A new working paper confirms that women with children earn less in the workforce, despite gains in education and experience over the past 30 years.

It’s no secret that all women are disadvantaged in the U.S. labor market, but mothers face an additional handicap: the “motherhood wage penalty.” This penalty is one of the biggest drivers of gender inequality in the workplace. And even as more women entered the workforce, there was little success in reducing this gap. A new working paper by Eunjung Lee and Joya Misra of University of Massachusetts, Amherst and Marta Murray Close of the U.S. Census Bureau finds that despite mothers’ gains in education and experience, the motherhood pay gap barely budged over the past 30 years.

The growing number of women in the U.S. workforce over this time period reshaped traditional gender and caregiving roles. Women have outnumbered men in college enrollment since the 1970s and now make up close to half the workforce. And in 2016, 70.5 percent of mothers with children younger than 18 were in the labor force. This development benefitted the U.S. economy and families alike. Between 1979 and 2003, women’s paid hours of work boosted Gross Domestic Product by an estimated 11 percent. Over the past three decades, women’s work also kept families afloat during an era in which men’s earnings fell by 9.5 percent.

Despite these strides, women with children earn less than women without children, and earn less than men with children as well. In fact, men experience bigger paychecks after the birth of a child. Other work on this topic zeroes in on the role that gender discrimination plays, and how assumptions around gender and caregiving shape employers’ perceptions of competency. Case in point: Mothers are less likely to be hired, promoted, or paid as much as their male colleagues even when controlling for factors such as hours and occupation.

The motherhood pay penalty is well-established, but researchers have not yet delved in depth into how it has changed over time. Over the past half-century, women-both those with and without children-have made significant advancements in education and labor force experience. It is also plausible that the growing numbers of mothers in the workplace may have reduced employers’ biases against women with children. These factors could mean that even if the motherhood pay penalty remains, it is less than it was in the past.

Unfortunately, this new working paper by Lee, Misra, and Close indicates this not the case. They find that the motherhood wage penalty has remained the same over the roughly 30-year period they studied, and actually gotten worse for mothers with one child. The gap has narrowed slightly for mothers with two or three children, but the gap remains almost the same as it was three decades ago once you control for education and experience

What can policymakers do to reduce or close the motherhood wage penalty? The authors suggest the need for better work-life policies, a conclusion that is backed up by a number of other studies. While other wealthy nations responded to women’s growing labor force participation over the past 30 years by implementing a number of work-family policies, including paid family leave and childcare subsidies, the United States at the federal level has only enacted a law requiring large- and medium-sized employers to offer unpaid family leave.

The authors suggest that it is time for policymakers to focus less on the choices individual women make, given that “changes mothers can make in their human capital investment, as well as their employment patterns, may not be enough to create real change.” Instead, Lee, Misra, and Close conclude that if we hope to lower the motherhood wage penalty, “policies aimed at supporting mothers’ employment may be a necessary next step.”