What the Research Shows

The largest study on birth order and personality, published in 2015, analyzed data from over 370,000 participants. The findings challenged decades of conventional wisdom.

Intelligence: There is a small but real effect. Firstborns score slightly higher on IQ tests, about one point on average.

Personality: The study found essentially no meaningful link between birth order and the Big Five traits. Firstborns were not more conscientious. Middle children were not more agreeable.

This doesn't mean birth order has zero influence. It means the influence is far smaller than family dynamics, genetics, peer relationships, and individual experiences.