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To examine the role of genetic factors in oppositional defiant disorder (ODD) and conduct disorder (CD), 38 variables realating to the relevant DSM-III-R criteria, as well as other angry and aggressive behaviors, were examined in 1177 Tourette syndrome (TS) and attention-deficit hyperactivity disorder (ADHD) probands, their first-degree relatives, and controls. Two techniques were used: (1) a genetic loading technique comparing the frequency of symptoms in groups with progressively less genetic loading for Gts and ADHD genes, and (2) comparison of the frequency of symptoms in relatives with, versus relatives without, TS or ADHD. When significant, the latter rules out ascertainment bias and inappropriate controls. For TS, the results were significant with most p values less than 10-8. The same trends were seen in the smaller number of ADHD families. A polygenic model is proposed inwhich TS and ADHD alone represent lesser degree of genetic loading and expression and TS + CD ADHD represents a higher degree of genetic loading and expression of genes common to all three disorders. These studies emphasize the important role of genetic factors in ODD and CD. The therapeutic implications are discussed. J Dev Behav Pediatr 16:142–157, 1995. Index terms: Tourette syndrome, attention-deficit hyperactivity disorder, conduct disorder, oppositional defiant disorder, aggression, genetic.