High concentrations of poverty, not racial segregation, entirely account for the racial achievement gap in U.S. schools, a new study finds.

The research, released Monday, looked at the achievement gap between white students, who tend to have higher scores, and black and Hispanic students, who tend to have lower scores. Researchers with Stanford University wanted to know whether those gaps are driven by widespread segregation in schools or something else.

They found that the gaps were “completely accounted for” by poverty, with students in high-poverty schools performing worse than those from schools with children from wealthier families.

“Racial segregation appears to be harmful because it concentrates minority students in high-poverty schools, which are, on average, less effective than lower-poverty schools,” concluded the paper by academics, led by Sean F. Reardon, professor of poverty and inequality in education and senior fellow at the Stanford Institute for Economic Policy Research.

The study examined scores from hundreds of millions of tests over the past decade by students in thousands of school districts. Researchers found a “very strong link” between racial school segregation and academic achievement gaps. Every school district with “even moderately high” segregation had a large achievement gap, they found.

The reason, they conclude, is because of exposure to poor schoolmates. After controlling for racial differences in school poverty, the study found that segregation no longer predicts achievement gaps.

“Racial segregation matters, therefore, because it concentrates black and Hispanic students in high-poverty schools, not because of the racial composition of their schools, per se,” the study says.

In an interview, Reardon explained that a district such as Atlanta has high racial segregation, with white students in generally wealthier schools than black students, and it also has high racial achievement gaps. But in Detroit, where all the students tend to be poor, the achievement gaps are smaller, he said.

Nonetheless, he concludes that because race and poverty are so closely related, the only way to close the gap is to racially integrate schools. He pointed to those who advocate that schools think less about integration and instead try to improve all schools. That hasn’t worked, he said.

“If you want to be serious about decreasing achievement gaps,” he said, “you have to take on segregation.”

On Monday, Stanford is launching a website that maps test scores and other data for every public school and school district in the United States. The site allows users to explore the relationship between test scores and socioeconomic status and segregation and to compare schools and districts in a state or with other similar places.