Since the early twentieth century, clinicians have suspected that heredity plays an important role in OCD susceptibility. Consistent with this, increased OCD prevalence was identified among first-degree relatives of case probands (11.7%), compared to relatives of controls (2.7%; the Hopkins OCD family study) 4. In the same study, the prevalence of OCD in siblings of early onset probands was 17.9% (λsib =7.8) and thus within the range found in other psychiatric disorders for which a heritable component was reported, such as bipolar disorder and panic disorder. Family studies of child and adolescent OCD probands report prevalence rates of 7% to 15% in first-degree relatives, and 20–30% of these probands had one or more first-degree relative with OCD 5. These findings are consistent with previous reports of an increased familial loading in probands with early age at onset. In addition, a recent population-based study suggested that (while there is no significant effect of a shared environment on OCD risk) the contribution of genetic factors to OCD risk is as high as 50% 6.
