Birth order tied to 150 health risks in a massive sibling study

A large study analyzing medical data from about 10 million U.S. siblings found birth order is associated with about 150 health conditions. First-borns showed higher rates of neurodevelopmental and psychiatric diagnoses (autism, ADHD, Tourette’s, OCD) and immune/allergic conditions like food allergies, while second-borns had higher rates of migraines, shingles, substance-use disorders, and gastritis. The study shows correlations at the population level, not causation, and has limitations including reliance on insurance claims and possible differences in diagnosis and healthcare-seeking behavior. Researchers emphasize birth order as a modest risk marker, with mechanisms like parental attention, early microbial exposure, and maternal changes across pregnancies remaining to be proven.