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Every day, millions of people write online restaurant reviews, leave product ratings, provide answers to an unknown user’s question, or contribute lines of code to open-source software, all without any direct reward or recognition. People help strangers offline as well, as when people anonymously donate blood or stop to help a stranded motorist, but these behaviors are relatively rare compared to the pervasiveness of online communities based on user-generated content. Why are mutual-help communities far more common online than in traditional offline settings that are not mediated by the Internet? We address this puzzle in two steps. We begin with empirical evidence from an online experiment that tests two mechanisms for the contagion of helping behavior:“generalized reciprocity” and “third-party influence”. We then use an empirically calibrated agent-based model to show how these mechanisms interact with …