Study of 10 Million Siblings Links Birth Order to 150 Health Conditions

A sweeping analysis of medical records spanning two decades has uncovered significant associations between birth order and long-term health risks across a population of 10 million American siblings. The collaborative investigation by researchers at the University of Chicago and Columbia University examined 569 disease categories within families, ultimately identifying 150 conditions with statistically meaningful links to whether individuals were born first, second, or later.

First-born children demonstrated elevated rates of neurodevelopmental and psychiatric conditions, with autism, ADHD, Tourette’s syndrome and obsessive-compulsive disorder appearing more frequently in this group. Additionally, immune-related and allergic disorders such as food allergies were disproportionately represented among older siblings, the data showed.

Younger siblings, particularly second-born children, exhibited higher prevalence rates for migraines, shingles, substance use disorders and various gastrointestinal conditions including gastritis. Dr. Marc Siegel, a senior medical analyst for Fox News, attributed some disparities to parental behavior, suggesting that intensive parental oversight of first-borns may contribute to heightened ADHD diagnoses.

Researchers emphasized that the study demonstrates correlation rather than causation, with findings potentially reflecting differences in healthcare access, diagnostic practices and parental awareness rather than purely biological mechanisms. The analysis also accounts for limitations inherent in insurance claim data and the variable spacing between sibling births.

The investigation concluded that birth order functions as a modest population-level risk indicator rather than a reliable individual health predictor. The research was published in Nature Health.