If you could imagine a world where people are more like animals, naked and primitive, with no means to actually switch sexes, what would define a transsexual? I'd assume that the only means would be through behavior. In animals, most of these behavioral quirks are related to breeding. In humans on the other hand, many of these behaviors are determined by cultural social structure which is artificial rather than natural. Because behavior in humans is largely arbitrary and not natural, it makes sense that some humans will naturally be inclined to behave outside of the socially defined expectation. In a social evolution sense, transsexuals might be defined as one of the things that forces a wide paradigm in the artificially enhanced behaviors which are determined by society. Sort of like a mirror echo.

RE: Criticism of a Gender Theory, and a Scientist Under Siege