Partial Ex-Post Verifiability and Unique Implementation of Social Choice Functions



This study investigates the unique implementation of a social choice function in iterative dominance in the ex-post term. We assume partial ex-post verifiability; that is, after determining an allocation, the central planner can only observe partial information about the state as verifiable. We demonstrate a condition of the state space, termed “full detection,” under which any social choice function is uniquely implementable even if the range of the players’ lies, which the ex-post verifiable information directly detects, is quite narrow. To prove this, we construct a dynamic mechanism according to which each player announces his (or her) private signal before the other players observe this signal at an earlier stage, and each player also announces the state at a later stage. In this construction, we can impose several severe restrictions, such as boundedness, permission of only tiny transfers off the equilibrium path, and no permission of transfers on the equilibrium path. This study does not assume either expected utility or quasi-linearity.