A large-scale study of over 10 million individuals reveals how sibling position may influence the risk of a wide range of diseases throughout life, highlighting biological and social factors affecting long-term health.
A large new analysis has added to evidence that birth order can shape health in ways that extend well beyond childhood, finding links between sibling position and a wide range of diseases. Researchers who examined data from more than 10 million people in two-child families reported associations between birth order and 150 conditions, ranging from neurodevelopmental disorders to digestive and immune-related illnesses. The findings, published in Nature Communications, suggest that being first-born or later-born may carry distinct health patterns across the life course.
According to the study, first-born children were more likely to be diagnosed with conditions such as autism, ADHD, Tourette syndrome, obsessive-compulsive disorder, acne and asthma. Later-born children, by contrast, were more likely to face substance misuse, gastrointestinal disease and certain infections, including shingles. The researchers said the pattern points to a mix of biological, immune and social influences rather than a single explanation.
One possible factor is early exposure to microbes. The authors suggested that younger siblings may benefit from microbial contact brought home by older brothers and sisters, which could help the immune system develop tolerance. They also found that this apparent protective effect was stronger when the age gap between siblings was smaller, lending further weight to the idea of direct transmission within the household.
Previous research has pointed in a similar direction. A Scientific Reports study of nearly 60,000 Finnish newborns found that later-born children had a higher risk of paediatric traumatic brain injury, while a meta-analysis in Nature Communications identified birth-order-related differences in neonatal DNA methylation, a chemical process that can influence how genes are switched on and off. Separate studies have also linked birth order to asthma, allergy and eczema, and to health differences that appear to change across the life course. Together, the findings suggest birth order may be one of several subtle influences on long-term health, alongside genetics, family environment and behaviour.
Disclaimer: This content is for informational purposes only and is not intended to be a substitute for professional medical judgment, advice, diagnosis, or treatment.
