Some Further Results on the Manipulability of Social Choice Rules

A typology of electoral environments in which strategic manipulation of voting rules can occur is proposed. From this typology, six different concepts of equilibria (or quasi-equilibria) are defined, including the usual ones. Then, focusing on three alternative elections, we derive some analytical representations for the vulnerability of a large number of voting rules to strategic manipulation by considering successively each type of equilibrium. Our results show that the hierarchy of the rules changes when the voting framework is modified. However, some rules are clearly dominated and some others (among which the Borda and the Nanson rules) appear to be uniquely the best in some specific contexts.     

A majority-rule characterization with multiple extensions

When May's necessary and sufficient conditions for majority rule as a binary voting rule are extended in a natural way to decisions over more than two options, the resulting conditions are consistent with the Borda and Black voting rules, but not with a variety of other voting rules for more than two options. This paper presents an alternative set of necessary and sufficient conditions for binary majority rule, which permits the plurality, Condorcet and (simplified) Dodgson rules, as well as the Borda and Black rules, but not the Copeland or Nanson rules, to be classified as extensions of binary majority rule to decisions over more than two options.I am indebted to Amartya Sen and anonymous referees for helpful suggestions     

Special majority rules and the existence of voting equilibria

This paper is concerned with voting rules in which one alternative defeats a second alternative if and only if it is preferred to the second alternative by a pre-specified proportion of the individuals who have strict preferences on the pair. In particular, the paper focuses on the potential of two alternative lower bounds (for the proportion used) as tools for studying the existence of voting equilibria (i.e., for studying the existence of alternatives which cannot be defeated). It establishes that there are certain important contexts where one of the two bounds does not directly reveal whether any voting equilibria exist, but the other one does.     

Juror perceptions of child eyewitness testimony in a sexual abuse trial

A mock child sexual abuse trial was used to study juror perceptions of child eyewitnesses. The child's age (5, 11, or 16) and level of involvement (victim or bystander) were varied across conditions in order to test their impact on juror perceptions and verdict. Results indicated a significant effect of level of involvement on the verdict; defendants in trials involving a bystander witness received higher guilt ratings than defendants in trials with a victim-witness. In addition, jurors perceived the child's eyewitness testimony to be more believable when the child was younger. Implications for the impact of these findings on real-life child sexual abuse cases are discussed.     

How frequently do different voting rules encounter voting paradoxes in three-candidate elections?

We estimate the frequencies with which ten voting anomalies (ties and nine voting paradoxes) occur under 14 voting rules, using a statistical model that simulates voting situations that follow the same distribution as voting situations in actual elections. Thus the frequencies that we estimate from our simulated data are likely to be very close to the frequencies that would be observed in actual three-candidate elections. We find that two Condorcet-consistent voting rules do, the Black rule and the Nanson rule, encounter most paradoxes and ties less frequently than the other rules do, especially in elections with few voters. The Bucklin rule, the Plurality rule, and the Anti-plurality rule tend to perform worse than the other eleven rules, especially when the number of voters becomes large.     

Strategic voting and nomination

Using computer simulations based on three separate data generating processes, I estimate the fraction of elections in which sincere voting is a core equilibrium given each of eight single-winner voting rules. Additionally, I determine how often each rule is vulnerable to simple voting strategies such as ‘burying’ and ‘compromising’, and how often each rule gives an incentive for non-winning candidates to enter or leave races. I find that Hare is least vulnerable to strategic voting in general, whereas Borda, Coombs, approval, and range are most vulnerable. I find that plurality is most vulnerable to compromising and strategic exit (causing an unusually strong tendency toward two-party systems), and that Borda is most vulnerable to strategic entry. I use analytical proofs to provide further intuition for some of my key results.     

Self-selective social choice functions

It is not uncommon that a society facing a choice problem has also to choose the choice rule itself. In such situations, when information about voters’ preferences is complete, the voters’ preferences on alternatives induce voters’ preferences over the set of available voting rules. Such a setting immediately gives rise to a natural question concerning consistency between these two levels of choice. If a choice rule employed to resolve the society’s original choice problem does not choose itself, when it is also used for choosing the choice rule, then this phenomenon can be regarded as inconsistency of this choice rule as it rejects itself according to its own rationale. Koray (Econometrica 68: 981–995, 2000) proved that the only neutral, unanimous universally self-selective social choice functions are the dictatorial ones. Here we introduce to our society a constitution, which rules out inefficient social choice rules. When inefficient social choice rules become unavailable for comparison, the property of self-selectivity becomes more interesting and we show that some non-trivial self-selective social choice functions do exist. Under certain assumptions on the constitution we describe all of them.     

