Gifts are important instruments for forming bonds in interpersonal relationships. Our study analyzes the phenomenon of gift contagion in online groups. Gift contagion encourages social bonds by prompting further gifts; it may also promote group interaction and solidarity. Using data on 36 million online red packet gifts on a large social site in East Asia, we leverage a natural experimental design to identify the social contagion of gift giving in online groups. Our natural experiment is enabled by the randomization of the gift amount allocation algorithm on the platform, which addresses the common challenge of causal identifications in observational data. Our study provides evidence of gift contagion: on average, receiving one additional dollar causes a recipient to send 18 cents back to the group within the subsequent 24 hours. Decomposing this effect, we find that it is mainly driven by the extensive margin -- more recipients are triggered to send red packets. Moreover, we find that this effect is stronger for "luckiest draw" recipients, suggesting the presence of a group norm regarding the next red packet sender. Finally, we investigate the moderating effects of group- and individual-level social network characteristics on gift contagion as well as the causal impact of receiving gifts on group network structure. Our study has implications for promoting group dynamics and designing marketing strategies for product adoption.

翻译：礼物是人际交往中建立纽带的重要工具。本研究分析了线上群体中的礼物传染现象。礼物传染通过促进进一步送礼来增强社交纽带，并可能促进群体互动与凝聚力。我们基于东亚某大型社交平台上3600万次在线红包赠予数据，利用自然实验设计识别了在线群体中赠礼行为的社交传染效应。该自然实验得益于平台对红包金额分配算法的随机化机制，有效解决了观察性数据中因果识别这一常见挑战。研究证实了礼物传染效应：平均而言，接收方每额外获得1美元，会在后续24小时内向群体回赠18美分。通过分解该效应，我们发现其主要源于广延边际——更多接收者被触发发送红包。此外，该效应对"手气最佳"接收者更为显著，表明群体中存在关于下一位红包发送者的规范。最后，我们考察了群体层面与个体层面社交网络特征对礼物传染的调节作用，以及接收礼物对群体网络结构的因果影响。本研究对促进群体动态及设计产品采纳营销策略具有启示意义。