Hannu J. Nurmi

We discuss the findings of the preceding chapters aiming at an overall judgement of the relative merits of the 18 procedures in the light of their (in)vulnerability to various voting paradoxes. No procedure is invulnerable to all the analyzed voting paradoxes, but there are differences in the variety of paradoxes that various procedures are vulnerable to. It turns out that for those emphasizing that a Condorcet Winner ought to be elected when s/he exists, the most plausible voting procedures are associated with the names of Copeland and Kemeny, while for those stressing the need to preserve the basic rationale of voting, viz., the participation condition, the most appealing system is the Borda count.

This chapter focuses on the possibility that some well-known voting procedures lead to specific types of monotonicity paradoxes in preference profiles that are characterized by the presence and election of a Condorcet winner. Moulin’s (Journal of Economic Theory 45:53–64, 1988) theorem establishes the incompatibility of Condorcet-consistency and invulnerability to the No-Show paradox in voting procedures when there are more than three alternatives to be chosen from. We ask whether this conclusion would also hold in the proper subset of profiles distinguished by the property that a Condorcet winner exists and is elected in the initial profile. Our focus is on 20 voting procedures designed to elect a single candidate. These procedures include both Condorcet-consistent and non-consistent rules. The former are, however, only briefly touched upon because their invulnerability to most types of monotonicity violations in the restricted domain is obvious.

This chapter focuses on the possibility that some well-known voting procedures are vulnerable to the Inconsistency paradox even in preference profiles that are characterized by a restricted domain where a Condorcet winner exists and is elected in each disjoint subset of voters but not in their union. Our focus is on 15 voting procedures known to be vulnerable to the Inconsistency paradox in unrestricted domains. These procedures include 10 Condorcet-consistent and 5 Condorcet-non-consistent rules. The former are, however, only briefly touched upon because their invulnerability to the Inconsistency paradox in the restricted domain is obvious.

The relative desirability of a voting procedure is assessed, inter alia, by verifying which axioms, or postulates, it satisfies or violates. One of these axioms is the subset choice condition (SCC). This axiom requires that if a candidate, x, is elected under a given voting procedure, f, in a profile consisting of n voters and k competing candidates (n, k > 1), then x ought to be elected by f also in such profiles over any proper subset of candidates that contain x and that preserve the pairwise preference relations of the original profile. Most known voting procedures violate, in general, this axiom. However, we were interested to find out which voting procedures satisfy or violate this axiom under a restricted domain assumption where a Condorcet winner exists and is elected in the initial profile by the investigated voting procedure. It turns out that, obviously, all conceivable Condorcet-consistent voting procedures satisfy SCC under this restricted domain assumption. However, most known non-Condorcet-consistent procedures continue to violate SCC even under the restricted domain assumption.

The No-Show paradox occurs whenever a group of identically-minded voters is better off abstaining than by voting according to its preferences. Moulin’s (Journal of Economic Theory 45:53–64, 1988) result states that if one wants to exclude the possibility of the No-Show paradox, one has to resort to procedures that do not necessarily elect the Condorcet winner when one exists. This paper examines 10 Condorcet-consistent and 10 Condorcet-non-consistent procedures in a restricted domain, viz., one where there exists a Condorcet winner who is elected in the original profile and the profile is subsequently modified by removing a group of voters with identical preferences. The question asked is whether the No-Show paradox can occur in these settings. It is found that only 2 of the 10 Condorcet-consistent procedures investigated (Minimax and Schwartz’s procedure) are invulnerable to the No-Show paradox, whereas only 3 of the 10 non-Condorcet-consistent ranked procedures investigated (Coombs’s, the Negative Plurality Elimination Rule, and the Majority Judgment procedures) are vulnerable to this paradox in the restricted domain. In other words, for a No-Show paradox to occur when using Condorcet-consistent procedures it is not, in general, necessary that a top Condorcet cycle exists in the original profile, while for this paradox to occur when using (ranked) non-Condorcet-consistent procedures it is, almost always, necessary that the original profile has a top cycle.

Voting is a common way to resolve disagreements regarding policies to be adopted or candidates to be chosen for various positions and is therefore a necessary ingredient of democratic government. Yet there are numerous voting rules that differ from each other in processing the ballots into voting results. In other words, it is possible that for a given set of voters having a fixed distribution of preferences among the competing alternatives, one would obtain the election of a different alternative as a result of using a different voting rule. We focus on the most obvious desiderata associated with voting procedures, viz., the avoidance of paradoxical outcomes.

