Affirmative action in college admissions is based on the premise that a diverse student body contributes to interactions among students from different backgrounds, which are in turn positively related to desirable outcomes of college. This study evaluates the merits of this rationale for affirmative action by examining the direct and indirect relationships between student-body diversity and students' gains in understanding people of other racial/ethnic backgrounds. Data from a nationally representative sample of 428 colleges and universities participating in the National Survey of Student Engagement (NSSE) indicated that student-body diversity was indirectly, but not directly, related to gains in understanding people from diverse backgrounds. Results supported the use of affirmative action in college admissions, indicating that student body diversity is directly related to greater interaction among diverse groups, but not the quality of interpersonal relations on campus. Diversity of the student body was indirectly related to gains in understanding diverse groups, acting through informal interactional diversity. (Contains 1 figure and 3 tables.)