New data released Wednesday by the U.S. Census Bureau show a very slim closing of the gender wage gap. The median women's earnings were 79 percent of men's in 2014, up ever so slightly from 78 percent in 2013.

That change is not statistically significant, the Census Bureau said. In fact, not since 2007 has there been any statistically significant narrowing of the wage gap. And even then, the increase was small—one has to go back to the '80s or '90s to see more rapid change.

The new numbers from the Census's Current Population Survey show that the real median earnings for full-time, year-round male workers was $50,383 last year, compared with $39,621 for full-time, year-round women.

Advocates of equal pay legislation were not enthusiastic about the results. "I suppose any increase is better than none, but it's frustrating to keep getting drops in the bucket when what women really need is an equal pay tidal wave," said Lisa Maatz, vice president of government relations for the American Association of University Women in an interview. 

The current gap between men's and women's annual wages is the smallest it has been yet, according to the AAUW. Still, change has been slower than some expected, particularly given that the gender wage gap has been getting increased attention over the past few years from presidential candidates, CEOs and leaders at the state level.

To eliminate—or at least unearth—pay disparities among male and female employees, some companies are embarking on pay audits, getting rid of salary negotiations and even going public with workers' pay. And though legislation at the federal level has long been stalled, California's recently passed Fair Pay Act (an attempt to close loopholes that exist in current law) is seen as the toughest in the nation.

To some extent, of course, the pay gap reflects general differences in employment for men and women. Overall, men tend to work in higher paying professions, take less time away from their careers for caregiving, and work more hours than women do.

Yet economists and other number-crunchers have done research controlling for such factors, and have found that not all of the gap can be explained by these differences. Even one year into their careers, young men and women see disparities in pay.

Other research has shown that the wage gap is, in many ways, really a motherhood gap. While unmarried or childless women's pay is nearly equal to men's, pay for working mothers is just 76 percent of men's. The study, conducted by University of Massachusetts-Amherst professor Michelle Budig, found that gap persisted even after controlling for things like experience, education and hours worked.

In other words, said the AAUW's vice president of research, Catherine Hill, employers' stereotypes about how mothers will approach their jobs could be playing a role. The gender pay gap, she said, is really a reflection of "the choices women make—and the choices other people think we're going to make."

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