The eldest is the responsible one. The middle child is overlooked and diplomatic. The youngest is charming and gets away with everything. And the only child? Spoilt, of course. Most of us can reel off these stereotypes, and many of us can point to a family where they seem to fit perfectly.
Yet when psychologists tested birth order on hundreds of thousands of people, the effects on personality almost disappeared. Here's what the biggest studies found, why the myth feels so convincing, and what actually shapes who you become.
Where the birth order idea came from
The idea has a long history. In the early twentieth century, the psychologist Alfred Adler argued that a child's position in the family shaped their outlook on life. Firstborns, he suggested, might feel "dethroned" when a sibling arrived, while youngest children might be pampered.
Later, the historian of science Frank Sulloway revived the theory in his 1996 book Born to Rebel, arguing that firstborns tend to be more conservative and conventional, while laterborns are more open and rebellious. The book was influential and widely discussed, and it gave the idea a fresh scientific gloss.
Both accounts were thoughtful, but they relied heavily on selected examples and smaller samples. The real test needed far bigger data.
The huge studies that checked
Two large studies published in 2015 are the most cited.
Damian and Roberts: nearly 400,000 students
Rodica Damian and Brent Roberts analysed data from around 377,000 American high school students. They looked at personality and intelligence alongside birth order, while controlling for things like family size and socioeconomic background.
They did find differences, but they were tiny. The average correlation between birth order and personality traits was about .02, which is close to zero in practical terms. Firstborns scored around one IQ point higher on average.
Rohrer, Egloff and Schmukle: three countries
Julia Rohrer, Boris Egloff and Stefan Schmukle examined data from more than 20,000 people in the United States, the United Kingdom and Germany. Crucially, they compared siblings within the same families as well as people across families.
They found no meaningful effects of birth order on extraversion, emotional stability, agreeableness, conscientiousness or imagination. The one consistent finding was a small advantage for firstborns in intelligence test scores, along with slightly higher self-reported intellect.
| Claim | What large studies found |
|---|---|
| Firstborns are more conscientious | No meaningful difference |
| Laterborns are more open and rebellious | No meaningful difference |
| Middle children are more agreeable | No meaningful difference |
| Youngest children are more extraverted | No meaningful difference |
| Firstborns score slightly higher on IQ tests | Yes, by roughly one to two points on average |
Why the myth feels so true
If the effects are so small, why do so many families seem to fit the stereotypes? A few reasons stand out.
- Age gets confused with order. Within a family, the eldest is always older, and older people tend to be more responsible and conscientious. This is part of the maturity principle, not a birth order effect.
- Family roles are real, but temporary. The eldest may be asked to babysit and act responsibly at home. That role doesn't necessarily carry into adult personality outside the family.
- Confirmation bias. We remember the responsible eldest and the charming youngest, and forget the families that don't fit.
- Family size and background. Larger families differ from smaller ones in many ways, which can create misleading patterns if they are not separated out.
- Parents rate their children differently. When parents describe their own kids, they may compare them to each other, exaggerating differences that outsiders wouldn't notice.
What about only children?
The stereotype of the lonely, spoilt only child is one of the most persistent. Research has not been kind to it. Reviews dating back to the 1980s found that only children were broadly similar to children with siblings on most measures of personality and social adjustment, and sometimes slightly ahead on achievement-related outcomes.
Modern studies generally agree that being an only child has little lasting effect on core personality traits.
The small IQ difference, explained
The one fairly consistent finding is that firstborns score slightly higher on intelligence tests. Researchers have proposed a few explanations, and none is settled:
- Undivided attention. Firstborns have a period with their parents' full attention before siblings arrive.
- Teaching younger siblings. Explaining things to others may reinforce learning.
- Changes in the family environment. Resources, time and energy are spread more thinly as families grow.
The difference is small, around one or two points on average, and individual variation is far larger. A laterborn sibling is very often the higher scorer in any given family.
What shapes personality more than birth order
If birth order isn't doing much, what is? The research points to a mix of influences:
- Genes. Studies of twins suggest that genetic differences account for a substantial share of variation in personality traits.
- Unique experiences. Friends, teachers, illness, opportunities and chance events shape siblings differently, even in the same home.
- Life roles in adulthood. Work, relationships and responsibilities continue to shape personality well past childhood.
- Culture and context. The norms and expectations around you shape how traits are expressed.
It also helps to remember that personality is not fixed. Our piece on whether personality can change covers how traits shift across life.
A better way to understand yourself
If you've always thought of yourself as "a typical middle child," it may be worth checking that story against a more reliable measure. Birth order labels are broad and mostly inaccurate. A structured questionnaire based on the Big Five can tell you far more about where you actually sit. You can take our free Big Five personality test and see whether your results match the stereotype.
The bottom line
Birth order makes for great family jokes, but large studies of hundreds of thousands of people find its effect on personality is close to zero. Firstborns do score slightly higher on IQ tests on average, but the gap is small. Your genes, experiences and adult life shape your personality far more than when you arrived in the family.
FAQ
Does birth order affect personality?
Large studies find that birth order has little to no meaningful effect on the Big Five personality traits. The stereotypes are mostly not supported by data.
Are firstborns smarter?
On average, firstborns score slightly higher on intelligence tests, by roughly one to two IQ points. The difference is small and many laterborns outscore their older siblings.
Is middle child syndrome real?
There is no good evidence that middle children have a distinct personality profile. Large studies have not found meaningful differences linked to being in the middle.
Are only children more selfish?
Research does not support this. Only children are broadly similar to children with siblings in personality and social skills.
