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.
© 2011 International Society for Sexual Medicine.