  When large social-media platforms allow users to easily form and
self-organize into interest groups, highly related communities can arise. For
example, the Reddit site hosts not just a group called food, but also
HealthyFood, foodhacks, foodporn, and cooking, among others. Are these highly
related communities created for similar classes of reasons (e.g., to focus on a
subtopic, to create a place for allegedly more "high-minded" discourse, etc.)?
How do users allocate attention between such close alternatives when they are
available or emerge over time? Are there different types of relations between
close alternatives such as sharing many users vs. a new community drawing away
members of an older one vs. a splinter group failing to cohere into a viable
separate community? We investigate the interactions between highly related
communities using data from reddit.com consisting of 975M posts and comments
spanning an 8-year period. We identify a set of typical affixes that users
adopt to create highly related communities and build a taxonomy of affixes. One
interesting finding regarding users' behavior is: after a newer community is
created, for several types of highly-related community pairs, users that engage
in a newer community tend to be more active in their original community than
users that do not explore, even when controlling for previous level of
engagement.
