Nestadtetal (2010)- found that 68% of MZ twins shared OCD compared with 31% of DZ twins.
Family studies- found that higher percentage of first degree relatives have this disorder, 10% compared to a prevalence rate of 2%.
OCD cannot be entirely genetic
Cromeretal (2007)- found that over half of OCD patients in their sample had experienced a traumatic event in their past. OCD was more severe if they had more than one trauma.