Echo chambers amplify cyberbullying across platforms and cultures

Social media platforms play pivotal roles in governing online aggression worldwide. However, cross-cultural investigations on the group-based transmission of cyberbullying and the development of universal mitigation strategies remain limited. In this study, 2.04 billion comments associated with 5.69 million posts and short videos from 1.75 billion users on X, TikTok, Weibo, and Douyin were analyzed. We identify 1,252 echo chambers involving cyberbullying behavior and 11,287 users. Echo chambers are tightly connected online communities in which users with similar views repeatedly reinforce shared perspectives and exclude opposing perspectives. Among the 12,352 users classified as cyberbullies, 61.05% were members of echo chambers. Compared with non-echo chamber contexts, cyberbullying content spreads more in echo chambers, with the maximum cascade depth reaching 6 versus 3 in non-echo chambers. Moreover, content persists longer, with an average of 9.3 versus 4.1 days, and diffuses more rapidly. A small subset of users with the top 1 % centrality contributed 15% of all cyberbullying comments, reflecting a highly uneven influence. These patterns are consistent across English and Chinese platforms, underscoring the cross-cultural generality of group-driven cyberbullying. Our findings reframe cyberbullying as a collective social phenomenon, and we propose culturally adaptable governance strategies, including early detection through network structures, friction-enhancing platform designs, targeted interventions for high-impact users, and the promotion of critical media literacy.