generation at one moment with 421 participants, so it does not transfer directly to only children in other countries or generations.

Taken together, being an only child on its own predicts little about personality. Meta-analyses finding only children significantly higher in achievement motivation and personal adjustment sit beside an experiment reporting disadvantages in a generation raised under a particular policy, and the narcissism stereotype was not confirmed in a representative sample. The authors of the 1986 meta-analysis proposed a model of the one-child family that combines parental attention, parental anxiety and family economic resources (Falbo & Polit, 1986).

There are reasons why “that is exactly my family” can feel true even though large studies find almost no difference. The first is age. Seen at the same moment, the firstborn is always older than the younger siblings, and personality changes with age, for example by becoming more conscientious (Rohrer et al., 2015). So an eldest child who seems more mature may simply be a few years older. Because almost everyone sees older siblings behaving differently from younger ones first-hand, and people tend to weigh anecdotes more heavily than data, the authors expected the idea that birth order shapes personality never to go away entirely (Damian & Roberts, 2015b). How personality changes with age is covered in the guide Can personality change?

Second, within a family we see siblings in comparison with each other. Rating siblings side by side can create a contrast effect that makes differences look bigger than they are (Damian & Roberts, 2015b). Third, we already hold expectations about birth order. People believed that personality differs by birth rank and rated firstborns as the most intelligent but least creative, and last-borns as the opposite. The same research found that earlier-born people do in fact complete more years of schooling and attain more prestigious occupations, and the authors held that beliefs about birth rank reflect what society regards as reality and may also influence that reality (Herrera et al., 2003). A similar mechanism is discussed in the blood type personality guide.

Fourth, roles within the family really do differ. Harris proposed that birth-order effects might be visible within the family yet not carry over to behaviour and relationships outside it (Harris, 2000, cited in Rohrer et al., 2015). In Korea, too, there are data showing that contact and support between parents and their adult children differed by birth order and gender. A study of 3,950 parents and 10,947 of their married children living elsewhere, from the 2006 Korean Longitudinal Study of Ageing (KLoSA), found that parents reported more frequent face-to-face contact with, and more financial support from, their first-born sons than their other sons and daughters, while daughters were in touch more often by phone, mail or email (Choi & Bin, 2016). This means the roles people take on at home can differ, not that their personality traits do.

The Korean term “K-jangnyeo” (K-eldest daughter) can be read in this light. It is used for eldest daughters describing how much responsibility and how many family expectations they took on from an early age, and in our search we found no study measuring it as a personality trait. That does not make the experience minor. It may reflect a heavy role and heavy expectations rather than a different personality, and that weight is worth talking through with family or with a counsellor. Talk of a neglected “middle child syndrome” is similar. One study of adults found middle-borns saying they felt less close to their family than firstborns and last-borns (Salmon & Daly, 1998, cited in Hardman et al., 2007), but a replication using the same method with three samples of children and one of adults found no such effect (Hardman et al., 2007).

Q. Do firstborns, middle children and youngest children really have different personalities? A. Large studies have found almost no meaningful difference in Big Five personality traits. A study of panels from the United States, Great Britain and Germany found no birth-order effect on extraversion, emotional stability, agreeableness, conscientiousness or imagination (Rohrer et al., 2015), and in a Chinese sample of 14,148 people the links were not significant either (Mu et al., 2024). Swedish conscription records of men, however, showed a gap of about 0.20 standard deviations between first- and third-borns in a combined non-cognitive score rated by psychologists in interviews (Black et al., 2018).

Q. Is it true that firstborns are smarter? A. On average, slightly. IQ was about 1.5 points lower with each step down in birth order, yet the younger of two siblings in the same family still had the higher IQ in 4 out of 10 pairs (Rohrer et al., 2015). In Norwegian data, the average gap between first- and second-borns was about 3 IQ points, yet birth order explained only about 3% of the IQ differences within families (Black et al., 2011).

Q. Is middle child syndrome real? A. The evidence is weak. The finding that middle-borns feel less close to their family (Salmon & Daly, 1998, cited in Hardman et al., 2007) did not appear in a replication with three samples of children and one of adults (Hardman et al., 2007), and the three-country panel analysis, which included families of three and four children and so middle children, found no significant birth-order effect on extraversion, emotional stability, agreeableness or conscientiousness (Rohrer et al., 2015).

Q. Are only children selfish or narcissistic? A. Large samples have not borne that out. In a representative panel of 1,810 people, only children were not more narcissistic than people with siblings (Dufner et al., 2020), and a meta-analysis of 141 studies found only children comparable in most respects to people with siblings (Polit & Falbo, 1987). An experiment with 421 people born around the start of China’s One-Child Policy did report differences in trust, competitiveness and more (Cameron et al., 2013), but that concerns one generation at one moment.

Q. What about the idea that eldest sons and daughters are more responsible? A. It has not been confirmed as a personality trait. In the three-country panels, birth order had no significant effect on conscientiousness (Rohrer et al., 2015). The roles and expectations people carry at home can still differ: in the Korean Longitudinal Study of Ageing, parents reported seeing their first-born sons more often and receiving more financial support from them than from their other children (Choi & Bin, 2016). The term “K-jangnyeo” can likewise be understood as the weight of the roles and expectations an eldest daughter has carried, rather than as a personality type.

Q. Is there a birth order personality test? A. mind.io does not offer a test that judges personality by birth order. You can look at your personality traits through your own answers in the Big Five personality test, and the MBTI test also shows your preferences and type from your answers. Neither result is linked to where you were born among your siblings. How the two tests differ is explained in the MBTI vs Big Five guide.

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