Here's a very interesting article about tests describing why women's salaries are generally lower than men's. Essentially, it has a two part answer: women tend not to ask for raises as often as men (self-esteem?) AND part of that is because women who do ask for raises are socially penalized more than men who ask for raises. I guess it boils down to a negative stereotype of the ambitious woman.
Which is why I have my doubts that Hillary Clinton could be elected president.