Familial Co‐Aggregation of Neurodevelopmental Conditions and Childhood‐Onset Type 1 Diabetes

Aim Childhood‐onset Type 1 diabetes (T1D) is associated with neurodevelopmental conditions (NDCs). We examined whether this association extends to relatives and assessed phenotypic, genetic, and environmental correlations between T1D and NDCs. Methods Using Swedish registers, we identified 4 066 634 individuals born 1973–2015 and linked them to siblings and cousins; 23 212 (0.57%) had T1D before 18 years. Logistic regression estimated odds of receiving a diagnosis of any and each of the examined NDCs among individuals with T1D and their relatives. Bivariate quantitative genetic models estimated phenotypic, additive genetic, shared environmental, and non‐shared environmental correlations. Analyses were conducted in April 2025. Results NDCs were more prevalent among individuals with T1D than in those without (10.3% vs. 6.4%). T1D was associated with higher odds of receiving a diagnosis of any NDCs (OR 1.43; 95% CI 1.37–1.49). Full siblings also had higher odds (OR 1.12; 95% CI 1.06–1.18), whereas no significant associations were observed among other relatives. Cross‐trait phenotypic correlations were small (0.06–0.08). Conclusion Individuals with childhood‐onset T1D and their full siblings had higher odds of recorded NDC diagnoses. Shared familial factors appeared to contribute only modestly, while differential ascertainment and family‐level consequences of T1D may also play a role. These findings support access to neurodevelopmental expertise in paediatric diabetes services.