Autistic traits in unaffected first-degree relatives of individuals with schizophrenia and bipolar disorder: implications for shared familial neurodevelopmental vulnerability.

Background: Autistic traits have increasingly been linked to schizophrenia (SCH) and bipolar disorder (BD) within a shared neurodevelopmental framework. However, whether these traits are similarly expressed in unaffected first-degree relatives of individuals with SCH and BD remains poorly understood. Methods: This cross-sectional study included 135 participants: first-degree relatives of individuals with schizophrenia (SCH-FDR, n=45), first-degree relatives of individuals with bipolar disorder (BD-FDR, n=45), and healthy controls (HCs, n=45). Autistic traits were assessed using the Autism Spectrum Quotient (AQ). Participants also underwent clinical psychiatric evaluation, including structured diagnostic assessment, to exclude psychiatric disorders and autism spectrum disorder. Results: Significant group differences were observed for AQ total score (p=0.002), Communication (p=0.004), and Imagination (p<0.001) subscales. Communication scores were significantly higher in SCH-FDR than in HCs (p=0.003). Imagination scores were higher in both BD-FDR (p=0.016) and SCH-FDR (p<0.001) groups compared with HCs. Total AQ scores were also higher in both relative groups than in HCs (BD-FDR: p=0.015; SCH-FDR: p=0.003). The proportion of participants with AQ scores ≥26 was higher in SCH-FDR (17.8%) and BD-FDR (13.3%) than in HCs (2.2%; p=0.045). Conclusions: Autistic traits were more pronounced in SCH-FDR and BD-FDR than in healthy controls. Differences were most evident in total AQ scores and the Imagination subscale across both relative groups, whereas communication difficulties were specifically elevated in schizophrenia relatives. These findings are consistent with the concept of shared familial neurodevelopmental vulnerability across schizophrenia and bipolar disorder.