Online platforms face pressure to keep their communities civil and
respectful. Thus, the bannings of problematic online communities from
mainstream platforms like Reddit and Facebook are often met with enthusiastic
public reactions. However, this policy can lead users to migrate to alternative
fringe platforms with lower moderation standards and where antisocial behaviors
like trolling and harassment are widely accepted. As users of these communities
often remain \ca across mainstream and fringe platforms, antisocial behaviors
may spill over onto the mainstream platform. We study this possible spillover
by analyzing around $70,000$ users from three banned communities that migrated
to fringe platforms: r/The\_Donald, r/GenderCritical, and r/Incels. Using a
difference-in-differences design, we contrast \ca users with matched
counterparts to estimate the causal effect of fringe platform participation on
users' antisocial behavior on Reddit. Our results show that participating in
the fringe communities increases users' toxicity on Reddit (as measured by
Perspective API) and involvement with subreddits similar to the banned
community -- which often also breach platform norms. The effect intensifies
with time and exposure to the fringe platform. In short, we find evidence for a
spillover of antisocial behavior from fringe platforms onto Reddit via
co-participation.

该研究使用差异法设计来研究被 Reddit 和 Facebook 封禁的三个社区迁移到边缘平台的用户，并发现与这些社区共同参与可能导致用户在 Reddit 上的毒性言论和违反平台规范的行为增加，并随着时间和曝光度而加剧。