Small classes in the elementary grades have been shown to boost students' academic performance. However, researchers continue to seek a consistent, integrated explanation of "why" small classes have positive effects. This article forwards the hypothesis that when class sizes are reduced, major changes occur in students' engagement in the classroom. Engagement is composed of "learning behavior" and pro- and antisocial behavior. Both are highly related to academic performance. We first review research on the relationship between class size and student engagement. Second, we review sociological and psychological theory about the behavior of individuals in groups to explain how student behavior can be affected by changes in class size. Both theory and empirical findings support our hypothesis, although additional research is required. High-priority questions needing further research are identified in the conclusion.