The Don Drapers of the world used to marry their secretaries. Now they marry fellow executives, who could very well earn more than they do.

With more marriages of equals, reflecting deep changes in American families and society at large, the country is becoming more segregated by class.

“It’s this notion of this growing equality between husbands and wives having this paradoxical effect of growing inequality across households,’’ said Christine Schwartz, a sociologist who studies the topic at the University of Wisconsin, Madison.

From Cinderella to Kate Middleton, fictional and real-life fairy tales have told of women marrying up. But it has been a long time since women said they went to college to earn a “Mrs. degree.’’ In more recent cultural touchstones — like “The Intern,’’ with Anne Hathaway, and “Opening Belle,’’ the novel and soon-to-be released Reese Witherspoon movie — the protagonists are highly successful women with husbands who don’t work. (Spoiler alert: Conflict ensues.)

These changes have been driven by women’s increasing education and labor force participation, new gender roles, and the rise of what social scientists call assortative mating.

Assortative mating is the idea that people marry people like themselves, with similar education and earnings potential and the values and lifestyle that come with them. It was common in the early 20th century, dipped in the middle of the century, and has sharply risen in recent years — a pattern that roughly mirrors income inequality in the United States, according to research by Robert Mare, a sociologist at the University of California, Los Angeles. People are now more likely to marry people with similar educational attainment — even after controlling for differences between men and women, like the fact that women were once less likely to attend college.

Even though the typical husband still makes more than his wife, the marital pay gap among opposite-sex couples has shrunk significantly in the decades since women started entering the workforce en masse. Today, wives overall make 78 percent of what their husbands make, according to an Upshot analysis of annual survey data from the Census Bureau. That’s up from 52 percent in 1970.

In opposite-sex marriages in which both spouses work outside the home some amount of time, 29 percent of wives earn more than their husbands do, up from 23 percent in the 1990s and 18 percent in the 1980s, according to the Bureau of Labor Statistics.

The marital pay gap still exists in part because women earn less than men in the economy as a whole, making 79 cents for a man’s dollar.

It reflects the stickiness of gender roles at work and at home: Marriage significantly depresses women’s earnings, and the arrival of children has an even stronger effect. Men, meanwhile, tend to earn more after having children, and studies show that’s because employers see mothers as less committed to work and fathers as doubly committed to bread-winning.

The nature of marriage itself is changing. It used to be about the division of labor: Men sought homemakers, and women sought breadwinners. But as women’s roles changed, marriage became more about companionship, according to research by two University of Michigan economists, Betsey Stevenson and Justin Wolfers. Now, people marry others they enjoy spending time with, and that tends to be people like themselves.

“Husbands and wives had different roles in different spheres, so that was the opposites-attract view of marriage,’’ Wolfers said. “Today you want people with shared passions, similar interests to you, similar career goals, similar goals for the kids.’’

Another reason people are finding mates like themselves is that they are marrying later, so they know more about their partners’ prospects and increasingly meet at work. People were least likely to marry those with similar educational backgrounds around the 1950s, according to Mare’s research, when people married very young. Americans are increasingly able to make their own romantic choices based on personal preferences, free from family or religious expectations, he found.

American society has also become more segregated geographically; people tend to live near others with similar educations and earnings. Researchers have linked the increase in so-called power couples, in which both partners have a college degree, to the fact that educated people are more likely now to live in big cities — so the people they date tend to be educated, too.

Technology could also play a role: Dating apps and sites let people filter their potential partners before they even have a conversation.

The change is happening internationally, too. In 40 percent of couples in which both partners work, they belong to the same or neighboring income bracket, up from 33 percent two decades ago, according to 2011 data from the Organization for Economic Cooperation and Development, which includes 34 countries. Two-thirds have the same level of education.

Despite being more common, these marriages are a break from tradition, and that can present problems.

Marriages in which the woman earns more are less likely to form in the first place, which accounts for 23 percent of the overall decline in marriage rates since 1970, according to a large study by University of Chicago and National University of Singapore economists.