I shared my friend’s observation with Helen Fisher, a senior research fellow at the Kinsey Institute who studies sexual attraction. She noted that even people who are satisfied with their sex lives often crave more nowadays. She told me about a study she conducted that asked participants who had had affairs why they did so. Fifty-six percent of her male subjects and 34 percent of her female subjects said they were “happy” or “very happy” in their partnerships but cheated anyway.

While past research has shown that men have higher rates of infidelity than women, those rates are becoming increasingly similar, particularly in younger people in developed countries, where recent studies have found no gender differences in extramarital sex among men and women under 40. This may be because younger women are more likely to be in peer marriages — and conditions in peer marriages make female infidelity more probable than in traditional ones. A large national study in the late 1990s found that women who were more educated than their husbands were more likely to engage in sexual infidelity than if they were less educated than their husbands. Studies also find that people who work outside the home and whose partners remain in the home cheat more — and the traditional gender roles in this situation are now frequently reversed. As women increasingly work in professions that are not female-dominated, they have more sexual opportunities with peers than ever.

Helen Fisher told me that women’s expectations for sexual fulfillment are changing so much that when she conducted a survey last year asking, “Would you make a long-term commitment to someone who had everything you were looking for but to whom you did not feel sexually attracted?” the least likely group to say yes was women over 60. At any age, companionship, it seems, is no longer enough of a draw on its own.

There’s a phrase I often use in therapy with couples: “competing needs.” What do partners do when they have needs that directly conflict with those of their spouses? What if both have to work on the same weekend or be out of town at the same time? Who goes to the school play or compromises without feeling resentful? It used to be that husbands and wives operated largely in their own spheres with so little overlap that these questions rarely came up. But women now make up almost half of the U.S. labor force, and 23 percent of married mothers with children under 18 have a higher income than their husbands. In fact, total income is higher in families in which the woman is the primary breadwinner. When a 2010 study of business-school graduates asked, “What is success to you?” surprisingly, it was more women than men who chose “career goals,” while more men than women picked “personal growth.”

As Sheryl Sandberg encourages women to “lean in” — by which she means that they should make a determined effort to push forward in their careers — it may seem as if women are truly becoming, as Gloria Steinem put it, “the men we want to marry.” But these professional shifts seem to influence marital stability. A study put out last year by the National Bureau of Economic Research shows that if a wife earns more than her husband, the couple are 15 percent less likely to report that their marriage is very happy; 32 percent more likely to report marital troubles in the past year; and 46 percent more likely to have discussed separating in the past year. Similarly, Lynn Prince Cooke found that though sharing breadwinning and household duties decreases the likelihood of divorce, that’s true only up to a point. If a wife earns more than her husband, the risk of divorce increases. Interestingly, Cooke’s study shows that the predicted risk of divorce is lowest when the husband does 40 percent of the housework and the wife earns 40 percent of the income.

Ian Kerner, a sexuality counselor and the author of “She Comes First,” sees couples struggle to find a ratio that works. “I work a lot with stay-at-home dads and men who work from home,” he said, “and one thing I hear a lot is that in theory they’re really happy balancing flexible work with stay-at-home responsibilities, while their wives are out working full-time in corporate jobs. But at the same time, a common complaint is that Mom comes home and feels guilty for being away all day, and so much time has to be made up connecting with the children, who take first priority, that these dads feel lost in the mix.” In many couples, Kerner says, the wives start to feel disgruntled because their husbands get to see more of the kids, and the husbands, whose wives are controlling more of the spending, start to feel “financially emasculated.” Sometimes, he says, a vicious cycle begins: The husband feels marginalized and less self-confident, which causes the wife to lose respect for and desexualize him. Under these circumstances, neither is particularly interested in sex with the other.