Birth Order and Personality: What Large Studies Find
PsyCove Editorial · 8 min read · Updated 2026-10-28
- The claim that firstborns are conscientious and laterborns rebellious does not survive the design that controls for family: within-family comparisons find effects near zero.
- The largest effect in a 20,186-person study was about a fiftieth of a standard deviation — detectable statistically, invisible in a person.
- A 377,000-student sample found no meaningful birth-order effect on the Big Five, though firstborns scored slightly higher on self-reported intellect.
- Siblings really do differ from one another, but the cause is mostly non-shared environment and specific events, not position in the queue.
- Family roles are real: the 'responsible one' and the 'wild one' are assignments that siblings negotiate, and assignments can be renegotiated.
The claim, and why it feels true
The modern version of the birth-order thesis comes from Frank Sulloway's 1996 book Born to Rebel, which argued that firstborns occupy a niche of authority and laterborns a niche of rebellion, and that this displacement produces measurable personality differences. The argument is elegant, and it matches almost everyone's family. That match is the problem: the pattern is easy to see in the family you grew up in and hard to see anywhere else. The test that would settle it is straightforward — if the same pattern held when siblings are compared with each other rather than across families, the thesis would stand, and it does not hold up that way.
There is a real phenomenon underneath the mistaken one. Families do assign roles — the responsible one, the funny one, the one who needs looking after — and those roles shape behavior for years. What the large studies separate is whether the role is caused by birth order or merely indexed by it. In most families the responsible one happens to be first, which makes the role look like a consequence of the position when it is often a consequence of the family's situation: a parent's illness, an early divorce, a gap in ages, or simply who was old enough to be given a task.
The design problem: between families or within one
The entire dispute reduces to one methodological question, and once you see it the literature becomes easy to sort. Are you comparing people who grew up in different families, or siblings who grew up in the same one? Between-family comparisons cannot separate birth order from everything that travels with it: family size, parents' age and resources, socioeconomic status, and the fact that a firstborn is by definition the child of younger, less experienced parents. Any of those can produce a difference that looks like an order effect and would be wrong to call one — which is why a between-family finding about firstborns is not evidence until it survives a within-family test.
- Family size: more children means less of everything per child, and larger families have more laterborns by construction.
- Parents' age and experience: a firstborn is by definition the child of the youngest version of the parents.
- Household resources: income, space, and adult attention all shift across the years a family is growing.
- Cohort: siblings born years apart grow up in different decades, schools, and neighborhoods.
Within-family designs hold all of that constant by comparing siblings to each other. This is a much harder design to run — you need families with several children and personality data on each — and it is the only one that answers the question people are actually asking.
What the large studies found
Two studies settle most of it. Rohrer, Egloff, and Schmukle (2015) analyzed 20,186 people across four large samples, using both between-family and within-family comparisons, and found no meaningful effect of birth order on any of the Big Five. The largest effect they could detect was about a fiftieth of a standard deviation, and their confidence intervals excluded anything larger than roughly a tenth — small enough that differences between siblings in the same family are not attributable to their order.
Damian and Roberts (2015) reached the same conclusion in a sample of more than 377,000 US high-school students. Birth order showed no meaningful association with the Big Five. It did show small associations with self-reported intellect and with academic ability, with firstborns slightly ahead — a difference of a couple of IQ points at most, and one that is plausibly about parental resources and the amount of adult attention available for a first child rather than about a personality effect of position.
The effect is not too small to matter. It is too small to be the thing you are describing when you describe your brother.
What birth order does predict
Near-zero is not zero, and the small effects that survive are worth naming precisely because they are the ones that get inflated. The consistent finding is cognitive rather than temperamental: firstborns average slightly higher on tests of intelligence and academic achievement, with the gap widest in large families and with short birth intervals. The leading explanation is resource dilution rather than position — a family with more children has less of everything per child, and the first child gets the version of the household that existed before the dilution.
- Conscientiousness, agreeableness, openness, extraversion, neuroticism: no meaningful birth-order effect in within-family designs.
- Self-reported intellect and academic ability: a small firstborn advantage, roughly a couple of IQ points.
