Ozy Frantz wonders if perhaps many people maybe "cis by default". That is, our societal default is for you to take on the gender role that matches your sex so people who don't have a strong inner impulse either way end up cis, but with a gender that they're not strongly attached to. And so:

If you don't have a gender identity, and you assume that your lack of a gender identity means other people don't have a gender identity, then trans people's behavior is ludicrous. Why the hell are all these people deciding they're women or men or something else?
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If my idea is correct, then it offers some hope in combating that sort of transphobia. We simply have to explain to cis-by-default people what a gender identity is and that they don't have one.

It's a tidy explanation for a lot of people's reactions to the idea of changing your gender and body dismorphic disorder has interesting parallels to phantom limb syndrome. But how might we test it?

I think the best evidence is the experiences of people who were raised as one gender even though that didn't match their biological sex. Take the fraction who live happy lives, comfortable in their gender and perhaps never even suspecting that their gender might be 'wrong', subtract the frequency of being trans in the general population [1], and you have the fraction of people who are cis by default.

The cases I've heard about are all people (ex: David Reimer) who feel very uncomfortable with their newly assigned gender, but it may be that we only hear about the problems and for most people it's fine. Wikipedia says that "a large proportion of XY infants born with cloacal exstrophy and raised as female from early infancy have requested reassignment to male gender in late childhood and adolescence" but doesn't give a citation. Where can I read more about this?

(It also sounds like this mostly happens to biologically male children, which means it really only tells us about the fraction of men who are cis by default.)

[1] In this case these are people where the reassignment at birth happened to match up with what they would have wanted later in life.