“Skeptics who deem the 77% estimate too optimistic also note that the figure only counts women working full-time (35 hours a week or more, for the full year) and doesn’t account for the fact that women are far more likely to take time off to start a family or work part-time while rearing one. Over a period of 15 years, according to a 2004 study by the Institute for Women’s Policy Research (IWPR), a full 52% of women in the prime earning age range of 26 to 59 go through at least one full calendar year earning nothing at all, compared to just 16% of men.”