title: Gender identity disorder in twins: a review of the case report literature.
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  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.
authors.affiliation: Department of Sexology and Gender Problems, Ghent University Hospital, Ghent, Belgium. gunter.heylens@uzgent.be
authors.forename: Gunter
authors.initials: G
authors.lastname: Heylens
authors.forename: Griet
authors.initials: G
authors.lastname: De Cuypere
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Zucker
authors.forename: Cleo
authors.initials: C
authors.lastname: Schelfaut
authors.forename: Els
authors.initials: E
authors.lastname: Elaut
authors.forename: Heidi
authors.initials: H
authors.lastname: Vanden Bossche
authors.forename: Elfride
authors.initials: E
authors.lastname: De Baere
authors.forename: Guy
authors.initials: G
authors.lastname: T'Sjoen
mesh_terms: D004200:Diseases in Twins; D005260:Female; D005783:Gender Identity; D006801:Humans; D008297:Male; D012737:Sex Factors; D019968:Sexual and Gender Disorders / Q000209:etiology / Q000235:genetics*; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic