Twin studies provide strong evidence for the genetic nature of GTS. The largest study included 30 monozygotic and 13 dizygotic pairs of twins [53]. These investigators utilized phone-based assessment and found that 77% of monozygotic twins were concordant for tic disorders (CT or GTS), but only 23% of dizygotic twins were concordant for these disorders [53]. Furthermore, the concordance rate of monozygotic twins reached 100% for GTS or CT when direct observational interviews were conducted [47]. In a second smaller study of 16 pairs of monozygotic twins, 56% of MZ twins were concordant for GTS and 94% were concordant for tic disorders [54]. High concordance rates for tic disorders in monozygotic twins suggest that GTS is a genetic disorder and that CMT and GTS are genetically related.