Juror Perceptions of Child Eyewitness Testimony in a Sexual Abuse Trial

ABSTRACT

A mock child sexual abuse trial was used to study juror perceptions of child eyewitnesses. The child's age (5, 11, or 16) and level of involvement (victim or bystander) were varied across conditions in order to test their impact on juror perceptions and verdict. Results indicated a significant effect of level of involvement on the verdict; defendants in trials involving a bystander witness received higher guilt ratings than defendants in trials with a victim-witness. In addition, jurors perceived the child's eyewitness testimony to be more believable when the child was younger. Implications for the impact of these findings on real-life child sexual abuse cases are discussed.     

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:

     

Minimal manipulability: anonymity and unanimity

This paper is concerned with the minimal number of profiles at which a unanimous and anonymous social choice function for three alternatives is manipulable. The lower bound is derived and examples of social choice functions attaining the lower bound are given. It is conjectured that these social choice functions are in fact all minimally manipulable social choice functions. Since some of these social choice functions are Pareto optimal, it follows that the lower bound also holds for Pareto optimal and anonymous social choice functions. Some of the minimally manipulable Pareto optimal and anonymous social choice functions can be interpreted as status quo voting.     

The reinforcement axiom under sequential positional rules

The reinforcement axiom roughly states that when an alternative is selected by two different constituencies, it must also be selected by their union. Hare and Coombs rules are special cases of sequential positional voting rules, which are known to violate this axiom. In this article, we first show that reinforcement can be violated by all such rules. We then evaluate, by the use of Monte Carlo simulations and the Fishburn–Gehrlein technique, the proportion of profiles at which this phenomenon occurs.     

Robustness against inefficient manipulation

This paper identifies a family of scoring rules that are robust against coalitional manipulations that result in inefficient outcomes. We discuss the robustness of a number of Condorcet consistent and “point runoff” voting rules against such inefficient manipulation and classify voting rules according to their potential vulnerability to inefficient manipulation.     

Optimality among restricted majority decision rules

When a group of decision makers with common interests faces a dichotomous choice, the task of deciding may be delegated to a committee consisting of a subset of the original group. This procedure is called a restricted majority decision rule. If each member of the original group is characterized by the probability of his deciding correctly, the expected utility from the decision is determined by which members are appointed to the committee. The conditions between enabling the comparison of alternative restricted majority rules are based on the voting profiles of the decision makers. The purpose of the current study is to propose an algorithm for identifying the optimal restricted majority rule amongst all restricted majority rules.I am indebted to Shmuel Nitzan for helpful comments and suggestions. All shortcomings are, of course, entirely my responsibility.     

Characterizing best–worst voting systems in the scoring context

An increasing body of theoretical and empirical work on discrete choice considers a choice design in which a person is asked to select both the best and the worst alternative in an available set of alternatives, in contrast to more traditional tasks, such as where the person is asked to: select the best alternative; select the worst alternative; rank the alternatives. Here we consider voting systems motivated by such “best–worst” choice; characterize a class of “best–worst” voting systems in terms of a set of axioms in the context of scoring rules; and discuss briefly possible extensions to approval–disapproval systems.     

Probabilistic Borda rule voting

An alternative voting system, referred to as probabilistic Borda rule, is developed and analyzed. The winning alternative under this system is chosen by lottery where the weights are determined from each alternatives Borda score relative to all Borda points possible. Advantages of the lottery include the elimination of strategic voting on the set of alternatives under consideration and breaking the tyranny of majority coalitions. Disadvantages include an increased incentive for strategic introduction of new alternatives to alter the lottery weights, and the possible selection of a Condorcet loser. Normative axiomatic properties of the system are also considered. It is shown this system satisfies the axiomatic properties of the standard Borda procedure in a probabilistic fashion.I thank Nic Tideman and Andrew Yates for helpful comments and especially appreciate detailed criticism from Keith Dougherty which greatly improved the exposition of the text. In addition, Don Saari, in his role as editor, suggested a number of clarifications and an important extension for which I am grateful. I accept full responsibility for any remaining shortcomings.     

