Why firstborns may be more likely than secondborns to be autistic or to have allergies

"In a wide-ranging preprint study that looked for 569 conditions across more than 10 million individuals and more than five million families, researchers found that firstborns were more likely to be diagnosed with neurodevelopmental conditions such as autism and attention deficit hyperactivity disorder (ADHD), as well as allergies."

"Those born second, on the other hand, were more likely to be diagnosed with substance use disorders, shingles and gastrointestinal disorders."

"The age gap between siblings also appears to matter. It seems that smaller age differences between siblings are protective against quite a few diseases, at least relative to larger age gaps."

"Sibling age gaps of less than four years were associated with a lower rate of allergies and asthma—likely because kids who are closer in age interact more often, sharing germs that improve their microbiome."

Research indicates that birth order significantly impacts health conditions. Firstborns are more susceptible to neurodevelopmental disorders like autism and ADHD, as well as allergies and psychoses. In contrast, secondborns show higher rates of substance use disorders and gastrointestinal issues. The age gap between siblings also plays a role, with smaller gaps linked to lower rates of allergies and asthma, likely due to increased interaction and shared exposure to germs that enhance the microbiome.

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