When states raise the number of math classes they require students to take in high school, black students complete more math coursework—and boost their earnings as a result. That’s the topline takeaway from new research by Joshua Goodman, an associate professor of public policy at Harvard’s Kennedy School of Government.

To understand the results, it’s helpful to have a little background. During the 1980s, a now-famous report called “A Nation at Risk” by Ronald Reagan’s National Commission on Excellence in Education opened this way:

Our Nation is at risk. Our once unchallenged preeminence in commerce, industry, science, and technological innovation is being overtaken by competitors throughout the world. This report is concerned with only one of the many causes and dimensions of the problem, but it is the one that undergirds American prosperity, security, and civility. We report to the American people that while we can take justifiable pride in what our schools and colleges have historically accomplished and contributed to the United States and the well-being of its people, the educational foundations of our society are presently being eroded by a rising tide of mediocrity that threatens our very future as a Nation and a people. What was unimaginable a generation ago has begun to occur--others are matching and surpassing our educational attainments.

At the time, most states required students to complete just one (and in some cases zero) math course to graduate. None required three. But in reaction to the report, states across the country began revamping their curriculum. By the time the class of 1994 was graduating, 40 states had raised minimum math requirements.

It wasn’t the first time the federal government had pushed states to rethink coursework. Back in the ‘50s, during the heat of the Cold War, the government devoted federal funding to helping states rethink science classes so the country would be better prepared to compete with the Soviets. And in recent years, the Obama administration pushed schools and even the entertainment industry to invest in STEM education, suggesting that students would need more and better science, technology, engineering, and math courses to compete in a global economy and that people with degrees in STEM fields earn more than many of their peers.

Yet Goodman, who used to teach high-school math in Watertown, Massachusetts, said during a phone interview that despite all the talk, there was little causal evidence to link how much math coursework students do with their earnings later on. So he started looking at Census data and high-school transcripts from the 1980s and ‘90s. Ultimately, he found big differences in outcomes for black students—but not for white students—after states raised minimum requirements. Earnings for black students that attended high school in states that had passed reforms went up by more than 3 percent. (Goodman focused on black and white students in part because his calculations relied on assigning students’ state of birth as a proxy for where they attended high school, and Asian and Hispanic students were much more likely to be foreign-born. Data limitations didn’t allow him to identify students who were born outside the U.S., but graduated from American schools.) That was especially true for black students who graduated from public schools with lots of students of color. He found that when states increased math requirements, each additional year of math had a return of about 10 percent, or about half the return to a year of high school.

The findings suggest several things, including that math coursework makes up a good portion of the gains a student makes through a year of schooling. Goodman isn’t suggesting that history and English don’t matter, but he is suggesting that math seems to matter a lot. And, crucially, that closing racial disparities in high-school math could help limit disparities in earnings later on.

It’s worth digging into that last point, and Goodman is up front about the fact that he doesn’t have all the answers, but when states raised math requirements, the increased coursework happened at relatively low levels. In other words, with the updated requirements, students were more likely to complete algebra or geometry, not higher-level courses like calculus. And one of the reasons white students weren’t really affected by curriculum changes is that they were already taking the classes anyway. Black students weren’t.

So why weren’t they? And what does that mean about the expectations that teachers and schools have for black students? Those questions are beyond the scope of the study, but the data suggests that when higher expectations are in place for everyone and students and teachers have fewer choices about which courses to take or push children toward (i.e., kids and school administrators can’t load a schedule with only history and English classes), achievement gaps shrink. And separate research has suggested that students of color tend to have less access to advanced coursework. So even if they wanted to take a calculus class, for instance, it might not be an option.

Interestingly, the reforms didn’t actually increase overall education-attainment levels, meaning students attending school after their states passed reforms weren’t more likely to earn a diploma. But the roughly 3 percent rise in earnings (which is associated with more math coursework and not increases in other subjects) for black students, Goodman said, are tied in part to the fact that those students were then more likely to work in occupations with higher cognitive-skill requirements. (If schools were able to close the racial disparities in more advanced classes like algebra II or calculus, he reasoned, racial gaps in earnings might shrink even further.)

Because of data limitations, Goodman acknowledged that it’s hard to say which state’s reforms were most effective. But the impact of raising course requirements doesn’t appear to be concentrated in one place, or to be tied to economic conditions or other education reforms. Similarly, when states reformed graduation requirements, they typically specified the number of math classes required and not the specific skills students needed, so it’s unclear how much specific curriculum mattered.

Goodman was, however, able to get some sense of whether the increase in earnings that black students saw after graduation as a result of higher state math requirements has been sustained in recent years. He found that the effects seemed to subside significantly as the years wore on. Goodman said that while the later data doesn’t rule out the effect math reforms seem to have on earnings today, it’s possible that students who’d taken more math classes were more valued in the ‘90s labor market than in today’s economy. Other research suggests that the demand for quantitative skills has flatlined somewhat, and that so-called softer skills like the ability to collaborate have increased in importance.

Still, he asserts in the study, the research presents “clear evidence that curricular reform has the potential to help close the racial and socioeconomic earnings gaps that are the focus of so much research and public policy.”