  We consider a community of users who must make periodic decisions about
whether to interact with one another. We propose a protocol which allows honest
users to reliably interact with each other, while limiting the damage done by
each malicious or incompetent user. The worst-case cost per user is sublinear
in the average number of interactions per user and is independent of the number
of users. Our guarantee holds simultaneously for every group of honest users.
For example, multiple groups of users with incompatible tastes or preferences
can coexist.
  As a motivating example, we consider a game where players have periodic
opportunities to do one another favors but minimal ability to determine when a
favor was done. In this setting, our protocol achieves nearly optimal
collective welfare while remaining resistant to exploitation.
  Our results also apply to a collaborative filtering setting where users must
make periodic decisions about whether to interact with resources such as movies
or restaurants. In this setting, we guarantee that any set of honest users
achieves a payoff nearly as good as if they had identified the optimal set of
items in advance and then chosen to interact only with resources from that set.
