What causes people to be transgender in the first place? The prevailing theories used to be psychosocial: That early traumas like dysfunctional family dynamics or childhood sexual abuse were responsible. "That is absolutely not true at all," says Dr. Johanna Olson, medical director of the Transgender Clinic at Children's Hospital Los Angeles. "But I still get people in my clinic who are trying to unravel what the traumatic incident was, that caused their kid to be trans."
Rather, a growing body of research is pointing to biological origins. The 2008 discovery by Australian researchers of a genetic variation in transgender women—their receptor gene for the sex hormone testosterone was longer, making it less efficient at communicating signals—set off speculation that insufficient uptake of male hormones in utero contributed to a "more feminised brain." And the brains of trans people do look different. Recent Spanish imaging studies have shown that the white matter of untreated trans men look much like those of biological males, and that the patterns of trans women's white matter fell about halfway between those of biological male and female control groups. But it's premature to draw conclusions from those studies, warns Olson, since "those parts of the brain are shaped by performance and experience," and so may be a product of nurture, not nature. And despite the big genetic finding, it's unclear what precise role genetics plays, since a recent survey of identical twins found that only in 20 percent of cases did both twins turn out transgender, despite having identical DNA.
"Trying to identify causes, whether they be genetic, hormonal, or something else entirely, those studies are underway," says Olson. "The question is, what contributes to the formation of gender identity? It's really complex."