Why do women still earn less than men? No seriously, why? The gender wage gap has been a problem ever since  I Love Lucy was a primetime hit, and we still don’t have a complete explanation.

“Because sexist bosses,” is the knee-jerk explanation, but it’s actually far more complicated than that.

New research out this week from the Institute for Women’s Policy Research, a feminist think-tank, reveals some surprising, and dismaying, trends that perpetuate this disparity.

Women who worked full time in 2013 made 82.1 percent as much as their male counterparts, an increase of about one percent from last year, the  IWPR reported .

That’s a major improvement from a few decades ago. In 1980, for example, women made just 63.9 percent what men did—$565 a week (in 2013 dollars) to men’s $885. The gender wage gap shrank rapidly from there because women started to earn more and more each year. Last year, women made $706 a week on average.

Men’s earnings, meanwhile, “fell, rose, fell, and rose again,” the organization writes, and ultimately stagnated. Last year they made $860—less than they earned in 1980. That is to say, the gender wage gap persists not because men keep earning more, but because women’s wages aren’t rising fast enough.

IWPR

So why aren’t women’s earnings growing at a faster clip? There are a few theories on this that regularly bounce around the Feminist Econ-Research Blogosphere (FERB).

One cause might be discrimination: Bosses who pay female workers less than they pay equivalent male workers for the same work and the same hours, because these bosses have somehow missed every memo ever.

But women’s wages grew every year until 2004, and they’ve declined slightly since then. Discrimination might be part of the problem, but it can’t explain the wage gap entirely because it’s unlikely that it’s become measurably more severe since 2004.

Another reason, as  my former colleague and honorary FERB member Jordan Weissmann pointed out , is the tendency of some industries to reward workers for putting in longer hours. These types of jobs might penalize women for taking maternity leave, for example, or for leaving the office right at 5 each day to pick up their kids from daycare. But not every employer watches the time clock so hawkishly—in fact Weissmann reports that female and male pharmacists, for example, make roughly the same per hour even when the women work fewer hours. So this “whoever burns the midnight oil gets a bonus” penalty can’t explain the entire gap, either.

Instead, there’s  yet another trend behind the wage gap. It could be that not enough women are taking “male” jobs—things like construction, computer programming, and civil engineering (none of which require anything particularly manly to succeed). Women, meanwhile, tend to disproportionately become kindergarten teachers and registered nurses. This chart from IWPR shows how pronounced this division can be:

IWPR

The problem? The male jobs pay more—way more. The median annual wage for computer programmers was $74,280 in 2012, according to the Bureau of Labor Statistics, and it was $53,400 for kindergarten teachers.

Now, within the FERB, there is also this idea called the “Index of Dissimilarity.” It basically just shows how many women would need to change fields in order to make all workplaces in America have about a 50/50 gender split. The Index had been decreasing steadily since the 1970s—more women have been joining traditionally male professions—but since 2000, it has stagnated.

IWPR

According to IWPR’s calculations, at this point just over half of women would have to change jobs to make fields like “brogineering” and “brogramming” resemble the gender composition of the real world.

“If women could get into male occupations, their wages would go up,” IWPR president Heidi Hartmann told me.

What’s driving this, according to the group’s research, is the growth in male-dominated professions. That is to say, jobs aren’t getting any more gender segregated, but gender-segregated jobs are becoming more common. (How often do you hear about a shortage of programmers, rather than librarians?)

Some people take this as yet another argument for urging more women to join higher-paying professions in technology and science. Others don’t think the Index of Dissimilarity is such a big deal—if women want to enter lower-paying fields, after all, why not let them?

And there’s another contingent that thinks the problem is that female jobs pay less in the first place. Years ago, the so-called ” comparable worth ” movement sought to rectify the gender wage gap primarily by boosting the salaries of female-dominated professions, with the idea that these jobs have historically earned less not because they provide less value but because they were performed by women. These days, such corrective efforts are limited mainly to the public sector and unionized professions.

The trouble arises when the choice to make less money isn’t entirely voluntary. It’s fine if a woman becomes a social worker because she wants to, but not if it’s because something, or someone, along the way tells her she can’t hack it as a scientist.