Strategy-proof preference aggregation: Possibilities and characterizations

An aggregation rule maps each profile of individual strict preference orderings over a set of alternatives into a social ordering over that set. We call such a rule strategy-proof if misreporting one's preference never produces a different social ordering that is between the original ordering and one's own preference. After describing two examples of manipulable rules, we study in some detail three classes of strategy-proof rules: (i) rules based on a monotonic alteration of the majority relation generated by the preference profile; (ii) rules improving upon a fixed status-quo; and (iii) rules generalizing the Condorcet–Kemeny aggregation method.     

An efficiency characterization of plurality rule in collective choice problems

We consider the problem of selecting alternatives from a set of feasible alternatives over which each agent is endowed with a strict preference. We show that there is one and only one rule that satisfies anonymity, neutrality, efficiency, tops-only, and reinforcement. The rule is known as plurality rule, which selects the alternative(s) most preferred by the largest number of agents. I would like to thank William Thomson for helpful suggestions and discussions. I am grateful to Biung-Ghi Ju, Hyungjun Kim, and Yan-An Hwang for detailed comments. I am also indebted to the Editor and an anonymous referee for valuable suggestions. As usual, I am responsible for any remaining deficiency.     

On the informational efficiency of simple scoring rules

We study information aggregation in large elections. With two candidates, efficient information aggregation is possible (e.g., Feddersen and Pesendorfer [5], [6] and [7]). We show that this result does not extend to elections with more than two candidates. We study a class of simple scoring rules in voting games with Poisson population uncertainty and three candidates. No simple scoring rule aggregates information efficiently, even if preferences are dichotomous and a Condorcet winner always exists. We introduce a weaker criterion of informational efficiency that requires a voting rule to have at least one efficient equilibrium. Only approval voting satisfies this criterion.     

Winning probabilities in a pairwise lottery system with three alternatives

Summary. The pairwise lottery system is a multiple round voting procedure which chooses by lot a winner from a pair of alternatives to advance to the next round where in each round the odds of selection are based on each alternatives majority rule votes. We develop a framework for determining the asymptotic relative likelihood of the lottery selecting in the final round the Borda winner, Condorcet winner, and Condorcet loser for the three alternative case. We also show the procedure is equivalent to a Borda lottery when only a single round of voting is conducted. Finally, we present an alternative voting rule which yields the same winning probabilities as the pairwise lottery in the limiting case as the number of rounds of the pairwise lottery becomes large.Received: 5 June 2003, Revised: 17 June 2004, JEL Classification Numbers: D71. Correspondence to: Jac C. HeckelmanWe thank Keith Dougherty and Andrew Yates for their comments.     

On the universally beneficial manipulation conjecture

The universally beneficial manipulation conjecture of Campbell and Kelly states that for a social choice rule, if everyone gains as a result of any optimal manipulation, then the rule satisfies universally beneficial manipulation, i.e., everyone gains as a result of any manipulation, optimal or not. We prove the conjecture for three alternatives and any number of individuals. We also construct a counterexample in a general form with more than three alternatives and at least two individuals.     

A strategy-proofness characterization of majority rule

Summary. A feasible alternative x is a strong Condorcet winner if for every other feasible alternative y there is some majority coalition that prefers x to y. Let (resp., denote the set of all profiles of linear (resp., merely asymmetric) individual preference relations for which a strong Condorcet winner exists. Majority rule is the only non-dictatorial and strategy-proof social choice rule with domain , and majority rule is the only strategy-proof rule with domain . Received: August 29, 2000; revised version: November 13, 2002 RID="*" ID="*"We are grateful to Wulf Gaertner and our two referees for insightful comments on a previous draft. Correspondence to: D. E. Campbell     

Constant scoring rules,condorcet criteria and single-peaked preferences

Summary A constant scoring rule asks each individual to vote for a given (and constant) number of alternatives and the alternative with the most votes is elected. A sequential constant scoring rule applies this principle in a process of sequential elimination. Constant scoring rules as well as sequential constant scoring rules fail to satisfy Condorcet criteria when individual preferences are unrestricted. The purpose of this paper is to show that, if we assume that preferences are single-peaked, then some constant scoring rules satisfy the Condorcet loser criterion and some sequential constant scoring rules satisfy the Condorcet winner criterion. The results we provide make possible the identification of these rules.I thank Maurice Salles and two anonymous referees for their helpful comments. The usual disclaimers apply.     

