Aggregation of coarse preferences

We consider weak preference orderings over a set A _n of n alternatives. An individual preference is of refinement?≤n if it first partitions A _n into ? subsets of `tied' alternatives, and then ranks these subsets within a linear ordering. When ?<n, preferences are coarse. It is shown that, if the refinement of preferences does not exceed ?, a super majority rule (within non-abstaining voters) with rate 1− 1/? is necessary and sufficient to rule out Condorcet cycles of any length. It is argued moreover how the coarser the individual preferences, (1) the smaller the rate of super majority necessary to rule out cycles `in probability'; (2) the more probable the pairwise comparisons of alternatives, for any given super majority rule. Received: 29 June 1999/Accepted: 25 February 2000     

Existence of a multicameral core

When a single group uses majority rule to select a set of policies from an n-dimensional compact and convex set, a core generally exists if and only if n = 1. Finding analogous conditions for core existence when an n-dimensional action requires agreement from m groups has been an open problem. This paper provides a solution to this problem by establishing sufficient conditions for core existence and characterizing the location and dimensionality of the core for settings in which voters have Euclidean preferences. The conditions establish that a core may exist in any number of dimensions whenever n ≤ m as long as there is sufficient preference homogeneity within groups and heterogeneity between groups. With m > 1 the core is however generically empty for . These results provide a generalization of the median voter theorem and of non-existence results for contexts of concern to students of multiparty negotiation, comparative politics and international relations.     

The sensitivity of weight selection for scoring rules to profile proximity to single-peaked preferences

Niemi (Am Polit Sci Rev 63:488–497, 1969) proposed a simple measure of the cohesiveness of a group of n voters’ preferences that reflects the proximity of their preferences to single-peakedness. For three-candidate elections, this measure, k, reduces to the minimum number of voters who rank one of the candidates as being least preferred. The current study develops closed form representations for the conditional probability, PASW(n,IAC|k), that all weighted scoring rules will elect the Condorcet winner in an election, given a specified value of k. Results show a very strong relationship between PASW(n,IAC|k) and k, such that the determination of the voting rule to be used in an election becomes significantly less critical relative to the likelihood of electing the Condorcet winner as voters in a society have more structured preferences. As voters’ preferences become more unstructured as measured by their distance from single-peakedness, it becomes much more likely that different voting rules will select different winners.A preliminary version of this paper was presented at the European Public Choice Society Conference in Berlin, Germany, April 15–18, 2004.     

How the size of a coalition affects its chances to influence an election

Since voting rules are prototypes for many aggregation procedures, they also illuminate problems faced by economics and decision sciences. In this paper we are trying to answer the question: How large should a coalition be to have a chance to influence an election? We answer this question for all scoring rules and multistage elimination rules, under the Impartial Anonymous Culture assumption. We show that, when the number of participating agents n tends to infinity, the ratio of voting situations that can be influenced by a coalition of k voters to all voting situations is no greater than $D_{m} \frac{k}{n}Since voting rules are prototypes for many aggregation procedures, they also illuminate problems faced by economics and decision sciences. In this paper we are trying to answer the question: How large should a coalition be to have a chance to influence an election? We answer this question for all scoring rules and multistage elimination rules, under the Impartial Anonymous Culture assumption. We show that, when the number of participating agents n tends to infinity, the ratio of voting situations that can be influenced by a coalition of k voters to all voting situations is no greater than , where D _m is a constant which depends only on the number m of alternatives but not on k and n. Recent results on individual manipulability in three alternative elections show that this estimate is exact for k=1 and m=3.

Arkadii SlinkoEmail:

     

Acyclic sets of linear orders: A progress report

Let f(n) be the maximum cardinality of an acyclic set of linear orders on {1, 2, … , n}. It is known that f(3)=4, f(4)=9, f(5)=20, and that all maximum-cardinality acyclic sets for n≤ 5 are constructed by an “alternating scheme”. We outline a proof that this scheme is optimal for n=6, where f (6)=45. It is known for large n that f (n) >(2.17)ⁿ and that no maximum-cardinality acyclic set conforms to the alternating scheme. Ran Raz recently proved that f (n)<c ⁿ for some c>0 and all n. We conjecture that f (n + m)≤f (n + 1) f (m + 1) for n , m≥ 1, which would imply f (n)<(2.591)^{n − 2} for all large n. Received: 12 April 2000/Accepted: 4 December 2000     

On the manipulability of approval voting and related scoring rules

We characterize all preference profiles at which the approval (voting) rule is manipulable, under three extensions of preferences to sets of candidates: by comparison of worst candidates, best candidates, or by comparison based on stochastic dominance. We perform a similar exercise for k-approval rules, where voters approve of a fixed number k of candidates. These results can be used to compare (k-)approval rules with respect to their manipulability. Analytical results are obtained for the case of two voters, specifically, the values of k for which the k-approval rule is minimally manipulable—has the smallest number of manipulable preference profiles—under the various preference extensions are determined. For the number of voters going to infinity, an asymptotic result is that the k-approval rule with k around half the number of candidates is minimally manipulable among all scoring rules. Further results are obtained by simulation and indicate that k-approval rules may improve on the approval rule as far as manipulability is concerned.     

