The research is drawn from a longitudinal study of the school experiences of 553 black and white students in a racially diverse, middle-class, and suburban unidentified district. That study, the Early Adolescent Development Study, collected detailed self-reported surveys during the 1996-97 school year from children ages 8 through 12 in grades three through five: 61 percent white, 39 percent black, with equal numbers of male and female students.

It’s a notable data set for a couple reasons, Cappella said, emphasizing that in the age range studied “children still form most of their friendships in classrooms and in schools. That was the case in 1996, and that's still the case in 2016.” The data in the Early Adolescent Development Study is also particularly useful for analyzing interracial friendships because it was conducted in a school district that at the time had relatively low levels of tracking and high levels of integration—an unusual combination—facilitating an analysis of factors such as cross-racial friendships. Further, because the composition of the class and the actual teacher didn’t change, “if there were changes in cross- and same-race friendships [during] that year, we can isolate the effect [to] some aspect of that classroom.”

After calculating the racial composition of the students’ classes, the study’s authors used an index to measure how many same-race friendships would be expected if friendships were randomly distributed. Despite the district’s high level of racial integration, researchers found that the number of same-race friends grew for both black and white children over the school year, with white and older students showing the largest increases.

In the fall of the third grade, black students had 15 percent fewer same-race friendships and white students had 2 percent more same-race friends than would be expected by random chance. By the spring, black third-graders had 5 percent fewer same-race friendships than would be expected by random chance and white third-graders had 6 percent more. Among fifth-graders, black students started out with 2 percent more same-race friends than expected, and white students started out with 23 percent more. By year-end, fifth-grade black students had 10 percent more friends of the same race than expected and white students had 33 percent more.

As the argument goes and studies prove, children of all backgrounds benefit from diversified classrooms and schools where they can interact with peers of different races and ethnicities. Teaching Tolerance, an educational project of the Southern Poverty Law Center, concluded in a comprehensive review of research on racial and ethnic diversity in schools that “a racially integrated student body is necessary to obtain cross-racial understanding, which may lead to a reduction of harmful stereotypes and bias.” But access to diversity is only the first step, not the destination, said Cappella, noting that the study points to the need for teachers to create classrooms where interracial friendships can develop and grow.