So why do women work fewer hours, choose less demanding jobs, and then earn less than men do? The answer is obvious: kids. A number of researchers have found that if you consider only childless women, the wage gap disappears. June O’Neill, an economist who has probably studied wage gaps as much as anyone alive, has found that single, childless women make about 8 percent more than single, childless men do (though the advantage vanishes when you factor in education). Using Census Bureau data of pay levels in 147 of the nation’s 150 largest cities, the research firm Reach Advisors recently showed that single, childless working women under 30 earned 8 percent more than their male counterparts did…

Here’s what the authors found: right after graduation, men and women had nearly identical earnings and working hours. Over the next ten years, however, women fell way behind. Survey questions revealed three reasons for this. First and least important, men had taken more finance courses and received better grades in those courses, while women had taken more marketing classes. Second, women had more career interruptions. Third and most important, mothers worked fewer hours. “The careers of MBA mothers slow down substantially within a few years of first birth,” the authors wrote. Though 90 percent of women were employed full-time and year-round immediately following graduation, that was the case with only 80 percent five years out, 70 percent nine years out, and 62 percent ten or more years out—and only about half of women with children were working full-time ten years after graduation. By contrast, almost all the male grads were working full-time and year-round. Furthermore, MBA mothers, especially those with higher-earning spouses, “actively chose” family-friendly workplaces that would allow them to avoid long hours, even if it meant lowering their chances to climb the greasy pole…

So it makes no sense to think of either the mommy track or the resulting wage differential as an injustice to women. Less time at work, whether in the form of part-time jobs or fewer full-time hours, is what many women want and what those who can afford it tend to choose. Feminists can object till the Singularity arrives that women are “socialized” to think that they have to be the primary parent. But after decades of feminism and Nordic engineering, the continuing female tropism toward shorter work hours suggests that that view is either false or irrelevant. Even the determined Swedes haven’t been able to get women to stick around the office.