We've all heard the claims: oldest siblings are bossy, younger ones are funnier to get noticed, middle children fear not having their voices heard. You can find thousands of articles online endorsing these stereotypes. However, a study with a large sample size finds that the effects are so small as to be nearly meaningless.
The perception that one's order in a family determines personality is so deep-seated that even people famous for their demolitions of pseudoscience attribute scientific revolutions to where the leading figures stood in their family. According to a paper in the Journal of Research in Personality, however, you'd do almost as well relying on astrology.
Professor Brent Roberts of the University of Illinois started off by comparing the IQ scores of 377,000 high school students based on their place in family chronology. Some studies have claimed first born children are more intelligent, although there is inevitable push-back from younger sibs. Roberts did indeed find a relationship with first borns scoring highest, but with just 1 IQ point between them you’d struggle to notice the difference.
The correlation was just 0.04, which Roberts describes as statistically significant but “infinitesimally small.”
Some studies have found a stronger relationship, but not only were these studies smaller, they often didn't control for factors that could have distorted the results, such as a family's economic status.
IQ differences might attract skepticism, but judging by the results of a simple Google search, people are far more likely than not to believe that your age relative to your brothers and sisters determines how you behave. Almost unbelievably, Roberts claims that many of the previous studies relied on asking children how they score on certain traits compared to others in their family. Funnily enough, different siblings gave different answers, while parents tended to describe their older children as scoring more strongly on characteristics that develop with age.
When assessments were done independently and on children of the same age, Roberts found that birth order was even less likely to influence personality than intelligence, with a correlation of 0.02. Moreover, Roberts conducted a secondary study on the subgroup of children who had exactly two siblings and had similar family circumstances. The results matched those of the study as a whole – differences exist, but were tiny compared to those driven by other factors. As Roberts notes, "In some cases, if a drug saves 10 out of 10,000 lives, for example, small effects can be profound. But in terms of personality traits and how you rate them, a 0.02 correlation doesn't get you anything of note.”
"The message of this study is that birth order probably should not influence your parenting, because it's not meaningfully related to your kid's personality or IQ," said Dr. Rodica Damian, the study's other author.