A massive study of health records from more than 10 million American siblings suggests that birth order may be associated with the risk of developing certain diseases and medical conditions later in life. Researchers from the University of Chicago and Columbia University analyzed two decades of U.S. health insurance data on more than 10.3 million children with one sibling.

Findings

The study examined 569 disease categories and found statistically significant differences between first-born and second-born children in 150 conditions. First-born children were more likely to have several neurodevelopmental and psychiatric conditions, including autism, attention-deficit/hyperactivity disorder, Tourette’s syndrome, and obsessive-compulsive disorder.

They also had higher rates of immune-allergic conditions. The study found that first-born children had a 20% higher risk of food allergies and a 9% higher risk of hay fever compared with their second-born siblings. Autism was associated with a 26% higher risk among first-born children, while ADHD was associated with a 7% higher risk.

Second-Born Children

Second-born children, meanwhile, showed higher rates of several other conditions. Researchers found a 19% higher risk of substance use disorders and a 14% higher risk of gastrointestinal conditions. Migraines and shingles were also among the conditions statistically more common among second-born children.

The researchers described the overall differences as widespread but modest. Despite the large number of records included in the analysis, the findings do not mean that a child’s position in the family determines his or her future health.

Original reporting: The Dallas Express — read the source article.