The Biological Roots of Gender Identity

Molly is the mother of two children, a 3-year-old boy and a 6-year-old transgender girl. The younger child is the “stereotype of what I thought having a son would be like,” she says. “He loves trucks, cars, construction, superheroes and destroying the house.”

Her first child, also born male, just did not develop in the same way. Rather, from the time her then-son acquired language as a toddler, he continuously protested that he was really a girl.

“It was, ‘I’m a girl. I’m a sister. I’m a daughter. I’m that girl on that show. I’m that girl in that book,” Molly recalls.

So … is there something biological going on here?

"I think we already know that biology definitively affects gender identity,” says Dr. Stephen Rosenthal,  medical director of UCSF’s Child and Adolescent Gender Center Clinic. Rosenthal rattled off studies in genetics, endocrinology and neuroanatomy, all suggesting that physiological differences play some role in gender identity development.

For example, one genetics study looked at 23 pairs of identical twins and 28 pairs of nonidentical twins, with each pair including at least one transgender person.

The study found, among the identical twins, that if one in a given pair was transgender, there was a 39 percent chance the other twin would also be transgender; in contrast, among the nonidentical twins, the study found no examples where both of the twins in a given pair were transgender.

In other words, sharing the same DNA meant a much greater likelihood that each twin would be transgender.

In the area of endocrinology, a metanalysis of 250 patients found that 5 percent of females with congenital adrenal hyperplasia, a condition that affects the adrenal gland's ability to make certain hormones, identified as male or had some level of gender dysphoria, a much higher rate than in the general population.

And research has also looked at parts of the brain that are shaped differently in men and women. A number of these studies, Rosenthal said, found that in transgender individuals, some of these anatomical structures more closely mapped to an individual’s gender identity than they did to the person’s natal sex. Rosenthal called these studies “compelling evidence that there is some intrinsic brain difference in someone who is transgender, versus someone who's not.”