  Counterspeech -- speech that opposes hate speech -- has gained significant
attention recently as a strategy to reduce hate on social media. While previous
studies suggest that counterspeech can somewhat reduce hate speech, little is
known about its effects on participation in online hate communities, nor which
counterspeech tactics reduce harmful behavior. We begin to address these gaps
by identifying 25 large hate communities ("subreddits") within Reddit and
analyzing the effect of counterspeech on newcomers within these communities. We
first construct a new public dataset of carefully annotated counterspeech and
non-counterspeech comments within these subreddits. We use this dataset to
train a state-of-the-art counterspeech detection model. Next, we use matching
to evaluate the causal effects of hostile and non-hostile counterspeech on the
engagement of newcomers in hate subreddits. We find that, while non-hostile
counterspeech is ineffective at keeping users from fully disengaging from these
hate subreddits, a single hostile counterspeech comment substantially reduces
both future likelihood of engagement. While offering nuance to the
understanding of counterspeech efficacy, these results a) leave unanswered the
question of whether hostile counterspeech dissuades newcomers from
participation in online hate writ large, or merely drives them into
less-moderated and more extreme hate communities, and b) raises ethical
considerations about hostile counterspeech, which is both comparatively common
and might exacerbate rather than mitigate the net level of antagonism in
society. These findings underscore the importance of future work to improve
counterspeech tactics and minimize unintended harm.
