The twin and family studies summarized in this review demonstrate that at least some forms of OCD have a genetic basis. However, given that none of the linkage studies and essentially all of the candidate genes studies provide only suggestive evidence for risk genes of moderate-to-large effect, whole-genome association studies of OCD are warranted as the next step in our understanding of the genetic basis of the disorder. Wholegenome association studies are preferred over more traditional linkage studies or candidate gene studies because they provide more power to identify risk genes of relatively small effect. The primary difference between genome-wide linkage studies and genome-wide association studies (GWASs) is that with linkage the investigator is looking for cotransmission of a specific DNA marker within a family, while in a genome -wide association study the investigator is looking for a population association between a DNA marker and disease. Linkage studies are better suited to identifying genes that have large effects, and GWASs are better when attempting to identify genes that have relatively small effects on the phenotype. These GWASs should examine both