Strategy-Proofness and the Tops-Only Property

A social choice function satisfies the tops‐only property if the chosen alternative only depends on each person's report of his most‐preferred alternatives on the range of this function. On many domains, strategy‐proofness implies the tops‐only property provided that the range of the social choice function satisfies some regularity condition. The existing proofs of this result are model specific. In this paper, a general proof strategy is proposed for showing that a strategy‐proof social choice function satisfies the tops‐only property when everyone has the same set of admissible preferences.     

Stability of Constitutions

Constitution is a pair of rules   ( s ,  S )   that are used in a voting situation. The rule  s  is used to vote about the existing alternatives and the rule  S  is used to vote about changing the rule  s  to some other rule    s '  . We consider what kind of constitutions are likely to emerge as prominent ones if the constitutions contain more than just two rules. In a constitution that contains any number of rules the  n th rule is used to decide about the    n  − 1 th   rule. We define a notion of stability for the constitutions and show that all stable constitutions roughly contain the same rule from    n  = 2   onwards. This is one explanation to the casual observation that the constitutions have usually only two rules.     

Nash Equilibrium and Welfare Optimality

If A is a set of social alternatives, a social choice rule (SCR) assigns a subset of A to each potential profile of individuals' preferences over A , where the subset is interpreted as the set of "welfare optima". A game form (or "mechanism") implements the social choice rule if, for any potential profile of preferences, (i) any welfare optimum can arise as a Nash equilibrium of the game form (implying, in particular, that a Nash equilibrium exists) and, (ii) all Nash equilibria are welfare optimal. The main result of this paper establishes that any SCR that satisfies two properties—monotonicity and no veto power—can be implemented by a game form if there are three or more individuals. The proof is constructive.     

Approval voting with endogenous candidates

We present a formal model of political competition under approval voting which allows for endogenous candidate entry. Our analysis yields a number of novel insights. First, we develop a notion of sincere voting behavior under approval voting, called relative sincerity. We then show that the relatively sincere voting behavior is consistent with the strategic calculus of voting. Second, we show that in a one-dimensional model with distance preferences, equilibria in relatively sincere strategies and without spoiler candidates always generate outcomes close to the median voter. Moreover, approval voting satisfies Duverger's Law in the sense that there are at most two winning positions! Finally, we extend our analysis to arbitrary policy spaces. In the general setting, approval voting is shown to be susceptible to the same kinds of problems as the plurality rule, such as the possibility of non-majoritarian outcomes, failure to elect the Condorcet winner and existence of spoiler candidates.     

The Borda rule,Condorcet consistency and Condorcet stability

Summary. The Borda rule is known to be the least vulnerable scoring rule to Condorcet inconsistency, Saari (2000). Such inconsistency occurs when the Condorcet winner (the alternative which is preferred to any other alternative by a simple majority) is not selected by the Borda rule. This note exposes the relationship between the Borda rule and the Condorcet q-majority principle as well as the Condorcet q-majority voting rule. The main result establishes that the Borda rule is Condorcet q-majority consistent when where k is the number of alternatives. The second result establishes that is the minimal degree of majority decisiveness corresponding to the Borda rule under sincere voting. The same majority is required to ensure decisiveness under the Borda rule and to ensure that a q-rule (the generalized q-majority Condorcet rule) is a voting rule. Received: April 8, 2002; revised version: July 17, 2002 Correspondence to:S. Nitzan     

On the evolutionary selection of sets of Nash equilibria

It is well established for evolutionary dynamics in asymmetric games that a pure strategy combination is asymptotically stable if and only if it is a strict Nash equilibrium. We use an extension of the notion of a strict Nash equilibrium to sets of strategy combinations called ‘strict equilibrium set’ and show the following. For a large class of evolutionary dynamics, including all monotone regular selection dynamics, every asymptotically stable set of rest points that contains a pure strategy combination in each of its connected components is a strict equilibrium set. A converse statement holds for two-person games, for convex sets and for the standard replicator dynamic.     

Extraneous Alternatives And Strategy-Proofness

It is well known that the Gibbard–Satterthwaite theorem cannot be circumvented by adding extraneous alternatives that are included in the individual preference information but are never selected. We generalize this by proving that, for any domain on which every strategy-proof rule is dictatorial, the addition of extraneous alternatives will not permit the construction of a non-dictatorial and strategy-proof rule if the new domain is a product set. We show how this result, and our other theorem, can be applied to seven families of social choice situations, including those in which more than one alternative is selected.     