Scoring rule voting games and dominance solvability

This article studies the dominance solvability (by iterated deletion of weakly dominated strategies) of general scoring rule voting games when there are three alternatives. The scoring rules we study include Plurality rule, Approval voting, Borda rule, and Relative Utilitarianism. We provide sufficient conditions for dominance solvability of general scoring rule voting games. The sufficient conditions that we provide for dominance solvability are in terms of one statistic of the game: sufficient agreement on the best alternative or on the worst alternative. We also show that the solutions coincide with the set of Condorcet Winners whenever the sufficient conditions for dominance solvability are satisfied. Approval Voting performs the best in terms of our criteria.     

Sociologists in action: The McMartin sexual abuse case,litigation, justice,and mass hysteria

This paper describes our involvement as jury consultants in one of the most notorious criminal trials in history—the McMartin child-molestation trial in Los Angeles. The McMartin trial was the longest and costliest criminal trial in American history. The prosecution spent $15 million and took nearly six years in making a criminal case against day-care workers, only to have the jurors declare them not guilty. The defendants in the McMartin trial were charged with molesting young children at a preschool in Manhattan Beach, Los Angeles County, California. In 1987, we had performed scientific defense voir dire jury selection to choose the most impartial jurors to try the two defendants, Raymond Buckey and Peggy Buckey McMartin. In performing scientific jury selection, both a community survey and pre-voir dire questionnaires served as an important empirical foundation to assess jurors’ attitudinal, demographic, and socioeconomic characteristics to develop the effective juror profiles for the trial. The authors received the 1994 Gustavus Myers Center Outstanding Book Award on Human Rights in North America.     

Analyzing Roll-Call Voting Tools for Content

Abstract

The roll-call voting tools most frequently used by legislative scholars are not equally useful for all. Roll-call votes selected for traditional measurement tools systematically exclude issues determined to be important to women legislators. Those who study women in politics are unable to employ roll-call voting tools with the assurance that they are using valid measures for their research. After analyzing the content of widely used voting tools and discovering that votes from controversial bills of interest to women were often excluded, I outline suggestions for employing alternative methodological approaches. Further, I add to a growing criticism of roll-call voting statistics by identifying another limitation to their use.     

Sincere versus sophisticated voting when legislators vote sequentially

Elsewhere (Groseclose and Milyo 2010), we examine a game where each legislator has preferences over (i) the resulting policy and (ii) how he or she votes. The latter preferences are especially important when the legislator is not pivotal. We show that when the game follows the normal rules of legislatures—most important, that legislators can change their vote after seeing how their fellow legislators have voted—then the only possible equilibrium is one where all legislators ignore their policy preferences. That is, each legislator votes as if he or she is not pivotal. The result, consistent with empirical studies of Congress, suggests that legislators should tend to vote sincerely, rather than sophisticatedly. In this paper we examine how outcomes change if we change the rules for voting. Namely, instead of a simultaneous game, we consider a game where legislators vote sequentially in a pre-determined order. We show that, opposite to the simultaneous game, an alternative wins if and only if a majority of legislators’ policy preferences favor that alternative. Our results suggest that if Congress adopted this change in rules, then sophisticated voting would become frequent instead of rare.     

Sincere voting in models of legislative elections

An assumption of sincere voting for one's most preferred candidate is frequently invoked in models of electoral competition in which the elected legislature consists of more than a single candidate or party. Similarly, such an assumption is more-or-less implicit in many normative discussions of the relative merits of alternative methods for electing a representative assembly. Voters, however, have preferences over policy outcomes—which are determined by the ex post elected legislature—and not over candidates per se. This paper examines the extent to which the sincere voting assumption is legitimate in a wide class of strategic models of legislative elections. The finding is negative, and this has direct implications for the interpretation of conlusions drawn from models—formal or otherwise—which impose sincere voting as an assumption.This paper is a revised version of CalTech Social Science Working Paper #637, and was written while I was a National Fellow at the Hoover Institution, Stanford University. Much of the paper is a result of the efforts of John Ledyard to understand what I was trying to say about the problem. I am extremely grateful for his help, and for his insistence that I look for a theorem and not simply a set of examples. I am also grateful to Kim Border for providing the structure of a proof for one of the results. Despite their imput, I retain all responsibility for any remaining errors and ambiguities.