Introduction. The etiology of gender identity disorder (GID) remains largely unknown. In recent literature, increased attention has been attributed to possible biological factors in addition to psychological variables.
Aim. To review the current literature on case studies of twins concordant or discordant for GID.
Methods. A systematic, comprehensive literature review.
Results. Of 23 monozygotic female and male twins, nine (39.1%) were concordant for GID; in contrast, none of the 21 same-sex dizygotic female and male twins were concordant for GID, a statistically significant difference (P = 0.005). Of the seven opposite-sex twins, all were discordant for GID.
Conclusions. These findings suggest a role for genetic factors in the development of GID. Heylens G, De Cuypere G, Zucker KJ, Schelfaut C, Elaut E, Vanden Bossche H, De Baere E, and T'Sjoen G. Gender identity disorder in twins: A review of the case report literature. J Sex Med 2012;9:751–757.