This chart shows the effect at work:
To do the study, Christin Munsch, a sociologist at the University of Connecticut, examined data for about 2,800 heterosexual married people under the age of 32 who participated in the National Longitudinal Survey of Youth, a big federal research project, between 2001 and 2011.
She found that the less that men and women earned in income compared to their spouses, the more likely they were to cheat. This was especially true of husbands.
Men were also more likely to cheat if they made significantly more than their spouses, while women who made a lot more than their husbands were less likely to be unfaithful.
People are, of course, different, and the data shouldn't be taken to mean that all marriages work best when both partners are working and for similar wages. In general, though, the couples with the least infidelity were those in which both partners were earning the same amount.
The survey didn't ask people directly whether they'd cheated. Munsch was able to infer whether they'd been unfaithful based on how they answered questions about the number of sexual partners they'd had in the past year. On the whole, about nine percent of the wives and 12 percent of the husbands had engaged in extramarital sex. She then compared husbands and wives with varying economic arrangements to other couples who were similar in terms of their race, education, salaries and churchgoing habits.
A finding not explained by economics
The fact that women and men who make less than their partners are more likely to cheat is counterintuitive. Infidelity frequently results in divorce, so this group is putting their access to their spouses' earnings at risk by straying. But people don't always behave in a way that just seeks to raise their income, and a difference in earnings might lead to resentment, Munsch theorized.
"People just don't like inequality in their relationships," Munsch said. "When I think about my own dating history, I'm not really interested in dating somebody who's much better looking or much worse looking, somebody who's much more successful or much less successful."
The small number of men who make less than their wives might feel a need to demonstrate their independence. Previous research has suggested that when the wife is the breadwinner in the household, the husband may feel that he is failing to fulfill his traditional responsibilities.
What can be done?
Munsch noted that one of the main reasons that women still earn less than men on average is that many stop working or work less when they have children. That suggests, she said, that policies that help mothers remain in the labor force could reduce infidelity.
"We've heard a lot of rhetoric of late about family values and the traditional breadwinner-homemaker model," she said. "Actually, these relationships are more stable when both people are bringing in income."
That said, Munsch stressed that cheating is rare across all kinds of households. With that in mind, she offered this advice to young people considering marriage: Think about how your prospective spouse would respond if you got a promotion or a big raise.
"See how your partner reacts to little successes that you have," she said. "You want to pick somebody who is happy for your successes."