A new study of American high school students may be troubling for advocates of racial integration-it finds that the larger a school, the more likely its students are to racially self-segregate instead of forming interracial friendships.

The study, conducted by University of Michigan sociology researchers Siwei Cheng and Yu Xie, suggests that the more options students have in choosing friends, the more likely they are to settle into close friendships with those who have similar racial backgrounds.

Cheng and Xie developed a theoretical computer model that simulated the formation of friendships among individuals in groups of increasing size. The model assumed that people prefer friends of their own race, and incorporated other factors that affect friendship formation like age, education, interests, personality, and cultural background.

They compared the theoretical model to real self-reported friendship data from 4,745 American high school students in grades 7 to 12 at 172 schools, collected during the 1994-1995 school year in the National Longitudinal Study of Adolescent Health. The students had listed up to ten of their closest friends from a school roster, and the researchers analyzed the formation of friendships among four mutually exclusive racial groups: Hispanic, non-Hispanic white, non-Hispanic black, and non-Hispanic Asian.

The results, published today in the Proceedings of the National Academy of Sciences, showed a significant effect of school size on self-segregation.

"We found that total school size had a major effect on the likelihood that students would form interracial friendships," said Xie in a statement. "Large schools promote racial segregation and discourage interracial friendships."

Xie and Cheng acknowledge individual choice in choosing friends. It's not necessarily that students consciously decide to make friends primarily with those who share their racial backgrounds-the structural influence of institution size simply makes it less likely for them to develop strong interracial friendships.

"In a smaller community, the number of potential friends is limited," Xie told USA Today. "It's harder to find a person who meets other criteria you want and of the same race." That makes it more likely for students to make interracial friendships by bonding over other common interests.

"But in a larger school ... it's more likely to find someone who will meet other criteria for a friend plus be of the same race. Race plays a bigger role in a larger community because you can satisfy other criteria, but in a smaller school other factors dominate the decision who is your friend."

The study may be limited by its context, since the represented sample is almost 20 years old and the analysis does not account for mixed-race students, who form an increasingly large proportion of the American population as demographics become more diverse. In addition, the research did not explore student attitudes about their peers of other races, or how the rates of interracial friendships changed over time for these students.

Still, Xie and Cheng believe their results shed light on interracial relations in other social contexts, like dating, marriages, political affiliations, and businesses. In any of these contexts, "size reduces social integration by allowing individuals to fully exercise their preexisting preferences."

In addition, they worry that the Internet might further encourage self-segregation. Even though people can connect to exponentially more people online in our globalized world, they could be more likely to do so in specific contexts like social networks and websites that limit their contact to people like themselves.

Racial integration of schools has long been a goal of American social justice advocates, since students are thought to become more open-minded when taught alongside those from other backgrounds. John Yun, director of the University of California Educational Evaluation Center in Santa Barbara, told Science Now that students in racially integrated schools think more critically as a result of exposure to different perspectives.

Research suggests that students from underprivileged minorities are more likely to perform at a higher academic level, earn higher wages over their lifetimes, and be healthier after attending racially mixed schools.

A 2009 study in the journal Sociology of Education found that interracial friendships in American universities were encouraged by living among students of other races and participating in various extracurricular activities.