Recently, considerable attention has been given to the relationship between TS and OCD. Approximately 55-75% of TS individuals are reported to experience some degree of obsessive-compulsive behavior. This is in contrast to a risk of between 0.4% and 4.0% for OCD in the general population. Although an increased occurrence of OCD in TS individuals is consistent with a common underlying genetic basis for the two disorders, it is not proof of this hypothesis. In order to provide a more rigorous test of this theory, data from family studies of individuals with TS and/or OCD have been examined. Researchers have reported an increased risk for OCD in first-degree relatives of individuals with TS and, similarly, an increased risk for tic behavior in relatives of individuals with OCD. A second source of evidence consistent with a genetic relationship between TS and OCD comes from the observation of comparable rates of OCD among relatives of individuals with both TS and OCD and relatives of individuals with TS but not OCD. If the two disorders were distinct genetic disorders, the expectation would be that significantly greater numbers of OCD individuals would be observed in relatives of individuals with both TS and OCD than in the relatives of individuals with TS but not OCD. This research project involves parallel twin studies of TS and OCD in order to conduct an even more powerful test of the hypothesis of a common genetic basis for the two disorders. If TS and OCD are alternative expressions of the same genetic disorder, the risk for OCD or TS to a co-twin of a twin affected with either disorder should be greater than that observed in the general population. Furthermore, specific predictions can be made with regard to the degree of differential risk in monozygotic (MZ) (identical twins) as opposed to dizygotic (DZ) (fraternal co-twins). Research plans also include the study of data from opposite-sexed twin pairs in order to examine gender differences in risk and descriptive data from all twin pairs. This information will be used to determine what characteristics, if any, may be used to determine which cases of TS and OCD are expressions of the same underlying genetic vulnerability. In sum, previously published research concerning the relationship between TS and OCD suggests that at least some forms of OCD may be etiologically related to TS. Although it does not seem that all OCD shares a common etiology with TS, at the present time little information is available to distinguish between subgroups of each of these disorders. The present research project will address these questions utilizing both same-sexed and opposite-sexed twin pairs in which at least one member is affected with TS and/or OCD. Potential benefits of such research include improvement of classification and, consequently, treatment of affected individuals. Michele C. LaBuda, Ph.D. Yale Child Study Center, New Haven, CT Award $20,000 Tourette Association of America Inc. – Research Grant Award 1990