The vast majority of clinically referred GTS individuals have comorbid psychiatric conditions; in fact only about 10–15% of individuals with GTS have no comorbidities [14, 15]. Obsessive Compulsive Disorder (OCD) and Attention Deficit Hyperactivity Disorder (ADHD) are the most common GTS-related comorbidities, each affecting over 50% of GTS individuals [11, 14, 16, 17]. These rates are significantly greater than those in the general population (e.g., 1.1 to 3.3% for OCD [18, 19], and 5% for ADHD in school age children [20]), suggesting that these disorders may also have some shared underlying etiological mechanisms with GTS. Understanding the etiology of comorbid GTS conditions is especially critical, as children with comorbid diagnoses exhibit greater psychopathological burden [21], have more severe tic symptoms [15], and are at higher risk for aggressive behaviors and frequent anger outbursts [14, 22, 23].
