In the most comprehensive review to date, van Grootheest et al6 summarized all published twin studies from 1929 through 2005 (Table I). Of note is that five of the six twin studies with adequate sample sizes32-36 (~100 twin pairs or more) attempted to estimate the heritability of obsessive-compulsive (OC) symptoms, not OCD. Only two studies29-30 were able to estimate the heritability of OCD as determined by DSM diagnostic criteria.
