A large US study has found associations between birth order and the likelihood of more than 150 health conditions, including allergies, psychiatric disorders and substance use. The findings matter because they suggest family dynamics and early-life development may contribute to health patterns, though the reported links do not establish that birth order directly causes these outcomes.

Researchers at the University of Chicago and Columbia University analysed two decades of medical data covering 10 million siblings in the United States. Reported patterns included higher rates of ADHD and autism among first-born children, while second-born children showed an increased risk of migraines; researchers suggested parental attention and immune-system development could be relevant factors.