We consider the _free-rider_ problem in peer-to-peer file sharing networks
such as Napster: that individual users are provided with no incentive
for adding value to the network. We examine the design implications of
the assumption that users will selfishly act to maximize their own
rewards, by constructing a formal game theoretic model of the system and
analyzing equilibria of user strategies under several novel payment
mechanisms.  We support and extend this work with results from
experiments with a multi-agent reinforcement learning model.