For the first time in history, American women are more likely to have bachelor's degrees than American men.

The shift, which happened in 2014 and which the Census Bureau pointed out last week, was a generation in the making. Women have earned the majority of bachelor's degrees granted every year since 1981. As older, less-educated women have died, more educated women have made up a greater share of the population.

Women, historically less educated than men, now totally dominate higher education: They make up 57 percent of college students and earned 57 percent of college degrees in 2013.

Girls have always excelled in school, but they used to be discouraged from seeking higher education. But as the feminist revolution brought more women into the workplace, young women and their families became more willing to invest in their economic futures. That allowed them to not only equal men's academic achievements but surpass them.

How women caught up to men in college-going

The story of how women started earning more college degrees than men is more complicated than you might think. Three economists — Claudia Goldin, Lawrence F. Katz, and Ilyana Kuziemko — explored how it happened in a 2009 paper on women's success in higher education.

They found that women didn't gradually increase their share of the college population over a century. They pulled almost even in the early 20th century, fell behind, caught up, and then began to dominate.

It was rare for anybody to go to college in the early 20th century, but men and women attended at roughly equal rates (although women were more likely to go to two-year teachers colleges and men to go to four-year universities). During the Depression, women began to fall behind as unemployed men went to college to get an edge in the labor market. When the GI Bill opened up college to many more white men after World War II, it cemented women's status as a minority; in 1950, they earned just 27 percent of bachelor's degrees.

Then women's college-going rates exploded in the 1960s and 1970s. Goldin, Katz, and Kuziemko argued this was driven by bigger social changes. Women married later and had access to better birth control, including the Pill; victories from the feminist movement meant that more careers, including many requiring higher education, were now open to them.

Young women started to expect to spend much more time in the labor force than their mothers or even their older sisters. In 1967, 41 percent of female college freshmen thought it was improper for a woman to work after marriage. Just seven years later, Goldin, Katz, and Kuziemko wrote, only 17 percent agreed. When you expect to work for more of your life, a college degree seemed like a more logical economic choice.

These changes explain why women caught up to men in college-going rates. But they don't fully explain why they pulled ahead.

Why women have surpassed men in earning degrees

One reason women might be outperforming men in college: They're more likely to cultivate behavior that makes them more likely to succeed.

By high school, girls report spending more time on homework, and they're less likely to face disciplinary action in school. These are learned behaviors, not innate, genetic differences. But when looking at other factors such as standardized test scores, those differences can explain much of the gender gap, Goldin, Katz, and Kuziemko write.

Other experts agree with this analysis. Middle school grades strongly predict whether students will eventually graduate from college, and students with higher grades are also less likely to report being disorganized or undisciplined — traits that are more common among boys than girls.

In a report for the think tank Third Way, two sociologists, Thomas DiPrete and Claudia Buchmann, found that the gap between boys and girls on social and behavioral skills such as paying attention, staying on task, and being sensitive to the feelings of others already exists by the end of fifth grade.

These differences probably aren't new: Girls have always gotten better grades than boys. So boys aren't really falling behind.

But before the 1960s, women considering college didn't expect to work for most of their lives, and so a college degree didn't seem particularly necessary or valuable. As those expectations changed, so did their estimate of the value of a college degree. And as more opportunities opened up to women, their historic edge in things like grades and class rank meant that they were prepared to take advantage of them.