This gradient of structural brain differences, from females to males, with transgender people in between, suggests that gender identity has a neural basis and that it exists on a spectrum, like so much of human behavior.

Some theorize that the transgender experience might arise, in part, from a quirk of brain development. It turns out that the sexual differentiation of the brain happens during the second half of pregnancy, later than sexual differentiation of the genitals and body, which begins during the first two months of pregnancy. And since these two processes can be influenced independently of each other, it may be possible to have a mismatch between gender-specific brain development and that of the body.

Is it really so surprising that gender identity might, like sexual orientation, be on a spectrum? After all, one can be exclusively straight or exclusively gay — or anything in between. But variability in a behavior shouldn’t be confused with its malleability. There is little evidence, for example, that you really can change your sexual orientation. Sure, you can change your sexual behavior, but your inner sexual fantasies endure.

In fact, attempts to change a person’s sexual orientation, through so-called reparative therapy, have been debunked as quackery and rightly condemned by the psychiatric profession as potentially harmful.

Of course, people should have the freedom to assume whatever gender role makes them comfortable and refer to themselves with whatever pronoun they choose; we should encourage people to be who they really feel they are, not who or what society would like them to be. I wonder, if we were a more tolerant society that welcomed all types of gender identity, what the impact might be on gender dysphoria. How many transgender individuals would feel the need to physically change gender, if they truly felt accepted with whatever gender role they choose?

At the same time, we have to acknowledge that gender identity is a complex phenomenon, involving a mix of genes, hormones and social influence. And there is no getting around the fact that biology places constraints on our capacity to reimagine ourselves and to change, and it’s important to understand those limitations.

The critical question is not whether there is a range of gender identity — it seems clear that there is. Rather, it is to what extent and in which populations gender identity is malleable, and to what extent various strategies to change one’s body and behavior to match a preferred gender will give people the psychological satisfaction they seek.