- Family size and birth interval: more consistently related to outcomes than order itself, and usually the confound hiding inside the order effect.
- Niche-picking in the family: real, observable, and not evidence that order caused it.
Why siblings differ so much anyway
The puzzle that keeps the birth-order story alive is genuine: siblings raised in the same house turn out remarkably different, and people reach for order as the explanation for the difference they can see. The finding that actually explains it is the one Turkheimer (2000) summarized as the first law of behavior genetics — all human behavioral traits are heritable — together with its corollary: the environment that matters most is the part siblings do not share. Roughly half of the variance in adult personality is genetic, and most of the rest comes from different peers, different teachers, different illnesses, different years of the same marriage. Shared family environment, the thing siblings have most in common, accounts for surprisingly little of what makes them different.
Birth order is a poor index of that non-shared experience, because two siblings two years apart in the same household live in different worlds for reasons that have nothing to do with queue position. The family moved, the parents' marriage changed, the money ran out and came back. All of that differentiates siblings far more than ordinal position does, and none of it is captured by being the eldest.
What to do with this
Three uses follow, and they are all about stripping an explanation down to what it can carry.
- Stop using order to explain yourself in the present tense. 'I am the eldest, so I take responsibility for everything' describes a role you learned, and roles can be renegotiated in a way that a birth position cannot.
- If you want to explain a difference between siblings, look for the events rather than the order: the move, the illness, the year one of you had a different teacher, the period when the household was under strain.
- Treat the role as the actionable layer. Families who can name the assignment — 'you were the peacemaker because it was not safe for anyone to be angry' — can change it in a way that 'I am the middle child' never allows.
The general lesson is about explanation rather than about families. An account that fits every family you have seen, cannot be tested inside a family, and survives mainly because it feels true is doing the work of a story. Stories are useful for talking about your childhood. They are poor guides to what caused it.
Frequently asked questions
Does birth order affect personality at all?
Effect sizes in within-family studies are close to zero — the largest in a 20,186-person analysis was about a fiftieth of a standard deviation. There are small firstborn advantages in measured intelligence and academic achievement, on the order of a couple of IQ points, and those are better explained by family resources than by position.
Why does everyone believe it then?
Because families assign roles, and the roles are visible while the causes are not. The responsible eldest is a common arrangement, so the pattern appears to confirm the theory in the one family each person can observe closely. The design that would test it — comparing siblings within a family — is not something anyone runs at the dinner table.
Are firstborns smarter?
Slightly, on average, by a couple of IQ points, with the gap largest in big families and with closely spaced children. The resource-dilution explanation fits better than a position effect: the first child gets more adult attention and the household's undivided resources, and later children get a thinner share.
What actually explains why siblings are so different?
Non-shared environment and genetics. About half the variance in adult personality is genetic, and most of the remainder comes from experiences siblings do not share — peers, teachers, illness, and the different versions of the same household they lived through. Shared family environment accounts for little of the difference.
Should I stop thinking of myself as the eldest?
Keep the role if it is useful, drop the causal claim. 'I learned to manage other people's feelings early' is a description you can act on. 'I am like this because I am the eldest' is a claim the evidence does not support, and it makes the pattern sound fixed when it is a habit.
References
- Rohrer, J. M., Egloff, B., & Schmukle, S. C. (2015). Examining the effects of birth order on personality. Proceedings of the National Academy of Sciences, 112(46), 14224–14229.
- Damian, R. I., & Roberts, B. W. (2015). The associations of birth order with personality and intelligence in a representative sample of U.S. high school students. Journal of Research in Personality, 58, 96–105.
- Damian, R. I., & Roberts, B. W. (2015). Settling the debate on birth order and personality. Proceedings of the National Academy of Sciences, 112(46), 14119–14120.
- Sulloway, F. J. (1996). Born to Rebel: Birth Order, Family Dynamics, and Creative Lives. New York: Pantheon Books.
- Turkheimer, E. (2000). Three laws of behavior genetics and what they mean. Current Directions in Psychological Science, 9(5), 160–164.