Here's an article that cites several different studies. Granted, I cited the higher end, it could be lower, by all means make up your own mind. They also add the caveat here that it only applies to metropolitan areas, but this is the vast majority of the population nowadays.
>... in 147 out of 150 of the biggest cities in the U.S., the median full-time salaries of young women are 8% higher than those of the guys in their peer group. In two cities, Atlanta and Memphis, those women are making about 20% more. This squares with earlier research from Queens College, New York, that had suggested that this was happening in major metropolises. But the new study suggests that the gap is bigger than previously thought, with young women in New York City, Los Angeles and San Diego making 17%, 12% and 15% more than their male peers, respectively. And it also holds true even in reasonably small areas like the Raleigh-Durham region and Charlotte in North Carolina (both 14% more), and Jacksonville, Fla. (6%).
> As for the somewhat depressing caveat that the findings held true only for women who were childless and single: it's not their marital status that puts the squeeze on their income. Rather, highly educated women tend to marry and have children later. Thus the women who earn the most in their 20s are usually single and childless.
http://content.time.com/time/business/article/0,8599,2015274...
Once again: women and men who want families make different choices.