When applied to groups, the revelation principle postulates a Bayesian-Nash behavior between agents. Their binding agreements are unenforceable or the principal can prevent them at no cost. The authors analyze instead a mechanism design problem in which the agents can communicate between...

In a public good environment with positively correlated types, we characterize optimal mechanisms when agents have private information and can enter collusive agreements. First, we prove a weak-collusion-proof principle according to which there is no restriction for the principal in offering...