An ingenious study by Kristen Schilt of the University of Chicago and Matthew Wiswall of New York University looked at whether men's and women's salaries changed after they had a sex change. The new men earned slightly more, and the new women quite a bit less, than they had in their previous gender. Schilt and Wiswall take this as unambiguous evidence of gender discrimination. Their crucial claim is that "while transgender people have the same human capital after their transitions, their workplace experiences often change radically."
We usually measure human capital only by level of education. For these subjects, that stayed the same. However, a more biological line of research about men's and women's differences at work emphasizes that testosterone is correlated with more aggressive work, including the desire to be paid more as a measure of status. Sex-change therapy includes significant hormone infusions, which are kept up after the surgery, to change the way the new men and women behave, as well as the way they look.
We can't tell from this study what hormonal difference there was between the new women and the men they were -- the main direction of wage change. We know at least enough to ask, though, whether in behavior that affects work the subjects of this study really were the same before and after.