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The gender pay gap is standard measure of women's economic inequality. At the dawn of second-wave feminism, it was 59 cents: women earned 59 cents for every dollar men earned. Today it's up to 77 cents, according to the National Committee on Pay Equity. That's progress, right? Here's even more rosy news: women without children now earn over 90% of men's wages. So maybe it is time to stop worrying about women and economics.

Not so fast. Let's start with the 90% statistic, which describes childless women at age thirty. Conservatives like to point to that one, concluding that what ails mothers is not discrimination but their own choices.

In fact, I have argued, what the 90% statistic really means is that women, if they want equality, should plan to die childless at thirty. Such women have earnings nearly as high as men's because most have not hit either of the two major forms of workplace gender bias.

The single strongest bias is the maternal wall. Motherhood triggers powerful assumptions that mothers are less competent and committed to their jobs. "I had a baby, not a lobotomy," protested a Boston lawyer, voicing the experience of many who find that, upon their return from maternity leave, they are given less work, no work or dead-end assignments. The resulting bias is a powerful drag on women's prospects: mothers are 79% less likely to be hired and 100% less likely to be promoted. They are offered an average of a whopping $11,000 less in salary, and held to higher performance and punctuality standards than men, according to a study by Shelley Correll and co-authors.

Most women at age thirty haven't hit the other major form of gender bias either: the glass ceiling. Glass ceiling bias reflects, first, that qualities associated with leadership--assertiveness, self-confidence, directive behavior -- are linked with masculinity. So women who exhibit them often are seen as socially clueless. To compound the problem, glass ceiling bias also means that women often have to prove themselves over and over again before they are even considered for leadership positions. Contemporary studies by social psychologists show that the glass ceiling is alive and well.

So the claim that fact that childless women at age thirty make nearly as much as men does not prove that women have gained equality. Neither does the gender pay gap. Although it is the standard measure, that statistic grossly overestimates women's economic equality. Why? Because it compares men who work full time with women who work full time. This is an accurate picture of men, but it is an extremely partial description of women. Fully one-quarter of employed women work part time.

The penalties associated with part-time work are an important contributor to women's relative poverty. The penalty for working part time in the U.S. is enormous: seven times as high as in Sweden, and twice as high as in the U.K., according to Janet Gornick and Marcia Meyers. A recent report by the Joint Economic Committee documented that two-thirds of part-timers are women, and that part-timers in sales earn only 58 cents on the dollar, as compared with full timers.

The last time I looked, when one compared all employed women with all employed men, including part-timers as well as full-timers, women only earned 59 cents for every dollar earned by men. Now that's a sobering statistic.

The old-fashioned gender pay gap statistic embeds the assumption that it is somehow "natural" and uncontroversial to impose sharp penalties on those who don't work "full" time. But what, after all, is "full" time? As Alice Kessler-Harris pointed out long ago, its definition has changed a lot. The one thing that has remained constant is that "full time" has always been defined as the amount of time a man typically works.

From the start of the Industrial Revolution until today, men have been able to work more hours than women outside the home because they work fewer hours inside it. And women still do twice as much housework, and four times as much routine housework, as men, according to Suzanne Bianchi and her co-authors. They also do three hours of child care for each hour men do.

Of course, women could just stop changing the diapers, doing the laundry, cooking the meals. But no one wants them to, because that kind of unpaid work is every bit as crucial for sustaining a productive economy as paid work is. So it's time to document, and to challenge, the highly artificial penalty imposed on anyone who does not work a full time schedule. The recent report by the Joint Economic Committee is a good first step. The second crucial step is to change the way we measure the gender pay gap, and to compare employed men and women, rather than restricting the analysis to full-timers. Only then can we get an accurate picture of the yawning gap between the earnings of men and those women.

This piece originally was published in On The Issues:

(c) Joan C. Williams