This is a big reversal from the 30 year trend between 1960 and 1990, when it was the men who were marrying down, educationally speaking. The difference is not yet huge; Pew researchers, using data from the American Community Survey and the Decennial Census, found that in 2012, 21 percent of wives had advanced further academically than their spouses, while 20 percent of husbands were the more educated ones. But it appears to be growing; counting just newlyweds (those married in the 12 months before the survey), more than a quarter of the women had chosen a partner with less education, while 15 percent of men did the same.
The trend is not necessarily due to the fact that women are smarter than men. More women than men have been graduating from college at all levels—bachelors, masters and doctoral—for several years, so it was simply a matter of time until the marriage pool reflected that.