Finding the majority-rule equilibrium under lexicographic comparison of candidates

This paper considers the well studied problem of the existence of an undominated point, under the assumption of lexicographic preferences of voters, as espoused by Taylor in [24]. We extend Taylor's model to situations were we allow for (i) voters to have different ranings of the issues in n-dimensional issue space and (ii) a candidate to be disregarded by a voter if his stand on any one or more of the issues involved in the election is perceived to be too extreme by the voter and (iii) combinations of (i) and (ii). We extend the results of Taylor by demonstrating the non-existence of an equilibrium point in these models in general and then showing that under special circumstances, specialized variants of the “median” point(s) represent equilibrium or undominated points in these models too. Thus a model of voting behavior results that is closer approximation of reality in that historically incumbents tend to win. The primary conclusion of the paper is to suggest that incumbents tend to have an advantage when the election process is characterized by a large presence of special interests or as information becomes more expensive to acquire. Received: 13 February 1996 / Accepted: 27 March 1997     

The Borda dictionary

Forn candidates, a profile of voters defines a unique Borda election ranking for each of the 2 ⁿ – (n + 1) subsets of two or more candidates. The Borda Dictionary is the set of all of these election listings that occur for any choice of a profile. As such, the dictionary contains all positive features, all flaws, and all paradoxes that can occur with single profile, sincere Borda elections. After the Borda Dictionary is characterized, it is used to show in what ways the Borda Count (BC) is an improvement over other positional voting methods and to derive several new BC properties. These properties include several new characterizations of the BC expressed in terms of axiomatic representations of social choice functions, as well as showing, for example, that the BC ranking ofn candidates can be uniquely determined by the BC rankings of all sets ofk <n candidates for any choice ofk between 2 andn.This research was supported, in part, by NSF grants IRI8415348, IRI-8803505 and a Fellowship from the Guggenheim Memorial Foundation.     

The number of weak orderings of a finite set

The number of Arrovian constitutions, when N agents are to rank n alternatives, is p(n) ^{p(n) N} , where p(n) is the number of weak orderings of n alternatives. For n≤15, p(n) is the nearest integer to n!/2(log2) ^{n +1}, the dominant term of a series derived by contour integration of the generating function. For large n, about n/17 additional terms in the series suffice to compute p(n) exactly. Received: 29 May 1995 / Accepted: 22 May 1997     

BETWEEN BROADCASTING POLITICAL MESSAGES AND INTERACTING WITH VOTERS

Politicians across Western democracies are increasingly adopting and experimenting with Twitter, particularly during election time. The purpose of this article is to investigate how candidates are using it during an election campaign. The aim is to create a typology of the various ways in which candidates behaved on Twitter. Our research, which included a content analysis of tweets (n?=?26,282) from all twittering Conservative, Labour and Liberal Democrat candidates (n?=?416) during the 2010 UK General Election campaign, focused on four aspects of tweets: type, interaction, function and topic. By examining candidates' twittering behaviour, the authors show that British politicians mainly used Twitter as a unidirectional form of communication. However, there were a group of candidates who used it to interact with voters by, for example, mobilizing, helping and consulting them, thus tapping into the potential Twitter offers for facilitating a closer relationship with citizens.     

On the selection of the same winner by all scoring rules

Different scoring rules can result in the selection of any of the k competing candidates, given the same preference profile, (Saari DG 2001, Chaotic elections! A mathematician looks at voting. American Mathematical Society, Providence, R.I.). It is also possible that a candidate, and even a Condorcet winning candidate, cannot be selected by any scoring rule, (Saari DG 2000 Econ Theory 15:55–101). These findings are balanced by Saari’s result (Saari DG 1992 Soc Choice Welf 9(4):277–306) that specifies the necessary and sufficient condition for the selection of the same candidate by all scoring rules. This condition is, however, indirect. We provide a sufficient condition that is stated directly in terms of the preference profile; therefore, its testability does not require the verdict of any voting rule.     

