The relative contributions of genetic and environmental factors to the development of gender identity are debated. We studied twins that are concordant and discordant for gender identity status in order to provide clarification of this issue. An extensive literature search yielded 26 cases of monozygotic twin pairs. These reports listed 16 male and 10 female sets discordant or concordant for transsexuality. In addition our Internet bulletin board and clinical contact requests for participants in a survey of twins in which one or both transitioned located 21 new pairs of same sex twins: 11 monozygotic male pairs, 3 monozygotic female pairs, 4 dizygotic male pairs, and 3 dizygotic female pairs.
From the literature, 8 of 16 (50%) monozygotic male pairs were found to be concordant for transsexual gender identity and 2 of 7 (29%) MZ female pairs were found concordant. From our survey, 5 of 11 male monozygotic pairs (45%) were concordant and among 3 female MZ pairs (0%) were identified as concordant for gender identity. No male or female dizygotic twin pairs were found to be concordant among our respondents.
Combining data from the present survey with that of past research, 13 of 27 monozygotic male twin pairs (48%) and 2 of 14 DZ male twins (14%) were found to be concordant for trans identity and 2 of 10 female MZ twins were found concordant (20%) while none of 3 DZ female twins (0%) were concordant. These findings support the thesis that there is a significant genetic contribution to the development of gender identity disorder with more influence in males than females.