Since the time of Freud, psychologists have been devoted to nailing down how exactly our birth order affects our lives, arguing that youngest children are more rebellious or more agreeable or that oldest children are more conscientious or smarter. But there are a lot of confounding factors when it comes to studies of sibling dynamics, including family income and parental education, whether you’re the youngest of two or five siblings, or whether you have an older brother versus an older sister.
According to a new, unusually large study of sibling relationships by psychologists from the University of Illinois at Urbana-Champaign, whether you are an older or youngest sibling has almost no effect on your personality or intelligence. The study, published in the Journal of Research in Personality, analyzed a nationally representative sample of 377,000 high school students using data from a longitudinal study that began in 1960. The researchers report that personality differences between firstborn and laterborn siblings may exist, but they are almost infinitesimal.
There was some evidence for associations between oldest children being slightly more extroverted and conscientious, but the differences were slim. There was only a minuscule advantage—“almost imperceptible”—in terms of IQ for first born children. However, in comparison to other factors related to personality and intelligence traits, such as family income, the effects were barely noticeable. The researchers write that “the attention given to the role of birth order on personality is, at best, disproportionate to its importance to the development of personality differences among siblings across families.”
The study had several limitations, including that it was based on self-reported personality traits, rather than observed ones, and that it didn’t control for large age gaps between siblings (growing up with an older sibling who’s already out of the house is a different environment than a sibling only a few years older). However, given that this is one of the largest studies of its kind, it’s likely to be slightly more reliable than past research that found significant personality differences among only a few hundred or a thousand subjects.