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But the numbers don’t tell the full story. Women are more likely to work in lower-paying occupations and leave the workforce when they have children, for example. When such circumstances are factored in — along with race and other demographic data -- about 40 percent of the gender wage gap is still unaccounted for, says Ariane Hegewisch of the Institute for Women’s Policy Research (IWPR), citing a 2007 study by Francine Blau and Lawrence Kahn.

That means that women earn 91 cents for every dollar, as compared to men, due to factors that have nothing to do with life choices — a difference that many researchers identify as discrimination.

Other studies also confirm a similar gap that’s not accounted for by life choices. Among college graduates, women earned 5 percent less than men one year after graduation, and the gap grew to 12 percent among full-time workers a decade after graduation, according to Catherine Hill, director of research at the American Association of University Women, or AAUW.

That said, the “91 cents on the dollar” figure also doesn’t give a full portrait of gender bias in the workforce. If an employer decides to give male employees overtime work because of gender, that wouldn’t be accounted for, Hill points out. Gender norms also affect the career choices that women make, and employers in female-dominated industries may be able to get away with paying less than they would otherwise, says Hegewisch. And fewer employers offer paternity leave than maternity leave, with few state and no federal laws mandating either.

Both AAUW and the IWPR have praised the Paycheck Fairness Act, which they believe could help empower women and pressure employers to close the gender wage gap. In particular, Hill and Hegewisch single out the new transparency requirements in the law, that would make it illegal for employers to retaliate against employees who discuss pay with colleagues. “Currently about half of all workers report that they are either contractually barred from discussing their pay or strongly discouraged, and the proportions are higher in the private sector,” Hill explains. As a result, women might not even realize how they’re being underpaid.