University of Chicago-led study ties birth order to 150 diseases in 10.3m children
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A large international study published in Nature has found associations between birth order and the risk of 150 diseases. The analysis, drawn from medical records of 10.3 million children, shows firstborns have higher odds of autism and ADHD, while later-born children face slightly elevated risks for substance use disorders and migraines. The study authors caution that the differences are small and cannot forecast individual health outcomes.
The Study
Researchers from the University of Chicago and other centers analyzed medical records of 5.1 million families with more than 10.3 million children. They used two methods: comparing first and second children from similar families and comparing siblings within the same family. The study, published in Nature, covered 418 diseases and found statistically significant ties for 150.
Disease Associations
Firstborns had higher rates of autism spectrum disorders, ADHD, Tourette syndrome, food allergy, allergic rhinitis, acne, and certain anxiety disorders. Later-born children showed slightly elevated risks for substance use disorders, gastritis and duodenitis, biliary tract diseases, migraine, and herpes zoster. The differences were consistent across both analytical methods.
Potential Causes
The researchers suggest several possible explanations. Younger siblings may encounter more diverse bacteria and viruses earlier, which could train their immune systems. Parents might seek more medical attention for firstborns, leading to higher diagnosis rates. Additionally, later-born children often enter adult environments sooner through older siblings' contacts. However, none of these hypotheses have been confirmed, and the observed differences were small.
What's Next
Further research is needed to explore whether these patterns hold across different populations. It remains unclear whether the associations reflect underlying biological pathways or are artefacts of parental behavior and healthcare access.
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University of Chicago-led study ties birth order to 150 diseases in 10.3m children
