Birth order may influence autism and allergy risk, study finds
A large preprint study of over 10 million individuals across more than five million families found firstborns are more likely to be diagnosed with neurodevelopmental conditions such as autism and ADHD, as well as allergies, while second-borns show higher rates of substance-use disorders, shingles, and gastrointestinal issues. Smaller age gaps between siblings appear protective against allergies and asthma, possibly due to increased microbial exposure, per the hygiene hypothesis. Limitations include reliance on insurance claims data and lack of uninsured populations; results have not yet undergone peer review.