  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 co-active 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 co-active 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.
