The gender wage gap isn't about women's choices. It's about how we value their work.

Every April, advocates "celebrate" Equal Pay Day — which represents the extra four months the average US woman would have to work if she wanted to earn as much as the average man did just in the previous year.

That's based on the average gender wage gap between male and female full-time workers, which currently stands at about 79 cents on the dollar for women. And it's actually a less rosy figure for women of color in particular: Black women make 67 cents on the dollar compared with white men on average, and Hispanic women make 61 cents. So their symbolic "Equal Pay Days" are still both months away.

Critics are quick to dismiss that number, and even call the gender wage gap a "myth," because it doesn't control for demographic information. They also say women's "choices" are largely to blame for the gap — women have more family care responsibilities and place a higher priority on flexible work schedules and part-time work, for instance, and that work often just pays less.

Some of this is true, but none of it is particularly useful if we want to think about how to help women earn more.

First of all, researchers have found that about 40 percent of the gap can't be explained by factors like occupation, education, or experience. Discrimination — whether it's intentional or not, and whether it's on a systemic level or an individual one — likely plays a role.

Second, even neutral-sounding factors like "occupation" and "education" actually work against women when it comes to income.

Research by the Institute for Women's Policy Research (IWPR) has found that out of the 119 occupations that we have full-time weekly earnings data for, women face at least a 5 percent wage gap in 111 of them. Women make more than men in only four occupations.

Occupations dominated by women also pay substantially less than occupations dominated by men, and that lower pay affects both men and women. Workers in female-dominated occupations make just 66 percent of what workers in male-dominated occupations make — which is a lot worse than the 79-cent average gap.

More or better education doesn't help. Women invest more in their educations, according to IWPR, but they get a worse return on that investment than men.

Librarians, for instance, are 82 percent women, and they have to get a master's degree. But computer support specialists, just 27 percent of whom are women, make more and only need an associate's degree. And the skills required to do the job are pretty similar; that's why IWPR suggests that tech companies could start closing their gender gap by actively recruiting and training women who work in different occupations but have comparable skill sets, like librarians.

IWPR also looked at "middle-skill" jobs that don't require a bachelor's degree, since about a third of job openings are "good" middle-skill jobs that pay more than $35,000 a year. But while more women work in middle-skill jobs than men, men dominate the jobs that actually pay well.

About half of the gender wage gap can be explained by "occupational segregation" — the striking tendency of many industries to be very heavily dominated by one gender or the other. Of the top 20 most common occupations for men and for women, only four overlap both genders.

Just look at health care, said Ariane Hegewisch, IWPR's program director of employment and earnings. The health care sector is heavily woman-dominated, even though most doctors are men.

Doctors, nurses, and technicians who work in health care have to learn a lot about science and technology to do their jobs. Yet, Hegewisch said, the Bureau of Labor Statistics defines "STEM jobs" (science, technology, engineering, and math) as male-dominated, high-paying fields like engineering and IT — but female-dominated, and lower-paying on average, health care jobs are labeled as "STEM-related."

"It shows something about how we value different jobs," Hegewisch said.

Occupational segregation also reveals how much, or rather how little, we value what's commonly thought of as "women's work" — child or elder care, teaching, and so on. Child care workers make poverty-level wages — perhaps in part because we as a society are so used to women taking care of children for free that we don't think we should have to pay people much to do it professionally.

But we need people to do this work, Hegewisch said. We should focus more on making these jobs better, and better-paid, not just shrug and say women could make more if they picked a different career path. "We need child care workers, we need teachers, we need care assistants," Hegewisch said. "We shouldn't want all of them to become engineers or IT people."

That means not paying people less for "women's work," and it also means promoting policies that help women stay and thrive in the professional workforce.

"Women work, but they still do more care work, for kids as well as elders and spouses," said Hegewisch. "And we have really made extraordinarily little progress on proper infrastructure for working families in the last ten years." That means policies like paid family leave and child care, which Hegewisch says could help keep women's earnings up after they have children.

So on the one hand, the wage gap is not just about which occupations women "choose," because almost all occupations still pay women less than men overall.

And on the other, it may be true that women often "choose" lower-paying occupations than men — but those choices are often tied to things like gender stereotypes, social pressure, and how well certain industries accommodate family responsibilities.

The "79 cents on the dollar" statistic doesn't mean that every woman makes 21 percent less than every man.

But it's a good shorthand for talking about all the complicated factors that cause women to earn less: systemically lower-paying jobs, fewer hours, fewer promotions, more social pressure to take time off for care work, and so on. It's an imperfect, yet still meaningful, representation of how and why women still aren’t treated as equals in the workplace. And it helps us talk about the issue, and keep talking about it, which is the small but essential first step to doing something about it.