I doubt I could find it, but when I was in college, I read a study that said one of the reasons men get paid more than women is because men are "generally" more aggressive at demanding raises, looking for higher-paying jobs, and threatening to 'walk' if they didn't get those raises or promotions. The study also showed that women, in general, tend to take a more passive stance, complain to coworkers rather than their boss, and worry too much about making waves in the workplace to ask for that raise. They also tend to get settled in their routine jobs and don't actively search for better-paying positions as often as men do.
Don't know how much of that study is true... :shrug: