Birth order and personality: what large studies found

What does the birth order theory claim?

The popular version is easy to state. Firstborns are said to be responsible, conscientious and used to being in charge, because they had their parents to themselves and were handed early responsibility. Middle children are said to be diplomatic and sociable, negotiating between the eldest and the youngest for attention. The youngest is said to be charming, rebellious and less serious, raised by parents who had relaxed their rules. Only children are said to be self-reliant and early to mature, having grown up mostly among adults.

The idea has a long history in psychology. It appeared in nineteenth-century work on the family backgrounds of eminent men, and it was developed in the early twentieth century into an account of the family as a small society in which each child finds a position that is not already taken. In broad strokes, the reasoning is that siblings compete for parental attention, so each of them specialises, and the specialisation hardens into character.

It is a good story. It fits the way families are remembered, it gives every reader a role, and it has been repeated in popular books for a century. The reasoning is also perfectly testable, because it makes a clear prediction: adults should differ on measured traits according to the position they were born into.

What did the large studies find?

The prediction has now been tested in samples big enough to detect small effects. Rohrer and colleagues analysed three large national panel studies and looked for birth-order differences on the broad personality dimensions. They found no birth-order effects on Extraversion, Emotional Stability, Agreeableness, Conscientiousness or imagination. The one association that did appear was a small decline in self-reported intellect with later birth position, about a tenth of a standard deviation, which held even after measured intelligence was controlled (Rohrer et al., 2015).

A study published the same year came at the question from a different direction. Damian and Roberts examined a large representative sample of United States high-school students and reported that the average absolute association between birth order and personality was .02, with .04 for intelligence, once age, sex, sibship size, socioeconomic status and family structure were controlled (Damian & Roberts, 2015). Two independent teams, different countries, different designs, and the same shape of answer.

The size of these samples is the point. Earlier work often had a few hundred participants, which is enough to produce an eye-catching result by chance and not enough to rule a small effect in or out. With tens of thousands of people, an effect of the size the popular theory implies would have been obvious. It was not there.

Note what is not being claimed. Nobody is saying that siblings are alike, or that growing up first in a family is the same experience as growing up last. The finding is narrower and more specific: adults' positions in their sibling order do not predict where they sit on the broad traits.

What is claimed for each birth position?

Here are the familiar claims set beside what the large studies report. The middle column is the popular description as it circulates. It is not a finding about people born into that position.

Why does birth order feel so convincing?

Because most people test it on their own family, and a family is the one place the comparison cannot work. When you look at your brothers and sisters, you are comparing people of different ages who were at different stages when you formed your impressions. The eldest really was more responsible at fifteen than the youngest was at nine. That is a difference in age, standing in for a difference in birth order, and it does not survive into a comparison of adults measured on the same scale.

Memory helps the story along. Families tell stories about themselves, and the roles in those stories get assigned early and repeated often: the sensible one, the wild one, the peacemaker. Everyone then notices the behaviour that fits the role they were given and lets the rest pass. Within-family labels are stable for reasons that have little to do with traits.

The descriptions themselves are also written to fit. Forer showed this in a classroom: he gave every student the same short sketch, drawn from a newsstand astrology book, told each student it was their own result, and they rated it about 4.26 out of 5 for accuracy, as the demonstration is usually reported (Forer, 1949). Birth-order descriptions work the same way. They are broad, mildly flattering, and read as personal because they arrive with your name on them.

• Age masquerading as position. Comparing siblings compares different ages, not different birth orders.
• Assigned family roles. The labels are handed out early and confirmed by everyone who repeats them.
• Selective memory. The evidence that fits a role is remembered; the rest is not.
• Descriptions that fit anyone. Responsible, diplomatic and charming are true of most adults some of the time.
• Family size and circumstance. Larger families differ from smaller ones in many ways, and those differences are easy to mistake for effects of position.

What does shape personality?

