The second study, published in 2009 ,30 obtained data from 2801 young-adult Norwegian twins by means of the Composite International Diagnostic Interview (CIDI). This study examined the heritability of five anxiety disorders (Generalized Anxiety Disorder, Panic Disorder, Phobias, Obsessive-Compulsive Disorder, and PostTraumatic Stress Disorder.) Valid anxiety data were available for 1385 twin pairs; however, there were only 57 pairs where one twin had a diagnosis of OCD. Because the prevalence of OCD was so low in this sample, the investigators included individuals who met criteria or subthreshold OCD (the number of pairs where at least one had a diagnosis of OCD or subthreshold OCD was 165). The estimate of heritability was 29%. However, these investigators reported that 55% of this heritability was due to a common factor shared by all five anxiety disorders. On the other hand, 45% appear to be due to factors that were specific to OCD.
