Time for some number crunching from our data expert, Mona Chalabi, from fivethirtyeight.com. And she has given us this number of the week.

AUTOMATED VOICE: Thirty-eight.

MARTIN: That is the percentage of American wives who earn more than their husbands. Mona Chalabi joins us from our studios in New York. Hey, Mona.

MONA CHALABI: Hi, Rachel.

MARTIN: OK, so 38 percent - what does that mean? Who are these people? Are they working couples, both people employed?

CHALABI: No, actually, in 1 in 3 of those cases, the woman's only earning more because her husband isn't earning anything at all. So if you only look at marriages where both spouses are working, then actually, you find that only 29 percent of women out-earn their husbands. But the number has gone up over time. So in 1987, only 18 percent of women were breadwinners in marriages where both partners were working. Although, I should point out, we're only talking about heterosexual, married couples here. Unfortunately, there just isn't data on same-sex or unmarried couples.

MARTIN: OK. So what effect does this have on these marriages?

CHALABI: Well, there's some research. So in 2013, the University of Chicago Booth School of Business published a paper that looked at 4,000 married couples in America. It found that once a woman started to earn more than her husband, divorce rates increased. Surprisingly, though, this data showed that whether the wife earns a little bit more or a lot more doesn't actually make much of a difference. So the researchers concluded from that that what really matters is the mere fact of a woman earning more.

MARTIN: Ok, so this seems crazy to me. I mean, (laughter) you're saying that when women earn more in a marriage, that's a reason that couples become unhappy, and they get divorced?

CHALABI: I mean, there seems to be a correlation, right? So the researchers are kind of looking for theories that can explain that middle bit to see if there really is a causation thing here. So everyone knows, on average, - or at least I think most people know - that American women spend more time on housework than men, about 44 minutes more every day. But here's the weird thing. The researchers found that the gap in housework got even larger when the woman was the primary earner.

MARTIN: So wait. So if the woman is earning a lot more money, or just more money, she's doing even more housework?

CHALABI: The gap between how much she's doing versus how much the man is doing is even bigger.

MARTIN: Why?

CHALABI: (Laughter). So they kind of turned from data to theory, like sociological theory here. They think that the explanation for that extra housework is that a high-earning woman is trying to make sure that her husband doesn't feel threatened. The idea is basically that men might feel a bit emasculated by a woman that earns more than them.

MARTIN: And so women do more housework to make up for that?

CHALABI: Yeah. But, I mean, I'm kind of reluctant to make the same leap here because they didn't actually ask the men whether or not they feel threatened. But they looked at the data on women's incomes relative to their husbands. And they said, and I'll quote here, "a threatening wife takes on a greater share of housework so as to assuage8 the husband's unease with the situation," unquote. But there's something else that's really weird here too. There's other research that suggests that when a woman out-earns her partner, it affects fidelity.

MARTIN: Really?

CHALABI: Yeah (laughter).

MARTIN: OK, tell me more.

CHALABI: There's a study from Cornell University that looks at data on young American couples. And actually, the good thing about this bit of research is that it included married and unmarried couples.

MARTIN: OK.

CHALABI: But the findings are pretty depressing. So the author found that a man is more likely to cheat on his partner if he is more financially dependent on her. And men who are completely dependent on their girlfriends or wives are five times more likely to cheat than men who earn the same amount as their partners. And the explanation given here was basically the same as the housework thing. So it's basically about kind of men feeling like they need to conform to society's definitions of masculinity.

MARTIN: OK, so it seems like a lot of these theories are focusing in on men's behavior...

CHALABI: Yeah.

MARTIN: When the wives are the breadwinners. But what about the women - is there any research about how women might behave differently in this dynamic?

CHALABI: Yeah. So we know that earning more than your partner might be kind of stressful for a woman. So in 2013, researchers from Washington University in St. Louis studied data from 200,000 married couples in Denmark. And they found that when women were earning more than their husbands, they were more likely to use anti-anxiety medications and also more likely to suffer from insomnia.

MARTIN: OK. So you've got this data talking about money and fidelity and specifically how it affects heterosexual marriages when the wife earns more than the husband. You know, does the data project out towards the future, how families are going to adapt to this? What is a new reality in the labor market?

CHALABI: I mean, not really. And one of the problems is that not only does the data not project out, but it kind of doesn't look backwards enough, right? So we don't know whether some of these couples were kind of unhappy from the start, right? Maybe a high-earning woman is more likely to divorce simply because she can because she has the financial independence to kind of walk away. So there's a lot of gaps, basically, in this information. But I think what the research does kind of point to is that economic realities are moving faster than societal norms. And by that, I'm not saying that, like, there's economic equality for man and women. That's just blatantly not the case. But it does look like progress is happening there faster than traditional gender norms seem to be changing.

MARTIN: That was Mona Chalabi, and you're listening to WEEKEND EDITION from NPR News.