Monotone Perfection

Abstract: This paper introduces the concept of perfect monotone equilibrium in Bayesian games, which refines the standard monotone equilibrium by accounting for the possibility of unintended moves (trembling hand) and thereby enhancing robustness to small mistakes. We construct two counterexamples to demonstrate that the commonly used conditions in Athey (2001), McAdams (2003) and Reny (2011) - specifically, the single crossing condition and quasi-supermodularity - are insufficient to guarantee the existence of a perfect monotone equilibrium. Instead, we establish that the stronger conditions of increasing differences and supermodularity are required to ensure equilibrium existence. To illustrate the practical relevance of our findings, we apply our main results to multiunit auctions and further extend the analysis to first-price auctions, all-pay auctions, and Bertrand competitions.