Births among women who finished high school

Of mothers with one to three years of college, 67 percent had at least one baby while unmarried.

Births among women with some college

Of mothers with four or more years of college, 32 percent had at least one baby while unmarried.

Births among women who completed college

The less-educated mothers also tended to have their children younger. Women with B.A.s were most likely to have children at age 29 or 30, while those with high-school degrees had them at 22.

“The clear line is whether you have a four-year college degree,” the study's lead author, Johns Hopkins sociologist Andrew Cherlin, told me. “There are two clear paths through adulthood—one for people who have a bachelor's degree and one for people who don't."

There are several things going on here:

First, the cornerstone theory of marriage no longer applies. Culturally, young adults of all social classes and income levels are less likely to think of marriage as the “cornerstone” of their lives—that is, the first thing they do as adults. Instead, people now think of it as a “capstone”— sort of a trophy for having earned a B.A., obtained a job, and generally learned to live on their own for a while. The national marriage age has gradually ticked up as a result. For people who don’t have all the stones leading up to the capstone, though, the entire order of operations gets messed up.

Second, marriage is increasingly something only educated people do. As my former colleague Jordan Weissmann wrote, the less a man earns these days, the less likely he is to have ever been hitched. College-educated people are increasingly only marrying other college-educated people, and they’re more likely to get married overall. One reason less-educated women are having children out of wedlock is that college-educated men are not interested in marrying them.

“The college-educated young adults can see a good future, where they're likely to find a good partner, pool two incomes, and they're willing to wait to have kids till they can do that,” Cherlin said. Meanwhile, the less-educated women “don't see the possibility of finding partners with good incomes. And many are unwilling to give up the opportunity to have a kid by waiting.”

Women without college degrees see a dearth of husband options around them. Incarceration rates are higher among the poor, and even low-income men tend to view marriage as something they should only do after they get a good job.