(While poor women are outpacing poor men, it is important to note that in the big picture, poor women are nevertheless far behind their richer counterparts. About 70 percent of women from a high socioeconomic status who were high school sophomores in 2002 had gotten bachelor’s degrees by 2013, compared to 17.6 percent of women from low socioeconomic status.)

This gender gap in college completion has been a long time in the making. In the early 1900s, when some elite colleges started opening up to women, women quickly got better grades than men, according to Claudia Buchmann, a professor of sociology at Ohio State and the co-author of The Rise of Women: The Growing Gender Gap in Education and What it Means for American Schools. In the 1970s, as more women started attending college, they started graduating at higher and higher rates, while men’s enrollment and graduation rates remained relatively flat. But until recently, the women attending college were mostly from elite families. Now, women from lower-income families are increasingly attending college.

Percentage of American 25-to-29-Year-Olds With a Bachelor’s Degree or Higher

Steven Johnson / The Atlantic

This is a positive development for women, because a college education is increasingly important in today’s economy. Out of the 11.6 million jobs created after the recession, 8.4 million of those went to those with at least a bachelor’s degree, according to the Center on Education and the Workforce at Georgetown. But while women across socioeconomic classes are embracing the idea that education is important and are pursuing postsecondary degrees, many men from lower-income households are not. “The puzzle is—why don't boys get it? There’s all this talk that we hear constantly, about the benefits of a college degree,” said Buchmann.

Some of the problem is that boys from low-income families appear to struggle more in school than girls do. They lag behind as early as kindergarten even though health tests show that, at the time of birth, they are just as healthy and cognitively able to learn as their sisters, a recent paper found. This is partly because they appear to be more affected by poverty and stress than girls are. “Boys are differentially sensitive to negative environments in general,” one of the paper’s authors, Northwestern professor David Figlio, told me. These findings dovetail with much-cited research out of the Equality of Opportunity Project that finds that childhood disadvantage is especially harmful for boys.

School quality is also more important for boys than for girls, Figlio said, and since many low-income families attend poor-quality schools, their sons, who are already lagging behind their daughters, fall even further behind. The paper found that lower-income boys often do worse in elementary and middle school than their sisters, and have more behavioral problems, which can lead them to disengage with school entirely or get kicked out.