From Alte at Traditional Catholicism:
Women increasingly need college degrees to earn a living wage because women pursue jobs that are heavily oversubscribed and require little talent. Their jobs often require interpersonal skills and a quick mind, which is difficult to judge on paper, and this leads to a downgrade of many desirable applicants. This is why I avoided such industries like teaching, despite having an obvious talent for it. The slight advantage my intelligence provides is greatly outweighed by my relative lack of credentials.
Men, on the other hand, are capable of earning a living wage in industries where productivity is the focus, so their abilities and talents are easily measurable, and their income can increase accordingly even without those credentials. This, in a nutshell, is the reason that men generally earn more than women outside of large cities (where most of the productive industries are), but less than them inside of large cities (where credentialism runs rampant).
I like this explanation mostly because it fits neatly into my prior thoughts on credentialism, education, and employment, although I’m not sure how you’d show this empirically.
Ducks and Economics 