Genetic studies have demonstrated that both biological and environmental factors are important in the etiology of OCD. A multitude of OCD family studies published since the 1930's provide strong evidence for an approximate four to ten-fold OCD risk increase among first-degree relatives of OCD-affected children and adults, respectively, as compared to relatives of controls.6-14 A review of twin studies concluded that obsessive-compulsive (OC) symptoms are heritable, with greater genetic influences in child-onset (45-65%), than in adult-onset OCD cases (27- 47%).15 This finding has been supported by subsequent twin studies16-18. Linkage study results have been somewhat encouraging,19 identifying peaks on chromosomes 3q,20 9p,21 10p,22, 23 15q20, 24 and 19q19 for OCD and on chromosome 14 for compulsive hoarding.25 Unfortunately, none of these peaks exceeded the threshold for genome-wide significance, and only the 9p peak has reached suggestive significance in more than one sample.19-21
