The racial disparities in school-discipline rates are well-known, as are the damaging effects that harsh disciplinary policies can have on school climates. Less clear is whether—and if so, how —these tendencies contribute to the race-based achievement gap, a problem so entrenched and pervasive that discussing it is almost cliché.

The achievement gap has narrowed since researchers started paying attention to it in the 1960s, but not by much. Myriad factors, many of them out of schools’ control, have stymied efforts to narrow it. Kids of color are less likely to have access to early-childhood education, which puts them at a disadvantage by the time they start kindergarten. They’re more likely to live in poverty and face socioeconomic barriers to success throughout their K-12 trajectories. Their parents tend to have lower educational attainment, meaning they may have less of the social and cultural capital needed to navigate the school system.

But a recent study published in the journal Social Problems concludes that differences by race in suspensions—over which schools do have some control—could be a key reason so many black children fall behind their white peers. In fact, as the study points out, progress on closing the achievement gap leveled off in 1990. And (coincidence or not), it was around that same time that schools started ramping up their disciplinary practices and, in many cases, embracing zero-tolerance tactics.

The study—which was authored by Edward Morris, a sociologist at the University of Kentucky, and Brea Perry, a sociologist at Indiana University—concludes that school suspensions account for roughly one-fifth of the white-black achievement gap. “Particularly for African American students in our data, the unequal suspension rate is one of the most important factors hindering academic progress and maintaining the racial gap in achievement,” Morris and Berry write, describing discipline patterns as an example of “hidden inequality embedded within routine educational practices.” During the 2011-12 school year, black children accounted for 16 percent of the U.S. student population but 32 percent of the students suspended and 42 percent of those expelled, according to Education Department data; nationwide, black students are suspended at roughly three times the rate as their white counterparts.

Suspensions and expulsions take a toll on student achievement in various ways, some of them obvious. For one, they take students out of school, which can easily hinder their academic progress. For another, they often weaken school bonds, disengaging children from their teachers and learning. Other research, also by Morris and Perry, has suggested that high suspension rates can undermine student achievement as a whole, even for children who weren’t personally suspended.

Still, little empirical evidence exists to demonstrate a direct link between suspensions and achievement—to show that the suspensions themselves are responsible for lower performance. While Morris and Perry’s latest study can’t prove that there’s a causal relationship between suspension and achievement, it does use longitudinal data to show a strong connection between the unequal suspension rates and the persistence of the race-based achievement gap.   

The researchers studied a sample of a little over 16,000 male and female middle- and high-schoolers at 17 campuses in Kentucky over a three-year period beginning in 2008. Most of the students included (59 percent) were white, while 25 percent were black and 10 percent were Latino. While the focus on Kentucky means the sample isn’t nationally representative, Morris and Perry argue that it’s “reasonably representative” of the Southeastern U.S., a region where exclusionary discipline is especially common.

The researchers found that, even within a single school, black students were six times as likely to be suspended as white students. One could argue the likelihood a child is disciplined in school depends more on things like income and family structure than it does on race. And in some cases, it does: The elevated rates of discipline among Latino students included in Morris and Perry’s study, for example, are “entirely explained by this group’s lower levels of socioeconomic status.” However, even after controlling for income—as well as other relevant factors, including gender and participation in special education—black students were suspended at three times the rate as their white peers, suggesting that race itself plays an important role in the discipline rates. Separate research, meanwhile, has found racial discrepancies in how discipline is administered, including for similar offenses.

These trends appear to play out in academic patterns. The researchers’ findings indicate that income level, family structure, and gender “only partially” explain the differences in reading-and-math achievement between black students who’ve been suspended and their classmates who haven’t. “Suspension does have meaningful and lasting adverse effects over time independent of early disparities between ever- and never-suspended students,” Morris and Perry write. Children who experienced one early suspension began three points behind their non-suspended peers in math and, after two years, had slipped to a nine-point deficit, according to their research. “We find that the effects of suspension are long lasting,” the researchers write, “setting into motion a trajectory of poor performance that continues in subsequent years, even if a student is not suspended again.”

Ultimately, the researchers suggest that the act of suspending a child who’s already at risk of underperforming academically could prove devastating. The students included in the sample who had experienced suspensions over two years on average began eight points behind their never-suspended peers in math. By the end of the three-year study period, the difference had grown to 11 points. This, according to Morris and Perry, suggests that such discipline puts already-at-risk students at even greater risk of “academic decline.”