Sibling influence on childhood neurodevelopmental disorders: A cohort study in Taiwan.

OBJECTIVE: Autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), cerebral palsy (CP), and developmental delay are major childhood neurodevelopmental disorders. Sibling status and birth order may influence neurodevelopmental outcomes, yet population-based evidence remains limited. This study examined their associations with childhood neurodevelopmental disorder risks in a nationwide birth cohort. METHODS: We analyzed 2,045,178 live births from Taiwan's Birth Registration Database (2004-2014), linked to National Health Insurance and Death Registry records through 2020. CP, developmental delay, ASD, and ADHD were identified using ICD codes. Children were classified by sibling status and birth order using maternal identifiers. Kaplan-Meier and Cox models estimated adjusted hazard ratios (aHRs), controlling for maternal age, birth period, perinatal factors, socioeconomic status, and urbanization. RESULTS: Compared with single children, those with siblings generally showed lower risks of CP and ASD, whereas associations with ADHD and developmental delay varied according to birth order. Later birth order was associated with lower recorded risks of ASD and ADHD. Similar patterns were observed after excluding multiple births and in analyses restricted to diagnoses before 6 years of age. CONCLUSIONS: Family structure, including single-child status and birth order, was associated with childhood neurodevelopmental disorder risks. Compared with children who had siblings, single children generally had higher risks across multiple neurodevelopmental outcomes. These findings suggest that family structure may represent an important early-life context for neurodevelopment and may help inform developmental surveillance in societies experiencing declining fertility rates and delayed childbearing.