Implementation in Weakly Undominated Strategies: Optimality of Second-Price Auction and Posted-Price Mechanism

We study the mechanism design problem of guaranteeing desirable performances whenever agents are rational in the sense of not playing weakly dominated strategies. We first provide an upper bound for the best performance we can guarantee among all feasible mechanisms. The bound is represented as the maximized value of the designer's objective subject to the inequality version of the standard envelope incentive conditions. We then prove the bound to be tight under certain conditions on the designer's prior over the agents' pay-off types in auction and bilateral-trade applications. In private-value auction and bilateral trade, the optimal mechanisms (a second-price auction and posted-price mechanism, respectively) satisfy dominant-strategy incentive compatibility, the classical notion of "robust" mechanisms. In an interdependent-value auction, we find that a second-price auction is optimal in revenue with interdependent values, which is neither dominant-strategy nor ex post incentive compatible, but satisfies the novel incentive compatibility introduced in this analysis.