Birth order has no measurable effect on personality traits,suggests an analysis of data on 20,000 people in Germany, the U.S., and the U.K., published in PNAS (Proceedings of the National Academy of Sciences).
Over the years many people have claimed that birth order can have a lasting impact on personality—that first-borns tend to be more dominant or perfectionists, for instance. But the researchers found no link between birth order and traits, including extraversion, agreeableness, and emotional stability. They did confirm previous findings that, population-wide, older siblings have slightly higher IQs. Even so, they estimated that a younger sibling “will still be smarter than his or her older sibling in 4 of 10 cases.”