The Copeland rule and Condorcet’s principle

Summary. The purpose of this note is to shed some light on the relationship between the Copeland rule and the Condorcet principle in those cases where there does not exist a Condorcet winner. It will be shown that the Copeland rule ranks alternatives according to their distances to being a Condorcet winner.Received: 30 July 2003, Revised: 31 December 2003, JEL Classification Numbers: D70, D71.Christian Klamler: I am very grateful to Daniel Eckert and Nick Baigent for their helpful comments.     

Ranking opportunity sets in a simple intertemporal framework

We extend the problem of ranking subsets (opportunity sets) of a set of alternatives to an intertemporal setting, whereby the agent makes a sequence of choice decisions over time. In particular, we show in a simple two-period model that an agent who satisfies five plausible axioms ranks opportunity sets in a lexicographic fashion. That is, the agent ranks opportunity sets based solely on the relative desirability of their first-period alternatives, and only if the first-period alternatives are thought equally desirable does the agent consider the second-period alternatives. We discuss this strong time-discounting result and the role of the axioms, and also show that the popular “independence” axioms employed in the existing “static” literature are inconsistent with some reasonable axioms regarding intertemporal choice.      

Set-rationalizable choice and self-stability

Rationalizability and similar notions of consistency have proved to be highly problematic in the context of social choice, as witnessed by a range of impossibility results, among which Arrow?s is the most prominent. We propose to rationalize choice functions by preference relations over sets of alternatives (set-rationalizability) and introduce two consistency conditions, and , which are defined in analogy to Sen?s α and γ. We find that a choice function satisfies if and only if it is set-rationalizable and that it satisfies and if and only if it is self-stable, a new concept based on earlier work by Dutta. The class of self-stable social choice functions contains a number of appealing Condorcet extensions.     

An axiomatic theory of political representation

We discuss the theory of gerrymandering-proof voting rules. Our approach is axiomatic. We show that, for votes over a binary set of alternatives, any rule that is unanimous, anonymous, and gerrymandering-proof must decide a social outcome as a function of the proportions of agents voting for each alternative, and must either be independent of this proportion, or be in one-to-one correspondence with the proportions. In an extended model in which the outcome of a vote at the district level can be a composition of a governing body (with two possible parties), we discuss the quasi-proportional rules (characterized by unanimity, anonymity, gerrymandering-proofness, strict monotonicity, and continuity). We show that we can always (pointwise) approximate a single-member district quota rule with a quasi-proportional rule. We also discuss a more general environment, where there may be more than two parties.     

The politics of progressive income taxation with incentive effects

This paper studies majority voting over quadratic taxation and investigates under which conditions marginal progressivity emerges as a voting outcome. In our model with endogenous income, there is no majority (Condorcet) winning tax schedule. We then investigate less demanding political equilibrium concepts in order to see under which conditions the set of equilibria is composed only of progressive tax functions. We follow three strategies: (i) reduction of the policy space to the tax functions that are ideal for some voter; (ii) elimination of weakly dominated strategies and the use of mixed strategies in a standard Downsian two-party competition game; (iii) assumption that political parties interact repeatedly and care about the size of their majority. Although each approach captures a different aspect of political behavior, they point to the same (simulation-based) conclusion that progressivity is more likely to emerge for most distributions of abilities and that it is actually the only possible voting outcome if the distribution is sufficiently concentrated at the middle.     

Are property rights enough?Re‐evaluating a big‐bang claim1

We investigate the claim made by proponents of the big‐bang strategy that the establishment of property rights in an economy in transition creates its own demand for the enforcement of laws to protect those rights. Our model contains a government engaging in activities to accomplish objectives that depend on public support for the enforcement of the rule of law and agents who interpret the level of activities of the government as indications of the government's intent to enforce the rule of law. Agents, using the level of government activities as an input to their decisions, choose whether to support the government's objectives. We establish conditions under which the level of activities chosen by the government maximizes its benefits, and simultaneously induces the constituency to support enforcement of the rule of law. These conditions provide a basis for the argument for the implementation of the big‐bang policy. When these conditions do not hold, however, we show that the level of activities that maximizes the government's benefits may have only a minor impact on support for the enforcement of the rule of law. Two characteristics play an important role in these conditions: the initial level of crime and the types of activities the government chooses to undertake. We present examples showing that the initial level of crime has the more dramatic effect on subsequent support for the rule of law.