Before revision:  To identify gift contagion , we leverage a natural experiment using a large sample of 36 million online red packets sent within 174,131 chat groups on WeChat, one of the largest social network services worldwide .
After revision:  To identify gift contagion , we leverage a natural experiment using a large sample of 36 million online red packet gifts on China's social site WeChat, we leverage a natural experimental design to identify the social contagion of gift giving in online groups .