Recent news has been filled with the Equal Pay movement (Hollywood stars and the US Women’s National Soccer team among the most prominent). The statistic oft quoted regarding the gender wage gap is that women working full time in the United States typically are paid just 79 percent of what men were paid, a gap of 21 percent. The gap has narrowed since the 1970s (when it was 59 cents for every dollar earned by a man), due largely to women’s progress in education and workforce participation and to men’s wages rising at a slower rate. But progress has stalled in recent years, and the pay gap does not appear likely to go away on its own.

Equally as concerning is the implications this gap has on the prospects of women in retirement....