タイトル：Robust Voting under Uncertainty (with Shmuel Nitzan and Takashi Ui)
This paper proposes a new normative consequentialist criterion for voting rules under Knightian uncertainty about individuals’ preferences to characterize a weighted majority rule (WMR). This criterion, which is referred to as robustness, stresses the significance of responsiveness, i.e., the probability that the social outcome coincides with the realized individual preferences. A voting rule is said to be robust if, for any probability distribution of preferences, it avoids the following worst-case scenario: the responsiveness of every individual is less than one-half. Our main result establishes that a voting rule is robust if and only if it is a WMR without any ties. This characterization of a WMR avoiding the worst possible outcome complements the well-known characterization of a WMR achieving the optimal outcome, i.e., weak efficiency regarding responsiveness. However, robustness is a stronger requirement than weak efficiency because a weakly efficient WMR allows ties with any tie-breaking rule.