There are lots of theories about why some couples who live together get married while others separate.

Some theories say couples are more likely to split up and less likely to marry when the male partner lacks full-time work or earns less than his female partner. Others say economic dependence strengthens couples’ commitment and sense of obligation to one another. Or maybe couples whose work hours are similar find it tough to juggle childcare and domestic chores.

Recent work by Patrick Ishizuka, the Frank H.T. Rhodes postdoctoral fellow at the Cornell Population Center, suggests it’s none of the above.

Ishizuka’s study is the first to offer empirical evidence that cohabitating couples are likely to get married only when they earn as much as their married peers. And when each partner in a cohabitating couple earns the same amount, they are less likely to separate, Ishizuka said.

His paper, “The Economic Foundations of Cohabiting Couples’ Union Transitions,” was published Feb. 22 in Demography.

“Once couples have reached a certain income and wealth threshold, they’re more likely to marry,” said Ishizuka, who researches work, families, and social inequality. “Economically disadvantaged couples are also more likely to separate.”

Ishizuka’s study validates a theory called “the marriage bar,” which says the closer a couple is to reaching the economic standards associated with marriage – like having enough money to buy a house – the more likely they are to get married. Qualitative studies have suggested that economically disadvantaged couples strongly value marriage, but they struggle to meet what they see as the high economic standard required to marry.

“They want to have a house and a car and enough savings to have a big wedding; and they also want to have stable jobs and a steady income,” Ishizuka said.

This new evidence reflects a growing socioeconomic divide in family life, he said. “Marriage is increasingly reserved for couples that have achieved a high economic standard. Rising divorce rates since the 1960s have also been steepest for individuals with less education.”

Ishizuka analyzed data from a series of nationally representative panels of the Survey of Income and Program Participation from 1996-2013, as well as monthly data from the Current Population Survey, a monthly survey of about 60,000 U.S. households conducted by the U.S. Census Bureau for the Bureau of Labor Statistics. He looked at how couples’ combined earnings compared to the median of married couples’ earnings in the state where they reside.

Ishizuka also found cohabiting couples who have equal earnings are more likely to stay together than couples with unequal earnings. “Equality appears to promote stability,” he said. “Equality in men’s and women’s economic contributions may hold these couples together.”

That could be because cohabitors value equality more than married couples do, or because cohabitors who make equal contributions are more likely to cooperate and invest in the relationship. “Equality might increase commitment or cooperation between partners since they’re bringing similar economic resources to the relationship,” Ishizuka said.

Cohabiters tend to have more egalitarian views on men’s and women’s roles than people who go from singlehood straight into marriage. That could explain why Ishizuka found no evidence showing men’s income or employment status being more important than women’s when it comes to predicting whether or not they marry or stay together. “It’s really the couple’s combined resources that seem to matter,” he said.