Researchers from several institutions in Melbourne, Australia and the University of California, Los Angeles collaborated in this ground-breaking study of 112 male-to-female transsexuals to look for a biological basis for how gender identity develops.

The researchers chose to measure variation in the androgen receptor gene, which is involved in the functioning of the sex hormone, testosterone. DNA samples from the transsexuals in the study group were compared with samples from 250 heterosexual men. Researchers discovered that the transsexuals were more likely than non-transsexuals to have a longer form of the androgen receptor gene.

"We think these genetic differences might reduce testosterone action and under-masculinise the brain during foetal development," Prince Henry's Institute researcher Lauren Hare said.

Lead researcher, Associate Professor Vincent Harley, head of molecular genetics at Prince Henry's Institute, commented; "There is a social stigma that transsexualism is simply a lifestyle choice. However, our findings support a biological basis of how gender identity develops."

Dr. Harley said researchers were recruiting transsexual people for another study and hoped to double the sample size and examine other genes, stating it was important to replicate the findings in other populations.

Researcher Trudy Kennedy, director of the Monash Gender Dysphoria Clinic, said the study supported other evidence that genetics and brain gender were important in transsexuality.

"This is something that people are born with and it's certainly not a lifestyle choice as some have suggested", she said.

The study research was jointly funded by the National Health and Medical Research Council and the US National Institutes of Health.

Source: The Age, Melbourne, Australia.