We assess the predictive power of survey measures of social capital with a new behavioral data set that examines whether citizens report a lost wallet to its owner. Using data from more than 17,000 “lost” wallets across 40 countries, we find that survey measures of social capital—especially questions concerning generalized trust or generalized morality — are strongly and significantly correlated with country-level differences in wallet reporting rates. A second finding is that lost wallet reporting rates predict unique variation in the outputs of social capital, such as economic development and government effectiveness, not captured by existing measures.