Civic honesty is essential to social capital and economic development but is often in conflict with material self-interest. We examine the trade-off between honesty and self-interest using field experiments in 355 cities spanning 40 countries around the globe. In these experiments, we turned in more than 17,000 lost wallets containing varying amounts of money at public and private institutions and measured whether recipients contacted the owners to return the wallets. In virtually all countries, citizens were more likely to return wallets that contained more money. Neither nonexperts nor professional economists were able to predict this result. Additional data suggest that our main findings can be explained by a combination of altruistic concerns and an aversion to viewing oneself as a thief, both of which increase with the material benefits of dishonesty.