Almost everyone knows their place in the family lineup, which is exactly why a new analysis of more than 10 million American siblings is likely to travel further than most epidemiology. Researchers at the University of Chicago and Columbia University screened 569 diseases in commercial insurance claims and found that birth order was significantly associated with 150 of the 418 conditions with sufficient cases to test.
The study, published in Nature Health, used Merative MarketScan claims data covering 2003 through 2024. The headline number is striking. The single largest effect was stranger still and caught the authors off guard.
