Some studies have suggested that firstborns show higher levels of intelligence as a result of being afforded more 'intellectual stimulation' by the family.
The order in which we and our siblings are born is widely believed to influence the people we become. For instance, there are many bestselling parenting books that claim birth order makes a difference to children's personality, intelligence, development and future success.
However, scientific evidence to support this claim has been inconclusive and controversial.
The authors of the new study note that "one of the most heated scientific disputes of all time" erupted over a disagreement on the extent to which birth order influences development. Psychoanalysis leaders Sigmund Freud (a firstborn child) and Alfred Adler (a middle child) clashed over the latter's claim that first- and lastborn children are burdened by neuroses as a result of their struggle for superiority, while middle children are more healthy, easygoing and rebellious.
The schism between the psychoanalysts was so deep that Adler was forced to resign his presidency of the Vienna Psychoanalytic Society, which met Wednesdays in Freud's apartment.
In modern times, scientists tend to favor evolutionary theory, which partly agrees with Adler's hypothesis, suggesting that siblings fulfill different niches in an effort to compete for parental attention. In this theory, the firstborn tends to be more responsible and concerned with parent-pleasing, whereas laterborn children are more rebellious and sociable.
Some modern studies have also found that firstborns show higher levels of intelligence, with researchers proposing this is because the amount of intellectual stimulation made available to children within the family is diluted with each successive child.
Largest ever study to look at influence of birth order on IQ and personality
The new study, conducted by researchers from the University of Illinois at Urbana-Champaign, featured a much larger representative sample size than any previous studies to look at this issue - 377,000 American high school students participated.
The team found that firstborn children have a higher IQ of just one point on average, compared with their younger siblings. However, the study authors say that while this difference is statistically significant, it is "meaningless."
The study also reports that some "infinitesimally small" personality differences correlate with birth order. By a correlation of 0.02, firstborns were found to have less anxiety and were more extroverted, agreeable and conscientious than laterborns.
University of Illinois psychology professor Brent Roberts explains what this 0.02 correlation means:
"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. You are not going to be able to see it with the naked eye. You're not going to be able to sit two people down next to each other and see the differences between them. It's not noticeable by anybody."
"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," concludes co-author Prof. Rodica Damian.