Numerous family studies on OCD and obsessional neurosis have been published since 1930 (Table II). Results from the majority of these studies demonstrate that at least some forms of OCD are familial, and the findings from twin studies summarized above provide evidence that this familiality is due in part to genetic factors. However, it is also evident that environmental/cultural factors influence OC behaviors and are also transmitted within families.29 These nongenetic factors unquestionably influence the manifestation of OC behaviors as evidenced from twin studies that consistently demonstrate that the concordance rate of MZ twins for OC behaviors and OCD is always less than 1.0. Understanding the impact of these environmental/cultural factors will be critical to the eventual elucidation of the risk factors important for the manifestation of complex disorders such as OCD. However, while it is clear that genes alone will not explain all of the observed inheritance of OCD, demonstrating familiality is an important step for the eventual determination of the importance of genetic risk factors.
