While the positive consequences of social capital and civil society are widely accepted and appreciated, the question of how they originate and can be sustained has received relatively little attention from scholars. In this study, we approach this question from a cross-national and individual-level perspective by examining how population heterogeneity in the form of ethnic and linguistic diversity affects citizenship behavior, measured by cognitive and interpersonal engagement about politics, membership in voluntary associations, and interpersonal trust. Based on data collected in 44 countries, our analyses show that heterogeneity does affect the quality of civil society in a country. However, indicators of population heterogeneity do not have uniformly positive or negative effects on individual-level measures of civil society—while they reduce some, they shore up others.