The communication cost of selfishness: ex post implementation

We consider the communication complexity of implementing a given decision rule when the protocol must also calculate payments to motivate the agents to be honest in an ex post equilibrium (agents’payo¤s are assumed to be quasi-linear in such payments). We …nd that the communication cost of sel…shness when measured with the average-case communication complexity may be arbitrarily large. For the worst-case communication complexity measure, we provide an exponential upper bound on the communication cost of sel…shness. Whether this bound is ever achieved remains an open question. We examine several special cases in which the communication cost of sel…shness proves to be very low. These include cases where we want to implement e¢ ciency or where we have only two agents, and the precision of agents’utilities is …xed.