Want a happier sex life? For parents, a new study suggests that one of the surest ways to do that is to equally share child care responsibilities.

Egalitarian couples who share between 40 to 60 percent of child care duties, from reading bedtime stories and diaper changing to arranging carpools and playing in the park, not only have higher quality relationships than more traditional couples where women do the bulk of the care work, they also have the best sex lives, the research shows.

“In nearly every case, compared to having the female partner doing all the child care, we found that couples who split it equally had better outcomes across the board,” said Dan Carlson, a sociologist at Georgia State University who co-authored the report and presented it at the American Sociological Association’s annual conference. “Egalitarian couples fought less. They had higher quality relationships. They were more satisfied with their sex lives, and they were more satisfied with the amount of sex they were having.”

Egalitarian and traditional couples, where men are primarily the breadwinners and women primarily responsible for housework and child care, had about the same amount of sex, Carlson said. But egalitarian couples were more satisfied with it.

“These are not minor differences at all,” Carlson said. “These are gulfs of difference between egalitarian and traditional couples in terms of relationship conflict, relationship satisfaction and for quality of sex.”

In fact, of all the couple arrangements Carlson studied, both the men and the women in traditional couples were the least satisfied with the quality of their relationships and their sex lives.

Even couples in the handful of reverse traditional arrangements, where men were doing most of the child care and women were breadwinning, were more satisfied than traditional couples, Carlson found. Except in one area: men in reverse traditional couples tended to rate the frequency of sex at the lowest level of satisfaction, while women rated it the highest.

The new research, one of only a handful to look at child care and sex, challenges conventional wisdom and classic economic theory, that couples who have specific duties in the “separate spheres” of work and home are happiest and have more sex.

Two decades ago, studies found that egalitarian couples spent less time together, had less sex than traditional couples, had more strain, were unhappier and more likely to divorce, Carlson said. They were called “sexless but equal” arrangements. Now, though the amount of sex that all American couples have has been on the decline for years, egalitarian couples are the only ones having more than they used to. And now, an unfair division of labor between work and home is one of the biggest predictors of divorce.

“Something rather miraculous has occurred in the last 20 years,” Carlson said. “Gender equality has become good for us.”

Carlson’s study also upends previous research about the relationship between sex and household duties.

One recent and widely publicized study found that the more housework men did, the less sex couples had, and derided egalitarian couples as “sexless but equal.” Another study, using the same data, found the opposite. (One study looked at types of housework, such as indoor “feminine” work like dishes and vacuuming, versus more “manly” outdoor work like yard work and auto repair and found the more men did “feminine” chores, the less sex couples had. Another looked at total hours spent on chores, and found, the more time men spent on housework, the more sex.)

Carlson said both studies were based on data collected in the late 1980s and early 1990s that may be outdated. His work relies on the more recent 2006 Marital and Relationship Survey of 487 primarily low to moderate-income married and co-habitating couples. Using that more recent data, Carlson and his co-authors released another report in 2014 that found couples who routinely shared housework had more sex than traditional couples, and expressed more satisfaction with it.

“Study after study shows that the trend in public opinion is increasingly toward a rejection of that traditional, separate spheres model, and a movement toward embracing shared responsibility for both paid and unpaid labor,” Carlson said. “A lot of this can boil down to people’s sense of fairness and satisfaction with their arrangements. If they’re satisfied and feel the division of labor is fair, that’s what leads to the positive outcomes in their relationships and sex lives.”

One such survey of 18 to 32-year-olds released in February found that, among the college-educated, 63 percent of men and 62 percent of women said they intended to equally share work and home duties with their partners. For those with a high school education, 82 percent of the men, and 59 percent of the women said they intended to form such egalitarian partnerships.

And time diary data shows that, though women are still doing the majority of housework and child care, both parents are spending more time with children, and the gap between mothers and fathers has narrowed. In the 1960s, women did four times the amount of child care that men did. Now, they do about twice as much.

In fact, Carlson and his colleagues analyzed historical trends and found that the amount of sex people are having has been on the decline for years, except for those in egalitarian relationships.

“The only couples having more sex than before are egalitarian couples,” Carlson said. “That speaks to the power of people having the partnerships they desire, as opposed to being forced into something because of job requirements, or because it doesn’t make sense for mom to work because dad makes more money and they can’t afford child care. That can build mom’s resentment.”

Despite the unrelenting demands of many American workplaces, and a lack of family-friendly policies like parental leave, egalitarian couples are still finding a way to make it work, Carlson said.

“Through sheer force of will, really,” he said.

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