Every year on Equal Pay Day, we hear that women in America make about 80 cents for every dollar men earn. For many women of color, the number is lower — black women make about 61 cents on the dollar compared with men, while Latina women make 53 cents.

But matters are actually worse than any of these numbers would suggest, according to a 2018 report by the Institute for Women’s Policy Research (IWPR), a think tank that looks at public policy through the lens of gender. Measures of the pay gap typically compare the wages of men and women working full time in a given year, as Emily Peck notes at HuffPost. But women are more likely to drop out of full-time work to take care of children or other family members.

To account for this, the report’s authors looked at women’s earnings across a 15-year period, and compared those with men’s. What they found was a pay gap nearly twice as big as what’s traditionally reported: averaged out over 15 years, women made just 49 cents for every dollar men made.

Men lost income when they dropped out of the labor force too, of course. But women were far more likely to drop out, and when they did, the wage penalty was more severe. Those inequalities, the report argues, are key to understanding the wage gap — and, ultimately, closing it.

Traditional measures of the pay gap miss the big picture

“The commonly used figure to describe the gender wage ratio—that a woman earns 80 cents for every dollar earned by a man—understates the pay inequality problem by leaving many women workers out of the picture,” authors Stephen J. Rose, a labor economist and fellow at the Urban Institute, and Heidi I. Hartmann, the founder of IWPR and an economist in residence at American University, argue in their report, titled “Still a Man’s Labor Market.” Specifically, it leaves out women who have dropped out of the labor force temporarily, often to care for family.

To include those women, Rose and Hartmann tracked the earnings of individual women and men over three 15-year periods: 1968 to 1982, 1983 to 1997, and 2001 to 2015. They used data from the Panel Study on Income Dynamics, which tracks a nationally representative sample of Americans over many years. For their analysis, Rose and Hartmann included everyone who earned money in at least one of the three time periods.

The gaps they found were stark: Between 1968 and 1982, women’s earnings were 19 percent of men’s. Between 1983 and 1997, they rose to 38 percent. Between 2001 and 2015, they rose again, but not by as much: Over that 15-year period, women made 49 percent of what men made.

The gaps are so big in large part because women were much more likely to take time off work. Between 2001 and 2015, 43 percent of women had a year with no earnings, while only 23 percent of men did.

Women also tended to suffer a greater wage penalty for taking time out of the labor force. The effect didn’t start right away: Women and men who took a year off work both made about 39 percent less in the years they did work than their counterparts who never took time out. But women who took four or more years off work made 65 percent less than women who never took time out; men who took the same amount of time, meanwhile, made 57 percent less than men who worked straight through.

The pay gap is also a “care chasm”

One big reason women drop out of the labor force at higher rates than men is that they’re far more likely to be the primary caregivers for children or sick relatives. In a 2013 Pew survey, 27 percent of mothers said they’d quit their job at some point to care for a family member; only 10 percent of fathers said the same.

In a 2016 Pew survey, meanwhile, women and men were about equally likely to say they’d recently taken time off to care for a sick family member, but women had taken slightly more time off than men. And women who did take leave were much more likely than men to say they were the primary caregiver for their sick relative.

Another way to look at these gender differences is to compare men and women who leave the labor force. In a 2004 study, just 2.4 percent of men between 20 and 64 who weren’t working cited care for family members as the reason, compared with almost 39 percent of non-working women, Nicholas Eberstadt, an economist at the American Enterprise Institute, wrote earlier this year — and the numbers have changed only slightly since then. Eberstadt calls this gap the “care chasm.”

Rose and Hartmann’s work drives home the fact that, as Vox’s Sarah Kliff has written, the gender wage gap is really a child care penalty — or, perhaps, just a care penalty, with women more likely to shoulder primary care responsibility for all family members, not just children.

Some women, just like some men, choose to take time off work to care for children or other relatives, and some find this care deeply rewarding. But whether they want to or not, women are more likely than men to face the expectation that they’ll step up to provide necessary care for a new baby or an ailing parent — in the 2016 Pew survey, 59 percent of women said family caregiving responsibilities fell mainly on women. (Interestingly, only 29 percent of men agreed.)

Meanwhile, paid family leave for many American workers is infinitesimal or nonexistent, meaning that for those who do need to take care of a family member, there’s often no choice but to quit work. And the fact that women face a steeper penalty than men for taking significant time off suggests that there’s more at work in the 15-year earnings gap than just women’s choices.

As Rose and Hartmann note, paid family leave, available and used by both men and women, would likely help close the gap. So would subsidies for child care and elder care. As Peck points out at HuffPost, there are also less intuitive solutions, like raising wages to give people more incentive to work.

Whatever the solutions are, though, Rose and Hartmann’s report is a reminder that when we think about policies to close the pay gap, we can’t just look at what working men and women are making in a given year. We have to look at the men and women who aren’t working, why they’re not working, and how that affects their careers over the long term.