The objective was to determine if the high frequency of behavioral problems in Tourette syndrome (TS) probands is the result of the pleiotrophic expression of the Gts genes or due to ascertainment bias. It was possible to distinguish between these two hypotheses by comparing the frequency of these behaviors in nonproband relatives with TS to relatives without TS. Twenty behavioral problems were prospectively assessed in a consecutive series of 361 TS probands, 113 nonproband TS relatives, 380 relatives without TS, and 68 controls, by the administration of a questionnaire based on the Diagnostic Interview Schedule and the DSM-III-R. Significance was set at a very conservative level of p < or = .001. Except for problems with smoking, reading and compulsive eating, all other behaviors were significantly more common in nonproband TS relatives than in relatives without TS. The relatives were also dichotomized on the basis of each of the behavioral problems. Regardless of the behavioral trait used, there was a higher frequency of all other behaviors in those who were positive versus those who were negative for that behavior. Ranking according to the number of behaviors that were significant indicated that mania, obsessive-compulsive behaviors and schizoid behaviors represented higher degrees of expression of the Gts genes than chronic tics. These results indicate that the Gts genes cause a spectrum of behavioral disorders in addition to chronic tics, indicate that TS is a behavioral spectrum disorder, and emphasize the role of genetic factors in a wide range of human behaviors.