This chapter concludes the analysis of the 20 voting procedures in terms of 5 voting paradoxes in restricted domains characterized by the existence of a Condorcet winner which at the same time is elected by the procedure under investigation. The restricted domain provides a perspective to how much difference various profile types make in terms of the possibility of encountering a voting paradox. In this analysis we contrast the general (unrestricted) domain with one where the initial outcome is stable. We illustrate the problems involved in the choice of an appropriate procedure by discussing the recent proposal for electoral reform suggested by Maskin and Sen.

Voting paradoxes occur in particular profile domains. For the avoidance of the paradoxes it is therefore important to know if the profiles typically encountered in practice are of such nature that the paradoxes are very unlikely or downright impossible. Ever since the publication of Arrow’s theorem, the role of domain restrictions has been appreciated. However, the earlier studies have mainly focused on conditions for rational collective choices through pairwise majority comparisons. In those studies the single-peaked preferences have been found to be an important type of preference similarity that guarantees complete and transitive collective outcomes. This booklet introduces and analyzes a similar preference restriction, viz., the existence in the profile of a Condorcet winner that is elected by the procedure under study. We examine the possibilities of various voting procedures to end up with voting paradoxes under these restricted domains. Thereby we refine the results that establish the vulnerability of some procedures to various kinds of voting paradoxes.

The (in)vulnerability of six ranked voting procedures which are not Condorcet–consistent (Borda count, Alternative vote, Coombs’ procedure, Bucklin’s procedure, Range Voting and Majority Judgment) to 13 paradoxes is examined in this chapter. For those systems that are vulnerable to some voting paradoxes the vulnerability is demonstrated through illustrative examples showing that there are profiles where the paradoxes in question happen when the respective procedures are in use. And for those systems that are invulnerable to some voting paradoxes the invulnerability is explained.

Responsiveness to electoral opinions is one of the hallmarks of democratic governance. We focus on a particularly strong type of unresponsiveness, viz., one where the complete inversion of all preferences in the electorate is accompanied with no change in the electoral outcome. It is known that the possibility of this extreme type of unresponsiveness, known as the Preference Inversion paradox or Reversal Bias, is associated with many voting rules. We set out to find out whether the paradox can be encountered when using various procedures under a restricted Condorcet domain, viz., one where a Condorcet winner exists and is elected by the procedure under study.

This book deals with 20 voting procedures used or proposed for use in elections resulting in the choice of a single winner. These procedures are evaluated in terms of their ability to avoid five important paradoxes in a restricted domain, viz., when a Condorcet winner exists and is elected in the initial profile. Together with the two companion volumes by the same authors, published by Springer in 2017 and 2018, this book aims at giving a comprehensive overview of the most important advantages and disadvantages of voting procedures thereby assisting decision makers in the choice of a voting procedure that would best suit their purposes.

This book deals with 18 voting procedures used or proposed for use in elections resulting in the choice of a single winner. These procedures are evaluated in terms of their ability to avoid paradoxical outcomes. Together with a companion volume by the same authors, Monotonicity Failures Afflicting Procedures for Electing a Single Candidate, published by Springer in 2017, this book aims at giving a comprehensive overview of the most important advantages and disadvantages of procedures thereby assisting decision makers in the choice of a voting procedure that would best suit their purposes.

The difficulty of making social choices seems to take on two forms: one that is related to both preferences and the method used in aggregating them and one which is related to the preferences only. In the former type the difficulty has to do with the discrepancies of outcomes resulting from various preference aggregation methods and the computation of winners in elections. Some approaches and results which take their motivation from the computability theory are discussed. The latter ‘institution-free’ type of difficulty pertains to solution theory of the voting games. We discuss the relationships between various solution concepts, e.g. uncovered set, Banks set, Copeland winners. Finally rough sets are utilized in an effort to measure the difficulty of making social choices.

It is well-known that different social choice procedures often result in different choice sets. The article focuses on how often this is likely to happen in impartial cultures. The focus is on Borda count, plurality method, max-min method and Copeland's procedure. The probabilities of Condorcet violations of the Borda count and plurality method are also reported. Although blatantly false as a descriptive hypothesis, the impartial culture assumption can be given an interpretation which makes the results obtained in impartial cultures particularly significant, viz. the probabilities of deviations in choice sets indicate how far apart are the intuitions underlying various choice procedures.