Two influences carry most of the weight, and neither is a position in a sibling order. The first is inheritance. A meta-analysis of twin, family and adoption studies put the heritability of personality at about forty percent, meaning that roughly two fifths of the differences between people in those samples tracked genetic differences (Vukasović & Bratko, 2015). It is an average across traits and study designs, and it describes a population rather than any one person.

The second is development across the life course. Pooling longitudinal studies shows average trait levels moving in a consistent direction through adulthood: people become more conscientious and more emotionally stable, mostly between 20 and 40, more agreeable mainly in later life, while social vitality rises in adolescence and declines in old age (Roberts et al., 2006). Deliberate programmes can shift trait measures as well, by about a third of a standard deviation over roughly six months (Roberts et al., 2017). How personality changes with age follows that path in detail.

Most of what is left is the part of the environment that siblings do not share. Different friends, different teachers, different opportunities and accidents. That is a real finding about families, and it is more interesting than the birth-order story: what separates two adults raised in one house is mostly the things that happened to only one of them.

How does Wellington measure personality instead?

Wellington is a science-based personality assessment from Therabot Labs LLC. It measures personality as continuous traits, built on the Big Five and HEXACO models, and writes the results back as a personal report rather than a type.

Wellington asks you about your own behaviour, which is the method with the evidence behind it. The free Snapshot is 60 questions, about 7 minutes, and covers six broad dimensions: Extraversion, Agreeableness, Conscientiousness, Emotional Stability, Openness to Experience, Honesty-Humility. Each score is the average of several statements rated on a five-point accuracy scale, reported as a percentile against the reference sample, in one of three bands, Quiet below the 30th percentile, Balanced to the 70th, High above. The bands and the Portrait names exist to make the result easier to talk about; the percentile is the measurement, and it is always shown alongside. Our norms are provisional until the norming sample is complete. The first ten questions are answered before anything is asked of you; after question ten you sign in with an email address so your answers are saved and your results can be shown. There is no card.

The Wellington Membership is $4.99 a month and adds the Portrait, 336 questions in 4 chapters, to reach 90 traits, including every Big Five facet and all 24 VIA character strengths. It continues toward all 255 traits with a short weekly module. In the written report, every trait has three pages, one per band, written in advance by the writing and psychology team, and your answers choose which one you read. Wellington never asks how many siblings you have, and family position is not part of any score. Wellington is a wellness tool for reflection, not a medical or psychological diagnosis, and it does not replace care from a qualified professional. It is not used for hiring, and you can export or delete your data at any time.

One thing a trait measure does that a birth-order description cannot is compare you with people outside your family. A percentile puts your Conscientiousness against a reference sample rather than against your brother, which is the comparison most people have actually been making. How accurate are personality tests? covers what else to check before trusting a result.

Questions people ask

Does birth order really affect personality?

Not measurably, on the evidence available. Three large national panel studies found no birth-order effects on Extraversion, Emotional Stability, Agreeableness, Conscientiousness or imagination, and only a small effect on self-reported intellect (Rohrer et al., 2015). A representative sample of United States high-school students found associations with personality that were very small (Damian & Roberts, 2015).

Are firstborns more conscientious than their siblings?

The large studies say no. Analysing three national panels, researchers found no birth-order effect on Conscientiousness (Rohrer et al., 2015). The one association that did appear was a small one on self-reported intellect, where earlier-born people rated their own intellectual ability slightly higher. That is a rating of ability, not a measure of how organised or dependable someone is.

Why do middle child and only child descriptions fit so well?

Because they are broad, kind and read as personal once they carry your name. Forer gave students an identical generic sketch, told each it was their own result, and they rated it about 4.26 out of 5 for accuracy, as the demonstration is usually reported (Forer, 1949). Family roles are also assigned early and repeated often, so the behaviour that fits the label is the behaviour everyone remembers.

If my siblings and I are so different, what explains it?

Mostly the environments siblings do not share, plus inherited differences. Twin and family studies put the heritability of personality at about forty percent of the differences between people (Vukasović & Bratko, 2015), and much of the rest tracks the experiences only one sibling had. Traits also keep moving through adulthood (Roberts et al., 2006).