Background. Previous reports of genetic influences on obsessive–compulsive disorder (OCD) symptoms have suggested moderate heritability. Family history studies of co-morbidity have found familial aggregation with tics, especially for early-onset OCD, and familial aggregation with anxiety disorders.

Method. Heritability of OCD and familial aggregation of OCD, tics and anxiety disorders were investigated in a community sample of 6-year-old twins using a two-phase design in which 4662 twin pairs were sampled and 854 pairs were assessed in the second phase by maternal-informant diagnostic interview using DSM-IV criteria.

Results. In the multivariate model combined additive genetic and common environmental effects were estimated as 47% for sub-threshold OCD, and the model was unable to distinguish these sources of familial aggregation. There were strong familial aggregations between sub-threshold OCD and tics and between sub-threshold OCD and other anxiety disorders (80% and 97% respectively), although again specific sources could not be distinguished.

Conclusions. The findings are consistent with the hypothesis of a tic-related early-onset OCD phenotype, but also with the hypothesis of an anxiety-related early-onset OCD phenotype.