Older vs younger siblings: a massive study links birth order to 150 health patterns
A large study of 10 million American siblings across 569 disease categories finds birth order is associated with about 150 health conditions. First-borns show higher rates of neurodevelopmental and psychiatric disorders (autism, ADHD, Tourette’s, OCD) and several allergies, while second-borns have higher rates of migraines, shingles, substance-use disorders, and gastritis. The authors stress these are population-level correlations, not proof of causation, and note that insurance-claims data (not biology alone) and factors like parental attention, early microbial exposure, and maternal changes between pregnancies may influence the patterns. The work, by researchers at the University of Chicago and Columbia University, appeared in Nature Health and should be interpreted as a modest risk marker rather than a definitive predictor.
