Perfect Bayesian implementation

Summary We study Social Choice Sets (SCS) implementable as perfect Bayesian equilibria of some incomplete information extensive form game. We provide a necessary condition which we callcondition . The condition is analogous tocondition C that Moore and Repullo [1988] show to be necessary for subgame perfect implementation in games of complete information, and it is weaker than the Bayesian Monotonicity condition stated in Jackson [1991]. Our first theorem establishes that Incentive Compatibility, Closure and Condition are necessary for implementation.Our second theorem establishes sufficient conditions. We show that any SCS which satisfies Incentive Compatibility, Closure and a condition called Sequential Monotonicity No Veto (SMNV) is implementable. SMNV is similar in spirit but weaker than the Monotonicity No Veto condition stated in Jackson [1991]. It is also similar to a combination of condition and No Veto Power, which Abreau and Sen show to be sufficient for implementation in subgame perfect equilibrium.This paper is a revised version of a chapter of my dissertation at Stanford University, Graduate School of Business. I would like to thank, without implicating, Faruk Gul, Bob Wilson and especially John Roberts for their constant advice and encouragement. I would also like to thank participants at the 1993 Summer Meeting of the Econometric Society and a referee for comments leading to substantial improvement in the paper. Financial support from Bocconi University is gratefully acknowledged.