Internalizing difficulties are prevalent in children with autism spectrum disorders (ASD), yet little is known about the underlying cause of this comorbidity. It is also unclear which types of autistic-like and internalizing difficulties are most strongly associated. The current study investigated the phenotypic and etiological associations between specific autistic-like traits and internalizing traits within a population-based sample. Parent-reported data were analyzed from 7,311 twin pairs at age 7 to 8 years. Structural equation modeling revealed distinguishable patterns of overlap between the three autistic-like traits (social difficulties, communication problems and repetitive/restricted behaviors) and four subtypes of internalizing traits (social anxiety, fears, generalized anxiety, negative affect). Although all phenotypic associations were modest (rph = 0.00–0.36), autistic-like communication impairments and repetitive/restricted behaviors correlated most strongly with generalized anxiety and negative affect both phenotypically and genetically. Conversely, autistic-like social difficulties showed little overlap with internalizing behaviors. Disentangling these associations and their etiological underpinnings may help contribute to the conceptualization and diagnosis of ‘comorbidity’ within ASD and internalizing disorders.
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