The order in which children are born may shape their risk of developing dozens of diseases, from autism and allergies to substance abuse.
Can the order in which kids are born determine a higher or lower risk of developing certain conditions?
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Birth order is associated with disease risk more broadly than previously recognised, a new study has found.
The researchers analysed data from over 10 million individuals in two-child families, and identified significant associations for 150 diseases, ranging from psychiatric disorders to digestive issues.
The findings, published in the journal Nature Health, reveal that first-born children are more likely to face neurodevelopmental challenges like autism and ADHD, as well as immune-related allergies.
They were also found to have higher odds of Tourette syndrome — a nervous system condition that begins in childhood — and obsessive-compulsive disorder (OCD), along with acne and asthma.
Younger siblings, on the other hand, showed a higher susceptibility to substance abuse, gastrointestinal problems, and certain infectious diseases like herpes zoster.
Why the differences?
The authors said that a mixture of physiological, immunological, and social mechanisms likely drives the associations between birth order and disease risk.
For immune-allergic and respiratory conditions such as hay fever, eczema, and asthma, the study noted that first-borns carry an excess risk because they lack the early microbial exposure that their younger siblings receive.
From an early age, younger siblings are exposed to a greater diversity of microbes brought home by their older siblings, and this early exposure is critical for developing immune regulation and tolerance.
The study also found that the protective effect for younger siblings is stronger when the age gap between siblings is smaller, reinforcing the idea of direct microbial sharing.
The increased risk for substance abuse among second-borns is often attributed to sociological factors, such as the modelling of an older sibling’s behavior or a natural tendency for later-born children to engage in risk-taking, the study noted.
