Roughly 50 years since the advent of the sexual revolution, we're still trying to find our way in a radically altered world. Our confusion fuels a seemingly endless, sometimes rancorous conversation about sex, family, gender roles at home and at the office, and the so-called work-life balance. The latest contribution to that conversation, Sheryl Sandberg's much-hyped book Lean In: Women, Work, and the Will to Lead, encourages women to stop holding themselves back in their careers. Instead, they should, well, "lean in," take charge, get noticed, do whatever they need to do to reach top-level positions in the working world.

It's useful advice — but only up to a point. Yes, ambitious women do continue to face countless social, cultural, and institutional obstacles that men don't have to contend with. And yes, more women might reach the pinnacle of power if they fought more single-mindedly to overcome, and ultimately tear down, those obstacles. Yet all the rousing pep talks in the world aren't going to eliminate the ambivalence that so often leads women to hold themselves back in the workplace. That ambivalence will begin to dissipate only when men begin to take on more of it for themselves.

The ambivalence about work that weighs on so many women is a product of history. Until the sexual revolution, the norm in family life was a division of labor: Men worked outside the home earning an income while women raised the kids and ran the household. Generally speaking, husbands and wives — fathers and mothers — understood what was expected of them and fulfilled those expectations without question.

Today things are different. In many families, the old division of labor has been scrambled but not overthrown entirely. Most men still work full time outside the home, but so do many women — though they're often still responsible for much more of the parenting and housework than their husbands. It is primarily this "second shift" that leads women to feel so much internal division about their choices and priorities.

What's the solution? To begin with, women need to accept that, strictly speaking, there is no "solution." Talk about women "having it all" is delusional. No one can — at least not if "all" means working a high-powered job and being a fully devoted parent (which these days often demands not only being physically present and emotionally available to children but also knitting booties, cooking healthful meals, shuttling kids to an endless string of "enriching" activities and play dates, and so on). The highest-powered jobs require enormous amounts of time and attention. Parenting also requires enormous amounts of time and attention. There are only so many hours in a day. Something has to give.

What should it be? One option is to give up having kids to focus entirely on career — which increasing numbers of women are doing. Another (when it's financially feasible) involves a return to the division of labor, though this time with interchangeable roles: In some families, the wife and mother will choose to stay home; in others, the husband and father will do so.

But there is an another option. Men could finally man up and take on the same trade-offs between parenting and career that so often weigh on women alone — and women could begin demanding that they do, making the expectation clear prior to marriage. In the second decade of the twenty-first century, there is simply no reason why it should be assumed that either spouse will automatically and by default be responsible for anything — including changing diapers, helping with homework, shopping for groceries, making dinner, doing laundry, washing dishes, taking out trash, caring for kids when they're sick — that isn't directly tied to physical necessity. And let's be honest: Carrying and giving birth to children is really the only thing inextricably linked to such necessity.

(Our breastfeeding fetishists would have us believe that nourishing newborns and infants should fall exclusively to the mother, too, with a partial exception for other caregivers to bottle-feed breast milk that the mother has pumped. But it's far from clear that women should abide by this advice. Formula feeding, which was widely practiced in the middle decades of the twentieth century, resulted in no public health crisis. It gets a bad rap today primarily for moral, not medical, reasons.)

A thoroughly modern marriage in which both parents work and strive to share domestic responsibilities can be challenging — and not only because reining in both spouses' loftiest career ambitions will have negative consequences on a family's earnings. If the more patriarchal form of marriage that prevailed prior to the sexual revolution was often unjust, an egalitarian marriage can resemble an interminable series of chores and negotiations, with all of the tedium and tension that this implies. At its worst, it can feel like both spouses work crushingly long swing shifts at a child-care facility.

But at its best, a marriage that truly seeks to fulfill the sexual revolution's promise of equality between the sexes can be a joy unlike any other — with both parents taking pleasure in the complementary satisfactions of work and family life, and suffering through the burdens of ambivalence, as genuine partners.