Twin studies of transsexuals
Dr NE Whitehead
In this collection (mostly from published literature), all possible sources are documented, but the collection is obviously very incomplete, although larger than anything else I have encountered. The point is that situations differ. If one member of an identical twin pair is transsexual, the other sometimes is, sometimes isn't. If genetics are not an important factor twins will usually differ. If genetics is overriding, they will almost all be the same.
The following collection shows the situation is intermediate - which suggests genes have some influence but it is not overwhelming. This is very typical of many, many other traits that have been studied. Only for very obviously physical traits such as eye colour are identical twins almost always the same.
In the following cases the identical twins (of either sex, but the majority are male) were found concordant (i.e. both were transsexual):
Anchersen, (1956), Mckee et al. (1976), Hyde and Kenna (1977), Buhrich et al. (1991), Zucker and Bradley (1995), Green (2000), Sadeghi and Fakhrai (2000) Total, 7 cases.
In the following cases there was discordance:
Green and Stoller (1971) (2 cases), McKee et al. (1976), (2 cases, but this is drawn from triplets, which are here classified as 2 cases of discordance and 1 of concordance), Buhrich et al. (1991) (3 cases), Garden and Rothery (1992), Zucker and Bradley (1995), Christopher (2001) Total, 10 cases.
Thus in 7/17 cases there was concordance. Since this is neither 0/17 or 17/17, (statistically unlikely to be either) the conclusion is that genetic effects are probably involved, but do not compel the trait. Change is therefore possible.
Anchersen, P, (1956) Problems of transvestism, Acta Psychiatrica Neurologica Scandinavica 106(suppl.): 249-256.
Buhrich,N; Bailey,JM; Martin,NG (1991): Sexual orientation, sexual identity, and sex-dimorphic behaviors in male twins. Behavior Genetics 21: 75-96.
Anon. (2001): Rev. Christopher in Washington. www.heroicstories.com/paulette2.html.
Garden,GMF; Rothery,DJ (1992): A female monozygotic twin pair discordant for transsexualism - some theoretical implications. British Journal of Psychiatry 161: 852-854.
Green,R (2000): Family cooccurrence of "Gender dysphoria": ten sibling or parent-child pairs. Archives of Sexual Behavior 29: 499-507.
Green,R; Stoller,RJ (1971): Two monozygotic (identical) twin pairs discordant for gender identity. Archives of Sexual Behavior. 1: 321-327.
Hyde, C, Kenna, J (1977): A male MZ twin pair concordant for transsexualism, discordant for schizophrenia. Acta Psychologica Scandinavica 56: 265-275.
McKee, EA, Roback, HB, Hollander, MH (1976): Transsexualism in two male triplets. American Journal of Psychiatry 133: 334-337.
Sadeghi,M; Fakhrai,A (2000): Transsexualism in female monozygotic twins: a case report. Australian and New Zealand Journal of Psychiatry. 34: 862-864.
Zucker,KJ; Bradley,SJ (1995): Gender Identity Disorder and Psychosexual Problems in Children and Adolescents. Guilford Press, New York. 440 pages.