Klaus Nehring

The structure of strategy-proof social choice -- Part I: General characterization and possibility results on median spaces

Abstract We define a general notion of single-peaked preferences based on abstract betweenness relations. Special cases are the classical example of single-peaked preferences on a line, the separable preferences on the hypercube, the “multi-dimensionally single-peaked” preferences on the product of lines, but also the unrestricted preference domain. Generalizing and unifying the existing literature, we show that a social choice function is strategy-proof on a sufficiently rich domain of generalized single-peaked preferences if and only if it takes the form of voting by issues (“voting by committees”) satisfying a simple condition called the “Intersection Property.” Based on the Intersection Property, we show that the class of preference domains associated with “median spaces” gives rise to the strongest possibility results; in particular, we show that the existence of strategy-proof social choice rules that are non-dictatorial and neutral requires an underlying median space. A space is a median space if, for every triple of elements, there is a fourth element that is between each pair of the triple; numerous examples are given (some well-known, some novel), and the structure of median spaces and the associated preference domains is analysed.

Arrow’s theorem as a corollary

Abstract Arrow’s Impossibility Theorem is derived from a general result on social aggregation in ‘property spaces’ (S3 convex structures) obtained in prior work. In the derivation, the specific structure of Arrowian aggregation as an aggregation of weak orders plays a purely combinatorial role.

Consistent judgement aggregation: the truth-functional case

Generalizing the celebrated “discursive dilemma”, we analyze judgement aggregation problems in which a group of agents independently votes on a set of complex propositions (the “conclusions”) and on a set of “premises” by which the conclusions are truth-functionally determined. We show that for conclusion- and premise-based aggregation rules to be mutually consistent, the aggregation must always be “oligarchic”, that is: unanimous within a subset of agents, and typically even be dictatorial. We characterize exactly when consistent non-dictatorial (or anonymous) aggregation rules exist, allowing for arbitrary conclusions and arbitrary interdependencies among premises.

Efficient and strategy-proof voting rules: A characterization

Abstract The paper provides a characterization of all efficient and strategy-proof voting mechanisms on a large class of preference domains, the class of all generalized single-peaked domains. It is shown that a strategy-proof voting mechanism on such a domain is efficient if and only if it satisfies a weak neutrality condition and is either almost dictatorial, or defined on a median space of dimension less than or equal to two. In more than two dimensions, weakly neutral voting mechanisms are still “locally” efficient.

Monotonicity implies generalized strategy-proofness for correspondences

Abstract. We show that Maskin monotone social choice correspondences on sufficiently rich domains satisfy a generalized strategy-proofness property, thus generalizing Muller and Satterthwaite’s (1977) theorem to correspondences. The result is interpreted as a possibility theorem on the dominant-strategy implementability of monotone SCCs via set-valued mechanisms for agents who are completely ignorant about the finally selected outcome. Alternatively, the result yields a partial characterization of the restrictions entailed by Nash implementability of correspondences.

Maximal elements of non-binary choice functions on compact sets

Abstract We give sufficient conditions for the existence of maximal elements on compact sets for ‘contraction consistent’ (but possibly non-binary) choice functions. Our result generalizes the Bergstrom-Walker theorem on the existence of maximal elements with acyclic preference relations.

The veil of public ignorance

Abstract A theory of cooperative choice under incomplete information is developed in which agents possess private information at the time of contracting and have agreed on a utilitarian “standard of evaluation” governing choices under complete information. The task is to extend this standard to situations of incomplete information. Our first main result generalizes Harsanyis (J. Polit. Econ. 63 (1955) 309) classical result to situations of incomplete information, assuming that group preferences satisfy Bayesian Coherence and Interim Pareto Dominance. These axioms are mutually compatible if and only if a common prior exists. We argue that this result partly resolves the impossibility of Bayesian preference aggregation under complete information.

The Condorcet set: Majority voting over interconnected propositions

Judgement aggregation is a model of social choice in which the space of social alternatives is the set of consistent evaluations (‘views’) on a family of logically interconnected propositions, or yes/no issues. However, simply complying with the majority opinion in each issue often yields a logically inconsistent collective view. Thus, we consider the Condorcet set: the set of logically consistent views which agree with the majority on a maximal subset of issues. The elements of this set turn out to be exactly those that can be obtained through sequential majority voting, according to which issues are sequentially decided by simple majority unless earlier choices logically force the opposite decision. We investigate the size and structure of the Condorcet set for several important classes of judgement aggregation problems. While the Condorcet set verifies a version of McKelveys (1979) celebrated ‘chaos theorem’ in a number of contexts, in others it is shown to be very regular and well-behaved.

Diversity and dissimilarity in lines and hierarchies

Abstract Within the multi-attribute framework of Nehring and Puppe [Econometrica, 70 (2002) 1155], hierarchies and lines represent the simplest and most fundamental models of diversity. In both cases, the diversity of any set can be recursively determined from the pairwise dissimilarities between its elements. The present paper characterizes the restrictions on the dissimilarity metric entailed by the two models. In the hierarchical case, this generalizes a classical result on the representation of ultrametric distance functions.

On Stalnaker's Notion of Strong Rationalizability and Nash Equilibrium in Perfect Information Games

Counterexamples to two results by Stalnaker (Theory and Decision, 1994) are given and a corrected version of one of the two results is proved. Stalnakers proposed results are: (1) if at the true state of an epistemic model of a perfect information game there is common belief in the rationality of every player and common belief that no player has false beliefs (he calls this joint condition ‘strong rationalizability’), then the true (or actual) strategy profile is path equivalent to a Nash equilibrium; (2) in a normal-form game a strategy profile is strongly rationalizable if and only if it belongs to C∞ , the set of profiles that survive the iterative deletion of inferior profiles.