n terms of biology, some scientists think it might be traced to the syncopated pacing of fetal development. “Sexual differentiation of the genitals takes place in the first two months of pregnancy,” wrote Dick Swaab, a researcher at the Netherlands Institute for Neuroscience in Amsterdam, “and sexual differentiation of the brain starts during the second half of pregnancy.” Genitals and brains are thus subjected to different environments of “hormones, nutrients, medication, and other chemical substances,” several weeks apart in the womb, that affect sexual differentiation.
This doesn’t mean there’s such a thing as a “male” or “female” brain, exactly. But at least a few brain characteristics, such as density of the gray matter or size of the hypothalamus, do tend to differ between genders. It turns out transgender people’s brains may more closely resemble brains of their self-identified gender than those of the gender assigned at birth. In one study, for example, Swaab and his colleagues found that in one region of the brain, transgender women, like other women, have fewer cells associated with the regulator hormone somatostatin than men. In another study scientists from Spain conducted brain scans on transgender men and found that their white matter was neither typically male nor typically female, but somewhere in between.
These studies have several problems. They are often small, involving as few as half a dozen transgender individuals. And they sometimes include people who already have started taking hormones to transition to the opposite gender, meaning that observed brain differences might be the result of, rather than the explanation for, a subject’s transgender identity