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

Key Points:

• A large preprint study analyzing over 10 million individuals found that firstborns are more likely to be diagnosed with neurodevelopmental conditions like autism and ADHD, allergies, childhood psychoses, and acne, while second-borns have higher rates of substance use disorders, shingles, and gastrointestinal issues.
• The study suggests that smaller age gaps between siblings (less than four years) are associated with lower rates of allergies and asthma, likely due to increased exposure to germs that strengthen the immune system, supporting the hygiene hypothesis.
• Researchers controlled for socioeconomic and genetic factors by comparing siblings within and across families, but limitations include reliance on insurance claims data, which may reflect parental diagnosis-seeking behavior rather than actual disease prevalence.
• Experts note that the study's exclusion of uninsured and Medicaid families may bias results toward wealthier populations, and some associations, such as increased substance use disorders in second children, may be influenced by environmental exposure rather than inherent birth order traits.
• Although the individual variations in health risks by birth order are small, their cumulative effects at the population level could be significant, emphasizing the nuanced role birth order plays in health outcomes.