Essentially, it is because women have choices about blending work, leisure, and home life which are not available to men.

Warren Farrell said in The Myth of Male Power

  •  Work full time
  • Care for children/family at home
  • Some blend of the two

Men have a different range of choices:

  • Work full time
  • Work full time
  • Work full time

This is part of an article by Kay S. Hymowitz in City Journal:

“Women still earn on average only about 75 cents for every dollar a man earns. That’s a huge discrepancy.”

Let’s begin by unpacking that 75-cent statistic, which actually varies from 75 to about 81, depending on the year and the study. The figure is based on the average earnings of full-time, year-round (FTYR) workers, usually defined as those who work 35 hours a week or more.

But consider the mischief contained in that “or more.” It makes the full-time category embrace everyone from a clerk who arrives at her desk at 9 am and leaves promptly at 4 pm to a trial lawyer who eats dinner four nights a week—and lunch on weekends—at his desk. I assume, in this case, that the clerk is a woman and the lawyer a man for the simple reason that—and here is an average that proofers rarely mention—full-time men work more hours than full-time women do. In 2007, according to the Bureau of Labor Statistics, 27 percent of male full-time workers had workweeks of 41 or more hours, compared with 15 percent of female full-time workers; meanwhile, just 4 percent of full-time men worked 35 to 39 hours a week, while 12 percent of women did. Since FTYR men work more than FTYR women do, it shouldn’t be surprising that the men, on average, earn more.

The way proofers finesse “full-time” can be a wonder to behold. Take a recent article in the Washington Post by Mariko Chang, author of a forthcoming book on the wealth gap between women and men. Chang cites a wage difference between “full-time” male and female pharmacists to show how “even when they work in the same occupation, men earn more.” A moment’s Googling led me to a 2001 study in the Journal of the American Pharmacists Association concluding that male pharmacists worked 44.1 hours a week, on average, while females worked 37.2 hours. That study is a bit dated, but it’s a good guess that things haven’t changed much in the last decade. According to a 2009 article in the American Journal of Pharmaceutical Education, female pharmacists’ preference for reduced work hours is enough to lead to an industry labor shortage.

The other arena of mischief contained in the 75-cent statistic lies in the seemingly harmless term “occupation.” Everyone knows that a CEO makes more than a secretary and that a computer scientist makes more than a nurse. And most people wouldn’t be shocked to hear that secretaries and nurses are likely to be women, while CEOs and computer scientists are likely to be men. That obviously explains much of the wage gap.

But proofers often make the claim that women earn less than men doing the exact same job. They can’t possibly know that. The Labor Department’s occupational categories can be so large that a woman could drive a truck through them. Among “physicians and surgeons,” for example, women make only 64.2 percent of what men make. Outrageous, right? Not if you consider that there are dozens of specialties in medicine: some, like cardiac surgery, require years of extra training, grueling hours, and life-and-death procedures; others, like pediatrics, are less demanding and consequently less highly rewarded. Only 16 percent of surgeons, but a full 50 percent of pediatricians, are women. So the statement that female doctors make only 64.2 percent of what men make is really on the order of a tautology, much like saying that a surgeon working 50 hours a week makes significantly more than a pediatrician working 37. …

In the literature on the pay gap and in the media more generally, this state of affairs typically leads to cries of injustice. The presumption is that women pursue reduced or flexible hours because men refuse to take equal responsibility for the children and because the United States does not have “family-friendly policies.” Child care is frequently described as a burden to women, a patriarchal imposition on their ambitions, and a source of profound inequity. But is this attitude accurate? Do women want to be working more, if only the kids—and their useless husbands—would let them? And do we know that more government support would enable them to do so and close the wage gap?

Actually, there is no evidence for either of these propositions. If women work fewer hours than men do, it appears to be because they want it that way. About two-thirds of the part-time workforce in the United States is female. According to a 2007 Pew Research survey, only 21 percent of working mothers with minor children want to be in the office full-time. Sixty percent say that they would prefer to work part-time, and 19 percent would like to give up their jobs altogether. For working fathers, the numbers are reversed: 72 percent want to work full-time and 12 percent part-time.

Inappropriate discrimination in the workplace should stop. But the gender wage gap will never stop, because men and women are different, and choose different things. This is not a sign of systemic injustice against women, but of the freedom women have to make those choices.

Knowing this should be empowering for women who want to earn more. There is nothing in ‘the system’ which works against them. They just have to make the same choices men do.