The Limits of Ex Post Implementation Revisited

Existence of ex post incentive compatible mechanisms in models with multi-dimensional signals and interdependent values is possible when there are no consumption externalities. An ex post incentive compatible mechanism is constructed in a model where a single indivisible object is allocated among several buyers with multi-dimensional information and interdependent values. It is the combination of informational externalities (i.e., interdependent values) and consumption externalities, rather than informational externalities alone, that leads to non-existence of ex post equilibrium when agents have multi-dimensional signals. As ex post equilibrium has been employed mostly in models with private goods, this is not a significant limitation.