Birth order linked to 150 health conditions in study of 10 million American siblings

Researchers analyzing data from 10 million American siblings identified associations between birth order and 150 health conditions. First-born children showed higher rates of autism, attention-deficit/hyperactivity disorder, and food allergies, while second-born siblings had higher rates of substance use disorders and migraines. The study did not establish whether birth order directly caused these health differences or whether other factors contributed to the observed patterns.

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