Before revision:  Given that the online monetary gift (also known as a "red packet") in WeChat groups is randomly split between group members, we are able to conduct a natural experiment to identify the causal effect of receiving online red packets on the contagion of gift exchange. Analyzing gifting behavior using a large-scale dataset of 3.4 million WeChatusers, we find that recipients on average repay 18.29\% of the amount they receive. Moreover, our analysis shows that recipients' contagion may be driven by different types of reciprocity. We further find thatthe "luckiest draw" recipients repay 1.5 times more than other recipients, and identify a group norm that luckiest draw recipients should send payment back to the group .
After revision:  Given that the online monetary gift (also known as a "red packet") in WeChat groups is randomly split between group members, we are able to conduct a natural experiment to identify the causal effect of receiving online red packets sent within 174,131 chat groups on WeChat, one of the largest social network services worldwide. 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. Moreover, this effect is much stronger for "luckiest draw" recipients repay 1.5 times more than other recipients, and identify a group norm that luckiest draw recipients should send payment back to the group .