  Previous research has documented the existence of both online echo chambers
and hostile intergroup interactions. In this paper, we explore the relationship
between these two phenomena by studying the activity of 5.97M Reddit users and
421M comments posted over 13 years. We examine whether users who are more
engaged in echo chambers are more hostile when they comment on other
communities. We then create a typology of relationships between political
communities based on whether their users are toxic to each other, whether echo
chamber-like engagement with these communities is associated with polarization,
and on the communities' political leanings. We observe both the echo chamber
and hostile intergroup interaction phenomena, but neither holds universally
across communities. Contrary to popular belief, we find that polarizing and
toxic speech is more dominant between communities on the same, rather than
opposing, sides of the political spectrum, especially on the left; however,
this mainly points to the collective targeting of political outgroups.
