Today, according to census data, in 64 percent of U.S. marriages with children under 18, both husband and wife work. There’s more gender-fluidity when it comes to who brings in the money, who does the laundry and dishes, who drives the car pool and braids the kids’ hair, even who owns the home. A vast majority of adults under 30 in this country say that this is a good thing, according to a Pew Research Center survey: They aspire to what’s known in the social sciences as an egalitarian marriage, meaning that both spouses work and take care of the house and that the relationship is built on equal power, shared interests and friendship. But the very qualities that lead to greater emotional satisfaction in peer marriages, as one sociologist calls them, may be having an unexpectedly negative impact on these couples’ sex lives.

A study called “Egalitarianism, Housework and Sexual Frequency in Marriage,” which appeared in The American Sociological Review last year, surprised many, precisely because it went against the logical assumption that as marriages improve by becoming more equal, the sex in these marriages will improve, too. Instead, it found that when men did certain kinds of chores around the house, couples had less sex. Specifically, if men did all of what the researchers characterized as feminine chores like folding laundry, cooking or vacuuming — the kinds of things many women say they want their husbands to do — then couples had sex 1.5 fewer times per month than those with husbands who did what were considered masculine chores, like taking out the trash or fixing the car. It wasn’t just the frequency that was affected, either — at least for the wives. The more traditional the division of labor, meaning the greater the husband’s share of masculine chores compared with feminine ones, the greater his wife’s reported sexual satisfaction.

Granted, some might view a study like this with skepticism. Correlations don’t establish causation, and especially when it comes to sex, there’s always a risk of reporting bias and selective sampling, not to mention the mood of a subject at the time of the survey. (Was she answering the questions while standing next to a big pile of garbage that hadn’t been taken out?) What’s more, while this study used the most recent nationally representative data that included measures of sexual frequency and a couple’s division of labor, it was drawn from information collected in the 1990s. (Julie Brines, an author of the chores study, explained, however, that many studies on housework since then show that not much has changed in terms of division of labor.) But as a psychotherapist who works with couples, I’ve noticed something similar to the findings. That is, it’s true that being stuck with all the chores rarely tends to make wives desire their husbands. Yet having their partner, say, load the dishwasher — a popular type of marital intervention suggested by self-help books, women’s magazines and therapists alike — doesn’t seem to have much of an effect on their libido, either. Many of my colleagues have observed the same thing: No matter how much sink-scrubbing and grocery-shopping the husband does, no matter how well husband and wife communicate with each other, no matter how sensitive they are to each other’s emotions and work schedules, the wife does not find her husband more sexually exciting, even if she feels both closer to and happier with him.

I first noticed this while doing a yearlong training in marriage therapy. I was seeing a couple who had been married for five years and wanted to work out some common kinks related to balancing their respective jobs, incomes and household responsibilities in, as the wife put it, “an equal way.” Over the course of treatment, the couple reported more connection, less friction and increased happiness. One day, though, when their issues seemed largely resolved and I suggested discussing an end to their therapy, the husband brought up a new concern: His wife now seemed less interested in having sex with him. He turned to her and asked why. Was she still attracted to him? After all, he wondered, why did she appear less interested now that their relationship seemed stronger in all the ways she wanted?

As with many parts of life, quality matters more than quantity.