On the Foundations of Ex Post Incentive-Compatible Mechanisms

This paper considers an interdependent-value robust mechanism design problem, where the principal has little knowledge about the agent's belief. Although ex post incentive-compatible (EPIC) mechanisms can implement allocations without any knowledge about the agent's belief, we show that, under a certain condition (order-reversing interdependence), there exists a non-EPIC mecha-nism that achieves a strictly higher expected revenue than any EPIC mechanism given whatever (admissible) belief structure the agent may enjoy. Conversely, with sufficiently small interdependence, such a non-EPIC mechanism does not exist: for some (admissible) belief structure, an EPIC mechanism achieves the highest expected reve-nue. (JEL D82, D83)