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Scientists have found “a significant genetic link’ between male-to-female transsexualism and a gene involved in testosterone action.

The implication of the research is that biological factors have a greater role than had previously been thought in formulating gender identity.

The study of 112 male-to-female transsexual volunteers – the biggest of its kind to date – found that they were more likely than non-transsexual males to have a long version of a gene that helps testosterone bind to cells, which could reduce its effects in the body.

While the genetic link was statistically significant, it was weak – 55% of the transsexuals volunteers had the long version of the gene, compared with 50% of non-transsexual men.

The biological bases for gender identity are largely unknown, “but we could speculate that [effective] testosterone signalling is required for male gender identity”, says Vincent Harley at Prince Henry’s Institute for Medical Research at Monash University in Melbourne, Australia, who led the work.

There has been debate over the causes of transexuality for decades. Early theories included psychosocial factors such as childhood trauma, but more recent studies have indicated that family history and genetic aspects are linked to the development of gender identity.

“There is a social stigma that transsexualism is simply a lifestyle choice,” says Harley. “However our findings support a biological basis of how gender identity develops.”

The work “supports the notion that transsexualism has a biological basis rather than being due to psychosocial factors in early childhood,” agrees Andrew Sinclair, director of early development and disease at the Murdoch Childrens Research Institute in Melbourne. “However the finding does not explain all male-to-female transsexuals, suggesting that multiple genetic factors are involved.”

Although this is the largest genetic study of transsexualism to date, the researchers now plan to see if the results can be replicated in a larger population.

[via New Scientist and Bird of Paradox]

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