The impossibility of unbiased judgment aggregation (updated version)

Abstract

All existing impossibility theorems on judgment aggregation over logically connected propositions have one of two restrictions: they either use a controversial systematicity condition or apply only to special agendas of propositions with rich logical connections. An important open question is whether judgment aggregation faces any serious impossibilities without these restrictions. Here we prove the first impossibility theorem without systematicity that applies to all standard agendas: there exists no judgment aggregation rule satisfying universal domain, collective rationality, anonymity and a new condition called unbiasedness. For many agendas, anonymity can be weakened. Applied illustratively to (strict) preference aggregation represented in the judgment aggregation model, our result implies that every unbiased social welfare function with universal domain depends only on a single individual.judgment aggregation, discursive dilemma, formal logics, impossibility theorem, unbiasedness, systematicity, agendas