Compatible Group Decisions

Abstract

Individuals may draw different conclusions from the same information. For example, members of a jury may disagree on the verdict even though each member possesses the same information regarding the case under discussion. This happens because individuals can hold different reasonable positions based on the information they share. The field of judgment aggregation studies how individual positions on the same information can be aggregated into a collective one. After a gentle introduction to judgment aggregation, I will offer an analysis of judgment aggregation problems using an argumentation approach. One of the principles of argumentation theory is that an argumentation framework can have several labellings. If the information the group shares is represented by an argumentation framework, and each agent’s reasonable position is a labelling of that argumentation framework, the question becomes how to aggregate the individual positions into a collective one. Whereas judgment aggregation focuses on the observation that the aggregation of individual logically consistent judgments may lead to an inconsistent group outcome, I will present an approach that not only ensures collective rationality but also social outcomes that are ‘compatible’with the individuals’ evaluations. This ensures that no individual member has to become committed to a group position that is in conflict with his own individual position. (Part of my presentation will be based on a joint work with Martin Caminada and Mikolaj Podlaszewski.)