An undominated mechanism for a class of informed principal problems with common values

In a class of informed principal problems with common values, we define iteratively a particular allocation which we call the assured allocation. It is comparatively easy to calculate and straightforward to interpret. It always exists, is unique and continuous in the priors. It is undominated, i.e. efficient among the different types of the principal subject to the agent's interim participation constraint. It is a perfect Bayesian equilibrium of the three-stage game in Myerson [16] and Maskin and Tirole [14]. It dominates the RSW allocation as defined in Maskin and Tirole [14] and coincides with it when the latter is undominated. It is the unique neutral optimum as defined in Myerson [16] when there are only two types. When the assured allocation is separating, then it is a neutral optimum with three or more types. It is an equilibrium of a game of competition in a market with adverse selection.