Well there’s discomfort with gender roles and discomfort with one’s sex (i.e. your body.) It’s been pretty well demonstrated that transsexual who get GRS have a fairly high degree of the latter. I think it’s likely the non-transitioning transsexuals and crossdressers have lower degrees of this—hence the “irresistable urge” part of it and the feeling that one has a tangible feminine side.

But I think discomfort with gender roles plays a bit part of it. I seem to remember some studies indicating a good percentage of men and women have personalities that fall outside the accepted norms for “masculine” and “feminine” personalities. It’s interesting to me that
there seem to be a lot of crossdressers from conservative social environments, where gender roles are typically more rigid, as well as the number of folks engaged in “logical/rational” profession, i.e. engineering, programming, etc. For the former group, “becoming” a woman may be a way to express aspects of their personalities that aren’t socially accept as a man. For the latter, I think crossdressing is a way of letting out the non-rational parts of their personalities that they normally keep in check. (It’s interesting that engineers, programmers, etc. are often over-represented in other “alternative” activities from the Rennaissance Faire to Burning Man, to Trekking, where they also have a
chance to become someone else.)

Lately, I’ve become intrigued by the question of why there are so few female crossdressers (although crossdressing play is more prominent in the lesbian community) when there are a good number of MTF transsexuals, especially if you factor in the stone butches and other strongly butch lesbians who to my outside eye often seem like they’re exhibiting transgender-ish behavior. The obvious answer is the greater flexibility in clothing, but that
ignores the fact that it’s intent that makes it crossdressing rather just wearing clothes of the opposite sex.

There’s an interesting female parallel to cross-dressing: tomboyism. From the research I’ve run across there’s two types of tomboyism, the first and widely common is “expansive” (i.e. wanting to play with dolls and trucks), while a minority of tomboys reject female behavior and sometimes even deny they’re girls. Lesbians (especially butches) have a much higher likelihood than “normal” of having been tomboys, especially the second type. But interestingly, bisexual women, who self-identified as andrgynous at suprisingly high rates,
had a strong likelihood of recalling being tomboys part of the time. Raven Kaldera, a transman, mentions having seen some female crossdressers and says they typically begin in their 20s. (Unfortunately, he doesn’t mention if any of them were tomboys, so I don’t know if it’s a resumption of behavior or something new. But my suspicion is the later
onset is because when the social pressures that typically put an end to tomboyism are weaker.)

The sum of this suggests to me that there’s a number of women who’ve got similar gender discomfort to us MTF crossdressers (at varying levels). Things do get murky because discomfort with gender role has been the focus of feminism, whereas with men there’s really not been a widespread equivalent. Women with gender role discomfort turn to feminism, where men with gender role discomfort may turn to crossdressing. Another factor is that MTF presentation involves “dressing up” whereas “masculine” women
generally are “dressing down” (short hair, not wear much, if any make-up, “practical” clothes, etc.) And of course the great range of acceptable clothing for women. So MTF crossdressing involves an overt awareness of what you’re doing, whereas women don’t necessarily have to been as self-aware, as in fact probably see it more as “not being
girly” than “being masculine.”