There is strong evidence that sexual orientation is largely tied to biology and that initial gender assignment is the strongest predictor of gender identity in the case of intersex children. Researchers have yet to precisely pinpoint the etiology of transsexualism, however. Various studies suggest that both biological and environmental variables may play a role in transgender development, says Eric Vilain, MD, PhD, chief of the division of medical genetics and professor of human genetics, pediatrics and urology at the University of California, Los Angeles.

In 1999, scientists identified anatomic brain differences between transsexuals and nontranssexuals (Journal of Psychosomatic Research). More recently, Vilain and his colleagues determined that genetics may have a mild to moderate effect on transgender development (Biological Psychiatry, 2009).

The biological evidence to date is not that strong, though, says Vilain. He points to another study in the April 2010 issue of the International Journal of Andrology showing that fetal exposure to a particular chemical appeared to have an effect on brain development that is linked to gender role behavior. It's quite possible that being transgender stems from a combination of genetic and environmental factors, Vilain concludes.

—Eve Glicksman