The relative importance of genetic and environmental factors in the etiology of obsessive-compulsive (OC) symptoms is unclear. Cognitive-behavioral models propose that shared environment (e.g., parenting style) is important. Family segregation studies suggest that nonadditive genetic factors may be involved. To investigate the etiology of OC symptoms, a meta-analysis was conducted of 37 twin samples from 14 studies, supplemented by a narrative review. Results indicated that in terms of mean effect sizes, (a) additive genetic effects and nonshared environment accounted for most of the variance in OC symptoms, (b) shared environment and nonadditive genetic effects made little or no contribution; (c) these findings did not vary with sex or symptom severity; (d) variance due to nonshared environment increased with age; (e) gene-environment interactions play an etiologic role; (f) OC symptoms are shaped by etiologic factors common to all types of OC symptoms but also have symptom-specific etiologies; and (g) OC symptoms are also shaped by very general etiologic factors (e.g., those influencing negative emotionality). Overall, the findings indicate that OC symptoms have a complex etiologic architecture that is not adequately explained by contemporary etiological models.
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