A Twin Study of Heritable and Shared Environmental Contributions to Autism

The present study examined genetic and shared environment contributions to quantitatively-measured autism symptoms and categorically-defined ASD. Participants included 568 twins from the Interactive Autism Network. Autism symptoms were obtained using the Social Communication Questionnaire and Social Responsiveness Scale. Categorically-defined ASD was based on clinical diagnoses. DeFries-Fulker and liability threshold models examined etiologic influences. Very high heritability was observed for extreme autism symptom levels (h(2)(g)=.92-1.20). Extreme levels of social and repetitive behavior symptoms were strongly influenced by common genetic factors. Heritability of categorically-defined ASD diagnosis was comparatively low (.21, 95% CI=0.15-0.28). High heritability of extreme autism symptom levels confirms previous observations of strong genetic influences on autism. Future studies will require large, carefully ascertained family pedigrees and quantitative symptom measurements