Judgment-aggregation theory has always focused on the attainment of rational collective judgments. But so far, rationality has been understood in static terms: as “coherence” of judgments at a given time, understood as consistency, completeness, and/or deductive closure. By contrast, this paper discusses whether collective judgments can be dynamically rational, so that they change rationally in response to new information. Formally, a judgment aggregation rule is dynamically rational with respect to a given revision operator if, whenever all individuals revise their judgments in light of some information (a learnt proposition), then the new aggregate judgments are the old ones revised in light of this information, i.e., aggregation and revision commute. We prove a general impossibility theorem: if the propositions on the agenda are sufficiently interconnected, no judgment aggregation rule with standard properties is dynamically rational with respect to any revision operator satisfying some mild conditions (familiar from belief revision theory). Our theorem is the dynamic-rationality analogue of some well-known impossibility theorems for static rationality. We also explore how dynamic rationality might be achieved by relaxing some of the conditions on the aggregation rule and/or the revision operator.
Christian List is Professor of Philosophy and Decision Theory at LMU Munich and Co-Director of the Munich Center for Mathematical Philosophy. He is also a Visiting Professor in Philosophy at the London School of Economics and a Fellow of the British Academy. He works at the intersection of philosophy, economics, and political science, with a particular focus on individual and collective decision-making and the nature of intentional agency. He was awarded the 2020 Joseph B. Gittler Award of the American Philosophical Association for his book Why Free Will is Real (Harvard University Press, 2019) and the 2010 Social Choice and Welfare Prize (jointly with Franz Dietrich) for his work on judgment aggregation. His previous book was Group Agency: The Possibility, Design, and Status of Corporate Agents (with Philip Pettit, Oxford University Press 2011). More information can be found on his webpage at: http://christianlist.net