Condorcet efficiency: A preference for indifference

The Condorcet winner in an election is the candidate who would be able to defeat all other candidates in a series of pairwise elections. The Condorcet efficiency of a voting procedure is the conditional probability that it will elect the Condorcet winner, given that a Condorcet winner exists. The study considers the Condorcet efficiency of weighted scoring rules (WSR's) on three candidates for large electorates when voter indifference between candidates is allowed. It is shown that increasing the proportion of voters who have partial indifference will increase the probability that a Condorcet winner exists, and will also increase the Condorcet efficiency of all WSR's. The same observation is observed when the proportion of voters with complete preferences on candidates is reduced. Borda Rule is shown to be the WSR with maximum Condorcet efficiency over a broad range of assumptions related to voter preferences. The result of forcing voters to completely rank all candidates, by randomly breaking ties on candidates that are viewed as indifferent, leads to a reduction in the probability that a Condorcet winner exists and to a reduction in the Condorcet efficiency of all WSR's. Received: 31 July 1999/Accepted: 11 February 2000     

An extension of a theorem on the aggregation of separable preferences

An Excess-Voting Function relative to a profile π assigns to each pair of alternatives (x,y), the number of voters who prefer x to y minus the number of voters who prefer y to x. It is shown that any non-binary separable Excess-Voting Function can be achieved from a preferences profile when individuals are endowed with separable preferences. This result is an extension of Hollard and Le Breton (1996). Received: 16 December 1996 / Accepted: 8 October 1997     

On the manipulation of social choice correspondences

Duggan and Schwartz (Soc Choice and Welfare 17: 85–93, 2000) have proposed a generalization of the Gibbard–Satterthwaite Theorem to multivalued social choice rules. They show that only dictatorial rules are strategy-proof and satisfy citizens sovereignty and residual resoluteness. Citizens sovereignty requires that each alternative is chosen at some preference profile. Residual resoluteness compels the election to be single-valued when the preferences of the voters are “similar”. We propose an alternative proof to the Duggan and Schwartz’s Theorem. Our proof highlights the crucial role of residual resoluteness. In addition, we prove that every strategy-proof and onto social choice correspondence concentrates the social decision power in the hands of an arbitrary group of voters. Finally, we show that this result still holds in a more general framework in which voters report their preferences over sets of alternatives.     

Stability of voting games

This paper generalizes the result of Le Breton and Salles (1990) about stable set (far-sighted core of order 1) for voting games to far-sighted core of arbitrary order. Let m be the number of alternatives, n be the number of voters and G(n,k) be a proper symmetric simple game in which the size of a winning coalition is greater or equal to k. It is shown that the far-sighted core of order d for G(n,k) is nonempty for all preference profiles and for all n and k with n/(n–k)=v ¹ iff m(d+1)(v–1).This paper is part of my dissertation. I am grateful to my thesis advisor Leonid Hurwicz for his guidance and encouragement. I would like to thank Edward Green, Lu Hong, James Jordan, Andrew McLennan, Herve Moulin and Marcel Richter for their very helpful suggestions. Especially a referee and Maurice Salles made many good comments. Of course, any errors that remain are the sole responsibility of the author.     

SYMMETRIC SCORING RULES AND A NEW CHARACTERIZATION OF THE BORDA COUNT

Young developed a classic axiomatization of the Borda rule almost 50 years ago. He proved it is the only voting rule satisfying the normative properties of decisiveness, neutrality, reinforcement, faithfulness and cancellation. Often overlooked is that the uniqueness of Borda applies only to variable populations. We present a different set of properties which only Borda satisfies when both the set of voters and the set of alternatives can vary. It is also shown Borda is the only scoring rule which will satisfy all of the new properties when the number of voters stays fixed. (JEL D71, D02, H00)     

The probability of ties with scoring methods: Some results

The main flaw of the Condorcet method is that a Condorcet winner does not always exist (when there is a top cycle). Moreover, in some cases, all candidates are contained in one cycle and, therefore, a society or committee using the Condorcet method risks to face a severe indeterminacy. An indeterminacy can also happen when using other methods: for example the Borda method, plurality voting or any scoring method; but the origin of the indeterminacy is completely different. It happens when all candidates are tied. We study the probability that all candidates are tied when using a scoring method. We show that it is equivalent to some random walk problems. Some analytical and numerical results show that, under the assumptions underlying our study, the probability is very small and decreases when the number of voters or candidates increases. Received: 18 December 1998/Accepted: 17 March 2000     

Political geography and interest-group power

The relationship between the distribution of an interest group's members (i.e., voters) across legislative districts and the group's ability to influence government policy is not well understood. We propose a simple model of interest groups and legislative decisionmaking to characterize this relationship formally, and analyze the model for the case of three competing interest groups. We find that an interest group's expected power is maximized when its members are evently divided across (m+1)/2 of the m districts, except when the group is somewhat small, in which case its expected power is maximized when it is evenly divided across all m districts. Expected power decreases sharply as a group's members become concentrated in fewer than (m+1)/2 districts.The author would like to thank the participants of the Workshop in Applications of Economics at the University of chicago fpr their helpful comments, and the Center for the Study of the Economy and the State for its financial support.     

Using elections to represent preferences

This paper takes voting theory out of the realm of mechanism design and studies elections as tools for representing preferences: every preference relation on a set of n elements is the outcome of pairwise voting by approximately 2 log₂ n voters with transitive preferences. Results like this one provide representation for preference relations not representable by utility functions. They also motivate definitions of the levels of intransitivity, nonlinearity and nonrepresentability (by utility function) of a preference relation. Received: 25 March 1999/Accepted: 19 June 2000