+(With the understanding that psychology is complicated and there's more to be said about what "as a first approximation" is even supposed to mean, but I need a few paragraphs to talk about the simple story before I can get to that.)
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+The idea is that male-to-female transsexualism isn't actually one phenomenon; it's two completely different phenomena that don't actually have anything to do with each other, except for the indicated treatment (HRT, surgery, social transition).
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+In one taxon, the "early-onset" type, you have same-sex-attracted males who have just been extremely feminine (in social behavior, interests, _&c._) their entire lives, in a way that causes huge social problems for them—the far tail of feminine gay men who end up fitting into the world better as straight women. _That's_ where the "woman trapped inside a man's body" trope comes from. This one probably is a brain-intersex condition. [TODO: cite that from Brown's bibliography]
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+That story is pretty intuitive. Were an alien AI to be informed of the fact that, among humans, some fraction of males elect to undergo medical interventions to resememble females, and aspire to be perceived as females socially, "brain-intersex condition such that they already behave like females" would probably be its top hypothesis for the cause of such behavior, just on priors.