  Fringe communities promoting conspiracy theories and extremist ideologies
have thrived on mainstream platforms, raising questions about the mechanisms
driving their growth. Here, we hypothesize and study a possible mechanism: new
members may be recruited through fringe-interactions: the exchange of comments
between members and non-members of fringe communities. We apply text-based
causal inference techniques to study the impact of fringe-interactions on the
growth of three prominent fringe communities on Reddit: r/Incel,
r/GenderCritical, and r/The_Donald. Our results indicate that
fringe-interactions attract new members to fringe communities. Users who
receive these interactions are up to 4.2 percentage points (pp) more likely to
join fringe communities than similar, matched users who do not.
  This effect is influenced by 1) the characteristics of communities where the
interaction happens (e.g., left vs. right-leaning communities) and 2) the
language used in the interactions. Interactions using toxic language have a 5pp
higher chance of attracting newcomers to fringe communities than non-toxic
interactions. We find no effect when repeating this analysis by replacing
fringe (r/Incel, r/GenderCritical, and r/The_Donald) with non-fringe
communities (r/climatechange, r/NBA, r/leagueoflegends), suggesting this growth
mechanism is specific to fringe communities. Overall, our findings suggest that
curtailing fringe-interactions may reduce the growth of fringe communities on
mainstream platforms.
