In the 1990s, scientists began to compare these sexually dimorphic regions in the brains of transsexuals and the rest of humanity. Early work in this area required the examination of brains postmortem; recent studies use images of the living brain.
The results show that when individuals of Sex A—despite having the chromosomes, gonads and sex hormones of that sex—insist that they're really Sex B, the gender-affected parts of the brain typically more closely resemble what's usually seen with Sex B.
Consider an obscure brain region called the forceps minor (part of the corpus callosum, a mass of fibers that connect the brain's two hemispheres). On average, among nontranssexuals, the forceps minor of males contains parallel nerve fibers of higher density than in females. But the density in female-to-male transsexuals is equivalent to that in typical males.
As another example, the hypothalamus, a hormone-producing part of the brain, is activated in nontranssexual men by the scent of estrogen, but in women—and male-to-female transsexuals—by the scent of androgens, male-associated hormones.
Two arguments against these findings come to mind. First, sex-reassignment surgery involves treatment with cross-sex hormones that alter the brain. This is true, but the differences in the brains of transsexuals are there before hormone treatments start. Secondly, maybe these findings aren't about the sex one identifies with but are instead about the sex one is attracted to. But no, the findings are the same in transsexual individuals who are attracted to the same or to the opposite sex.
