ABSTRACT

The relative contributions of genetic and environmental factors to the development of gender identity have been debated. Twins were studied that are concordant or discordant for gender identity status in order to provide clarification of this issue. An extensive library search yielded reports of 27 male and 16 female sets concordant or discordant for transsexuality. An Internet bulletin board search and clinical contact requests for participants in a survey of twins in which one or both transitioned located 69 new twin pairs. In addition to asking about matters associated with gender, these new twins were asked about their transition, rearing, and sexual practices. Combining data from the present survey with those from past-published reports, 20% of all male and female monozygotic twin pairs were found concordant for transsexual identity. This was more frequently the case for males (33%) than for females (23%). The responses of our twins relative to their rearing, along with our findings regarding some of their experiences during childhood and adolescence show their identity was much more influenced by their genetics than their rearing.