Why Women Make Less Than Men

First, the Atlantic magazine announced "the end of men." Then a Time cover story in March proclaimed that women are becoming "the richer sex." Now a Pew Research Center report tells us that young women have become more likely than young men to say that a high-paying career is very important to them. Are we really in the midst of what Pew calls a "gender reversal?"

One stubborn fact of the labor market argues against the idea. That is the gender-hours gap, close cousin of the gender-wage gap. Most people have heard that full-time working American women earn only 77 cents for every dollar earned by men. Yet these numbers don't take into account the actual number of hours worked. And it turns out that women work fewer hours than men.

The Labor Department defines full-time as 35 hours a week or more, and the "or more" is far more likely to refer to male workers than to female ones. According to the department, almost 55% of workers logging more than 35 hours a week are men. In 2007, 25% of men working full-time jobs had workweeks of 41 or more hours, compared with 14% of female full-time workers. In other words, the famous gender-wage gap is to a considerable degree a gender-hours gap.

The main reason that women spend less time at work than men—and that women are unlikely to be the richer sex—is obvious: children. Today, childless 20-something women do earn more than their male peers. But most are likely to cut back their hours after they have kids, giving men the hours, and income, advantage.

One study by the American Association for University Women looked at women who graduated from college in 1992-93 and found that 23% of those who had become mothers were out of the workforce in 2003; another 17% were working part-time. Fewer than 2% of fathers fell into those categories. Another study, of M.B.A. graduates from Chicago's Booth School, discovered that only half of women with children were working full-time 10 years after graduation, compared with 95% of men.

Women, in fact, make up two-thirds of America's part-time workforce. A just-released report from the New York Federal Reserve has even found that "opting-out" by midcareer college-educated wives, especially those with wealthy husbands, has been increasing over the past 20 years.

Activists tend to offer two solutions for this state of affairs. First is that fathers should take equal responsibility for child care. After all, while men have tripled the number of hours they're in charge of the kids since 1970, women still put in more hours on the domestic front. But even if we could put a magic potion in the nation's water supply and turn 50% of men into Mr. Mom, that still leaves the growing number of women with no father in the house. Over 40% of American children are now born to unmarried women. A significant number—though not a majority—are living with their child's father at birth. But in the next few years when those couples break up, which is what studies show they tend to do, guess who will be left minding the kids?

Which brings us to the second proposed solution for the hours gap: generous family-leave and child-care policies. Sweden and Iceland are frequently held up as models in this regard, and they do have some of the most extensive paternity and maternity leave and publicly funded child care in the world.

Yet even they also have a persistent hours and wage gap. In both countries, mothers still take more time off than fathers after the baby arrives. When they do go back to work, they're on the job for fewer hours. Iceland's income gap is a yawning 38%—that is, the average women earns only 62 cents to a man's dollar. Even Sweden's 15% gap—though lower than our 23% one—is far from full parity.

All over the developed world women make up the large majority of the part-time workforce, and surveys suggest they want it that way. According to the Netherlands Institute for Social Research, in 2008 only 4% of the 70% of Dutch women who worked part-time wished they had a full-time job. A British Household Panel Survey interviewing 3,800 couples discovered that among British women, the happiest were those working part-time.

A 2007 Pew Research survey came up with similar results for American women: Among working mothers with minor children, 60% said they would prefer to work part-time, while only 21% wanted to be in the office full-time (and 19% said they'd like to give up their job altogether). How about working fathers? Only 12% would choose part-time and 70% wanted to be full-time.

Some counter that the hours gap would shrink if employers offered more family-friendly policies, such as flexible hours and easier on-off ramps for moving in and out of the workforce. We don't know if there is a way to design workplaces so that women would work more or men would work less or both. What we do know is that no one, anywhere, has yet figured out how to do it. Which means that for the foreseeable future, at least when it comes to income, women will remain the second sex.

Ms. Hymowitz is a fellow at the Manhattan Institute and the author, most recently, of "Manning Up: How the Rise of Women Has Turned Men Into Boys," just published in paperback by Basic Books.

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