We study the effect of birth order on personality using Swedish population data. Earlier-born men are more emotionally stable, persistent, socially outgoing, willing to assume responsibility, and able to take initiative than later borns. Firstborn children aremore likely to bemanagers and to be in occupations requiring leadership ability, social ability, and Big Five personality traits.We find a significant role for the sex composition within the family. When investigating possible mechanisms, we find that negative effects of birth order are driven by postnatal environmental factors. We also find evidence of lower parental human capital investments in later-born children.