For some game-theoretic solution concepts, such as dominant strategies, Nash equilibrium, and undominated strategies, only dictatorial social choice functions are implementable on a full domain of preferences with at least three alternatives. For other solution concepts, such as the iterative removal of weakly dominated strategies, undominated Nash equilibrium, and maximin, it is possible to implement non-dictatorial social choice functions. Which aspects of solution concepts accounts for these differences? We answer this question by providing a characterization of solution concepts which lead to impossibility results.

This paper identifies several social choice correspondences which are or are not fully implementable in economic environments when agents are incompletely informed about the environment. We show that in contrast to results in the case of complete information, neither efficient allocations nor core allocations define implementable social choice correspondences. We also identify conditions under which the Rational Expectations Equilibrium correspondence is implementable. We extend the concepts of fair allocations and Lindahl allocations to economies with incomplete information, and show that envy-free allocations and Lindahl allocations are implementable under some conditions while fair allocations are not.

In this paper, we analyze optimal contracts in an infinitely repeated agency model in which both the principal and agent discount the future. We show that there is a stationary representation of the optimal contract when the agent's conditional discounted expected utility is used as a state variable. This representation reduces the multi-period problem to a static variational problem which can be analyzed using standard variational techniques. This reduction is used to obtain several properties of the contract.

The main result of this paper is that the multiple equilibrium problem in mechanism design can be avoided in private-value models if agents do not use weakly dominated strategies in equilibrium. We show that in such settings, any incentive-compatible allocation rule can be made the unique equilibrium outcome to a mechanism. We derive a general necessary condition for unique implementation that implies that the positive result for private-value models applies with considerably less generality to common-value settings.