Several observations support this hypothesis. First, it has been reported that comorbid conditions are more frequent in GTS than in other less severe tic disorders. In a recent Swedish study of school-aged children, 66% of the GTS cases had comorbid ADHD compared to 33% of children with chronic vocal tics, 12% of children with chronic motor tics, and just 4% of children with transient motor tics [17]. A similar gradient was observed in the rates of OCD, as well as in the total number of comorbid disorders [17]. Another study found 44% of children with GTS had comorbid ADHD, compared with 23% of children with chronic tics, and 54% of children with GTS had comorbid OCD, compared with 8% of children with chronic tics [11]. Second, an increased genetic burden appears to influence the risk of GTS and comorbid disorders. In a prospective study of children at risk for GTS, children of two GTS-affected parents had three times greater risk of developing ADHD compared to the children of one affected parent, and two times greater risk of developing either tics,
