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US study on birth order and disease risks

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2026-08-11 02:45 UTC → 2026-08-11 12:31 UTC · added ~~removed~~

Researchers in the United States published a large-scale study in Nature Health analyzing medical records from over 10 million individuals across approximately 5 million two-child families. Using insurance claims data spanning roughly 20 years, the study mapped links between birth order and various health outcomes. Findings indicate that first-born children face higher odds of neurodevelopmental conditions, such as autism, ADHD, and Tourette syndrome, as well as immune-allergic diseases like asthma and food allergies. Conversely, second-born children showed higher risks for substance abuse, gastrointestinal disorders, migraines, and certain infections like shingles. Detailed analysis identified statistically significant differences in 150 out of 418 analyzed diseases. Researchers suggested several contributing factors, including the potential for microbial exposure from older siblings to protect younger children from allergies, and the influence of parental attention or age gaps between siblings on neurodevelopmental diagnoses. The study notes that while these associations exist, they “do not prove causality,” and results may be influenced by socioeconomic factors. Recent reports have highlighted how these patterns may be linked to early microbial exposure. Specifically, research suggests that the presence of diverse microbes brought home by older siblings may influence health outcomes, reinforcing the theory that early-life environmental factors play a role in immune system development.

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