Today, Senate Republicans voted down the Paycheck Fairness Act (PFA). The PFA would have strengthened the Equal Pay Act (EPA), making it harder for employers to discriminate by gender when paying their employees. Among other things, the bill, which passed in the House in January of 2009, would have required employers to justify pay disparities between men and women, prevented employers from punishing employees who share information about their salaries, and would have required the Department of Labor to conduct and publicize information about pay disparities.

Despite these benign-seeming proposals, the bill had prominent critics, and, sadly, those critics seem to have prevailed. In a September editorial, The Washington Post claimed that it would “allow employees and courts to intrude too far into core business decisions.” In The New York Times, Christina Hoff Sommers of the American Enterprise Institute wrote that “mountains of research [show] that discrimination plays little role in pay disparities between men and women.” And Diana Furchtgott-Roth of the Hudson Institute criticized the bill for responding to a “false problem,” claiming to debunk the “myth of the 77 cent pay gap.”

It’s true that proponents of wage-gap reform often justify its necessity by citing the somewhat misleading statistic that women are paid only 77 cents for every dollar that a man earns (see Obama’s statement from last July or Valerie Jarrett’s September editorial in the Post). The statistic is misleading not because it’s mythical, but because factors that are unrelated to discrimination contribute to this difference, like the fact that occupations dominated by women—such as nursing or teaching—tend to be lower-paying, or that women often choose to take time off to have children, thus slowing the accumulation of professional experience. These conditions might be unfortunate (it’s not as though women chose to go into these fields because they pay less), but they do not necessarily illustrate injustice.

But, even accounting for legitimate factors for pay inequality, there is significant evidence that women are still paid less than men. For example, consider the finding, in a report published by the American Association of University Women (AAUW), that, just one year after graduating from college, women earn on average only 80 percent of what men earn. The fact that women tend to chose majors that result in lower-paying professions explains some of the difference, but, even among male and female graduates within a single major, women almost always earn less. (History is the only major of the 12 majors included in the report in which female graduates earn more than male graduates.)

Ten years after graduating, after controlling for “job and workplace, demographic and experience, and education and training,” an unexplained pay gap of 12 percent between men and women remains, the AAUW report states. Such unexplained gaps also exist within the federal government. A report issued by the Government Accountability Office found that an 11-cent wage gap existed between men and women in the federal workforce and that 7 cents of that gap was not explicable by education or experience. Even in Furchtgott-Roth’s analysis of wage disparities among different data sets—which she adjusts to incorporate “appropriate” variables that she deemed missing from the data—in no instance were women’s earnings on par with men’s. Is it acceptable that, according to her adjusted analysis, the Bureau of Labor Statistics should report only a 13-cent difference (rather than the stated 20-cent difference)? How about no difference?

And, while the pay gap is shrinking, the rate at which male and female salaries are converging appears to be slowing down. A Cornell study from 2006 found that the wage gap between men and women decreased much faster in the 1980s than in the 1990s. Of course, this sorry fact might be explained by any number of factors unrelated to discrimination—many of which are considered in the paper—but the largest factor, according to the authors, was that, in the ’80s, the “unexplained” gender wage gap fell much faster than in the ’90s.

There is something inherently unsatisfying about legislation justified by an “unexplained” inequality—especially since the laws intended to prevent such inequalities from arising are already on the books. But the very fact that the Equal Pay Act has been around for almost 50 years and women still earn less than men for the same work, should generate support for this act. Just because we can’t explain the problem does not mean that we shouldn’t sharpen the tools that people have to fight it.