Several years ago, while observing a parenting group in Minnesota, I was struck by a confession one of the women made to her peers: She didn't really care that her husband did the dishes after dinner. Sure, it was swell of him, and she had friends whose husbands did less. But what she really wanted, at that point in her day, was for her husband to volunteer to put the kids to bed. She would have been glad to sit in the kitchen on her own for a few minutes with the water running and her mind wandering. Another woman chimed in: "Totally. The dishes don't talk back to you."

According to the American Time Use Survey—which asks thousands of Americans annually to chronicle how they spend their days—men and women now work roughly the same number of hours a week (though men work more paid hours, and women more unpaid). Given this balanced ledger, one might guess that all would finally be quiet on the domestic front—that women would finally have stopped wondering how they, rather than their husbands, got suckered into such a heavy load. But they haven't. The question is: Why?

Part of the problem is that averages treat all data as if they're the same and therefore combinable, which often results in a kind of absurdity. On average, human beings have half an Adam's apple, but no one thinks to lump men and women together this way. Similarly, we should not assume that men and women's working hours are the same in kind. The fact is, men and women experience their time very differently.

For starters, not all work is created equal. An hour spent on one kind of task is not necessarily the equivalent of an hour spent on another. Take child care, a task to which mothers devote far more hours than dads. It creates much more stress in women than other forms of housework. In "Alone Together" (2007), a comprehensive look at the state of American marriage, the authors found that if women believe child care is unevenly divided in their homes, this imbalance is much more likely to affect their marital happiness than a perceived imbalance in, say, vacuuming.

Or consider night duty. Sustained sleep deprivation, as we know, consigns people to their own special league of misery. But it's generally mothers, rather than fathers, who are halfway down the loonytown freeway to hysterical exhaustion, at least in the early years of parenting. According to the American Time Use Survey, women in dual-earner couples are three times more likely to report interrupted sleep if they have a child under the age of 1, and stay-at-home mothers are six times as likely to get up with their children as are stay-at-home fathers.