Before revision:  Our natural experiment is enabled by WeChat's random gift amount algorithm, with which the amount that a user receives does not depend on her own attributes. We find that, on average, receiving one more dollar causes a recipient to send 18 cents back to the group within the subsequent 24 hours.
After revision:  Our natural experiment is enabled by the randomization of the gift amount allocation algorithm on WeChat, which addresses the common challenge of causal identifications in observational data. Our study provides evidence of gift contagion: on average, receiving one more dollar causes a recipient to send 18 cents back to the group within the subsequent 24 hours.