Birth order linked to 150 health conditions in study of 10 million American siblings

Whether someone is the older or younger sibling may be linked to certain health outcomes later in life, according to a massive new study.

Analyzing medical data from 10 million U.S. siblings over two decades, researchers from the University of Chicago and Columbia University found links suggesting that birth order may be connected to specific long-term health risks.

Researchers examined 569 disease categories across families and among siblings, identifying 150 conditions that were significantly associated with birth order.

Regarding the higher rate of substance use disorders among second-born children, Siegel suggested emotional drivers, saying, “it may be the same idea: ‘I want attention. I’m looking for attention. I wanna do something to get a rise out of you.'”

While the research highlights patterns across a large population, the study comes with limitations.

The findings demonstrate correlations rather than direct causation, meaning the trends don’t necessarily stem from physiological changes, early microbial exposures or social dynamics.

Additionally, the data relies on commercial health insurance claims, which can reflect differences in diagnosis, healthcare-seeking behavior and parental attention rather than true biological functions alone.

Factors such as the age of parents between births and varying age gaps between siblings also complicate these associations. Ultimately, researchers emphasized that birth order serves as a modest population-level risk marker rather than a definitive health predictor.

The research was published in the journal Nature Health.