While a large social-choice-theoretic literature discusses the aggregation of individual judgments into collective ones, there is much less formal work on the transformation of judgments in group communication. I develop a model of judgment transformation and prove a baseline impossibility theorem: Any judgment transformation function satisfying some initially plausible conditions is the identity function, under which no opinion change occurs. I identify escape routes from this impossibility and argue that the kind of group communication envisaged by deliberative democats must be (...) "holistic": It must focus on webs of connected propositions, not on one proposition at a time, which echoes the Duhem-Quine "holism thesis" on scientific theory testing. My approach provides a map of the logical space in which different possible group communication processes are located. (shrink)

Several recent results on the aggregation of judgments over logically connected propositions show that, under certain conditions, dictatorships are the only propositionwise aggregation functions generating fully rational (i.e., complete and consistent) collective judgments. A frequently mentioned route to avoid dictatorships is to allow incomplete collective judgments. We show that this route does not lead very far: we obtain oligarchies rather than dictatorships if instead of full rationality we merely require that collective judgments be deductively closed, arguably a minimal condition of (...) rationality, compatible even with empty judgment sets. We derive several characterizations of oligarchies and provide illustrative applications to Arrowian preference aggregation and Kasher and Rubinsteinís group identification problem. (shrink)

Suppose the members of a group (e.g., committee, jury, expert panel) each form a judgment on which worlds in a given set are possible, subject to the constraint that at least one world is possible but not all are. The group seeks to aggregate these individual judgments into a collective judgment, subject to the same constraint. I show that no judgment aggregation rule can solve this problem in accordance with three conditions: “unanimity,” “independence” and “non-dictatorship,” Although the result is a (...) variant of an existing theorem on “group identification” (Kasher and Rubinstein, Logique et Analyse 160:385–395, 1997), the aggregation of judgments on which worlds are possible (or permissible, desirable, etc.) appears not to have been studied yet. The result challenges us to take a stance on which of its conditions to relax. (shrink)

Our aim in this survey article is to provide an accessible overview of some key results and questions in the theory of judgment aggregation. We omit proofs and technical details, focusing instead on concepts and underlying ideas.

In this paper we study collective identity functions that deal with formation of clubs. Usually the choice offered to individuals is to cast a vote in favor of qualification or not, and the final outcome is qualification or non-qualification. In this context we show that independent collective identity functions are naturally characterized by voting rules, and in particular, consent rules can be represented by one single collection of weighted majorities. In addition, we consider the extended model where voters are allowed (...) to abstain and we distinguish between disqualification and a neutral outcome. We show that this environment allows agents to act in different capacities that cannot be distinguished in the original formulation of the problem. (shrink)

A widely held rights-based view asserts that individuals should be permitted to self-designate characteristics such as race and gender. But some argue that there are opinions that oppose the use only of self-designation, and that these should not be ignored. Kasher and Rubinstein (Logique Anal 160:385–395, 1997) demonstrated that the latter view is equivalent to accepting that one or more of five conditions must be violated. This paper extends their analysis to allow for more than two categories, and focuses on (...) simplifying the conditions without jeopardising their conclusion. In particular, we use significantly weaker conditions of independence and of symmetry than are common in other extensions of the original proposition. (shrink)

In this paper, we introduce an Arrovian social choice framework with the process of nomination. We consider a two-stage social choice procedure in which some alternatives are first nominated by aggregating the opinions of nominators, and then the society makes a collective choice from the nominated alternatives by aggregating the preferences of voters. Each nominator’s opinion is a positive, negative, or neutral view as to whether each alternative deserves to be eligible for collective decision making. If a voter is a (...) nominator, his preference space is restricted by his opinion as follows: he always prefers positive alternatives to neutral ones and neutral alternatives to negative ones, according to his opinion. When each nominating voter has such a preference space, we first characterize Arrow-consistent preference domains at the second stage of the social choice framework. Second, we find a resolution of Arrow’s impossibility theorem when at least one nominating voter exists. (shrink)

A Collective Identity Function (CIF) is a rule which aggregates personal opinions on whether an individual belongs to a certain identity into a social decision. A simple CIF is one which can be expressed in terms of winning coalitions. We characterize simple CIFs and explore various CIFs of the literature by exploiting their ability of being expressed in terms of winning coalitions. We also use our setting to introduce conditions that ensure the equal treatment of individuals as voters or as (...) outcomes. (shrink)

Judgment aggregation theory, which concerns the translation of individual judgments on logical propositions into consistent group judgments, has shown that group consistency generally cannot be guaranteed if each proposition is treated independently from the others. Developing the right method of abandoning independence is thus a high-priority goal. However, little work has been done in this area outside of a few simple approaches. To fill the gap, we compare four methods based on distance metrics between judgment sets. The methods generalize the (...) premise-based and sequential priority approaches to judgment aggregation, as well as distance-based preference aggregation. They each guarantee group consistency and implement a range of distinct functions with different properties, broadening the available tools for social choice. A central result is that only one of these methods (not previously considered in the literature) satisfies three attractive properties for all reasonable metrics. (shrink)