Birth order linked to 150 health conditions in major US study

A large-scale epidemiological study involving data from approximately 10 million siblings across more than 5 million US families has identified significant correlations between birth order and various long-term health patterns. Researchers from the University of Chicago and Columbia University analyzed 569 disease categories using US health insurance data spanning from 2003 to 2024.

The study, published in the journal Nature Health, found significant associations with 150 different conditions. First-born children showed a higher prevalence of neurodevelopmental and psychiatric diagnoses, including autism, ADHD, Tourette syndrome, obsessive-compulsive disorder, anxiety, and depression. They also exhibited an increased susceptibility to immune-mediated allergic reactions, such as food allergies and rhinitis.

In contrast, second-born children were statistically more likely to experience migraines, shingles, substance use disorders, and gastrointestinal issues. Researchers emphasized that these findings represent correlations rather than proven direct causation.