In sum, these studies of OCD probands and their relatives cumulatively provide strong evidence that some but possibly not all forms of OCD are familial. This was confirmed in a meta-analysis of five family studies of OCD probands published prior to 2001 involving 1209 firstdegree relatives68 in which a significantly increased risk of OCD among relatives of probands was observed (Mantel-Haenszel summary OR=4.0 (95% CI=2.2-7.1)). The unadjusted aggregate risk for relatives of OCD probands was 8.2%, compared with 2.0% for relatives of relatives. Although these family study findings are consistent with a genetic etiology of OCD, by themselves they only demonstrate that OCD is familial; not that genetic factors are necessary for the manifestation of the illness. However, taken together with the evidence from twin studies, there is compelling evidence that genetic factors play an important role in the manifestation of some forms of OCD.
