Rank-Preserving Multidimensional Mechanisms

We show that the mechanism-design problem for a monopolist selling multiple, heterogeneous objects to a buyer with ex ante symmetric and additive values is equivalent to the mechanism-design problem for a monopolist selling identical objects to a buyer with decreasing marginal values. Symmetric and incentive-compatible mechanisms for heterogeneous objects are rank preserving, i.e., higher-valued objects are assigned with a higher probability. In the identical-objects model, every mechanism is rank preserving. This facilitates the equivalence, which we use in three applications.