The overall gender wage gap stands at 20%, but when you adjust for work experience, credentials, occupation, and number of hours worked per week, it quickly shrinks to 4% ~ 7%. In fact, when gender preferences such as workplace flexibility and employee benefits are taken into account, the wage gap disappear completely. (ses: one of many studies backing this up).

I think one of the major explanations for this trend is the fact that women are able to "mary up" or otherwise subsidize their own income with the income from a man ("I'll get this art degree when I go to college, instead of something a little more useful, because I'll have a man to support me and my dreams of becoming an artist").

The overall wage gap would probably shrink dramatically if society had very clear expectations that women couldn't use men for their money (either through dating or marriage), but instead had to earn it themselves.

This would force women to make the right decision and put in the work themselves to earn a decent income, instead of taking the easy way out and expecting a man to pay their way through life.