ADHD is one of the most common childhood-onset developmental disorders [20], characterized by hyperactivity, impulsivity, and inattentiveness. ADHD has a significant genetic component, with 70–80% of risk believed to be due to genetic causes [32]. The familial relationship between GTS and ADHD is not fully understood, but it is becoming clear that, unlike the relationship between GTS and OCD, the most common form of ADHD does not appear to have a shared underlying susceptibility with GTS. Comings and Comings [33] proposed that ADHD represented a variant expression of the same genetic factors responsible for GTS [33, 34]. However, several later studies did not find support for this hypothesis [2, 12, 35]. While an increased rate of ADHD has been observed among the first degree relatives of GTS probands, even when the probands themselves did not have ADHD, that increase appears to be due to an increased rate of comorbid GTS+ADHD among those relatives who also had a diagnosis of GTS. In other words, rates of ADHD alone were not elevated in the relatives of GTS probands who did not themselves
