  Online communities often overlap and coexist, despite incongruent norms and
approaches to content moderation. When communities diverge, decentralized and
federated communities may pursue group-level sanctions, including defederation
(disconnection) to block communication between members of specific communities.
We investigate the effects of defederation in the context of the Fediverse, a
set of decentralized, interconnected social networks with independent
governance. Mastodon and Pleroma, the most popular software powering the
Fediverse, allow administrators on one server to defederate from another. We
use a difference-in-differences approach and matched controls to estimate the
effects of defederation events on participation and message toxicity among
affected members of the blocked and blocking servers. We find that defederation
causes a drop in activity for accounts on the blocked servers, but not on the
blocking servers. Also, we find no evidence of an effect of defederation on
message toxicity.
