  While online social media offers a way for ignored or stifled voices to be
heard, it also allows users a platform to spread hateful speech. Such speech
usually originates in fringe communities, yet it can spill over into mainstream
channels. In this paper, we measure the impact of joining fringe hateful
communities in terms of hate speech propagated to the rest of the social
network. We leverage data from Reddit to assess the effect of joining one type
of echo chamber: a digital community of like-minded users exhibiting hateful
behavior. We measure members' usage of hate speech outside the studied
community before and after they become active participants. Using Interrupted
Time Series (ITS) analysis as a causal inference method, we gauge the spillover
effect, in which hateful language from within a certain community can spread
outside that community by using the level of out-of-community hate word usage
as a proxy for learned hate. We investigate four different Reddit
sub-communities (subreddits) covering three areas of hate speech: racism,
misogyny and fat-shaming. In all three cases we find an increase in hate speech
outside the originating community, implying that joining such community leads
to a spread of hate speech throughout the platform. Moreover, users are found
to pick up this new hateful speech for months after initially joining the
community. We show that the harmful speech does not remain contained within the
community. Our results provide new evidence of the harmful effects of echo
chambers and the potential benefit of moderating them to reduce adoption of
hateful speech.
