The relative contributions of genetic and other factors to the development of gender identity are still being debated. We studied twins, over the age of 10 who were concordant or discordant for gender identity status in order to provide clarification of this issue. An extensive literature search yielded 24 studies of monozygotic (MZ) twin pairs (14 male, 10 female) who were discordant or concordant for transsexuality. In addition, Internet requests for participants and clinical contacts of the author located 55 new pairs of twins: 18 monozygotic male pairs, 16 monozygotic female pairs, 8 dizygotic (DZ) male pairs and 13 dizygotic female pairs. The dizygotic twins included 2 male/female pairs.
From the literature, 8 of the 14 (57%) monozygotic male pairs were found to be concordant for transsexual gender identity and 4 of the 10 (40 %) female pairs were concordant. From our survey, 5 male monozygotic pairs out of 18 (28%) were identified as concordant for gender identity. Three of 16 sets of female MZ twins (19 %) were identified as concordant. Among 8 male and 13 female dizygotic twin sets, none were found to be concordant for transsexuality.
Combining data from our independent findings with those from past publications, 13 of 32 male monozygotic twin pairs (38%) were found to be concordant for transsexual identity and 7 of 26 (27%) female MZ twins were found concordant. In comparison, concordance among either male or female DZ twins was only found in one of 27 cases (04%).
These findings support the suggestion that there exists a significant genetic contribution to the development of gender identity disorder.