We also found that ADHD was highly heritable in our TS families, although, in contrast to OCD, there was no significant genetic correlation between TS and ADHD. In our parent-offspring analyses, the primary predictor of ADHD among TS-affected offspring was a parental diagnosis of ADHD rather than parental diagnoses of TS or OCD. Together with the heritability analyses, these results suggest that TS and ADHD are most likely genetically separate disorders, with the high rates of ADHD among TS-affected individuals resulting partly from increased but separate parental transmission of TS and ADHD susceptibility. These findings also have implications for genetic studies, suggesting that the presence or absence of ADHD is not of direct relevance for genetic studies of TS and should not be considered either as an inclusion or exclusion factor for such studies. Note that this does not necessarily extend to the more complex phenotype of TS+OCD+ADHD, which is highly heritable, and may in fact be an appropriate alternate phenotype for genetic studies. On the clinical side, the association between maternal ADHD and offspring ADHD and/or OCD is of
