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Arrow's impossibility theorem is a key result in social choice showing that no ranked-choice voting rule [note 1] can produce logically coherent results with more than two candidates. Specifically, any such rule violates independence of irrelevant alternatives : the principle that a choice between and should not depend on the quality of a third, unrelated outcome . [1]
The result is often cited in discussions of election science and voting theory, where is called a spoiler candidate. As a result, Arrow's theorem implies that a ranked voting system can never be completely independent of spoilers. [1]
The practical consequences of the theorem are debatable, with Arrow himself noting "Most [ranked] systems are not going to work badly all of the time. All I proved is that all can work badly at times." [2] [3] However, the susceptibility of different systems varies greatly. Plurality and instant-runoff suffer spoiler effects more often than other methods. [4] [5] [6] Majority-choice methods uniquely minimize the effect of spoilers on election results, limiting them to rare [7] [8] situations known as cyclic ties. [6]
While originally overlooked, a large class of systems called rated methods are not affected by Arrow's theorem or IIA failures. [3] [9] [10] Arrow initially rejected these systems on philosophical grounds, but reversed his opinion on the issue later in life, arguing score voting is "probably the best". [3]
Arrow's theorem is named after economist and Nobel laureate Kenneth Arrow, who demonstrated it in his doctoral thesis and popularized it in his 1951 book. [1]
Arrow's work is remembered as much for its pioneering methodology as its direct implications. Arrow's axiomatic approach provided a framework for proving facts about all conceivable voting mechanisms at once, contrasting with the earlier approach of investigating such rules one by one. [11]
Arrow's theorem falls under the branch of welfare economics known as social choice theory, which deals with aggregating preferences and information to make fair and accurate decisions for society. The goal is to create a social ordering function—a procedure for determining which outcomes are better, according to society as a whole—that satisfies the properties of rational behavior. [1]
Among the most important is independence of irrelevant alternatives, which says that when deciding between and , our opinions about some irrelevant option should not affect our decision. [1] Arrow's theorem shows this is not possible without relying on further information, such as rated ballots (rejected by strict behaviorists). [12]
As background, it is typically assumed that any non-degenerate (i.e. actually useful) voting system non-dictatorship:
Most proofs use additional assumptions to simplify deriving the result, though Robert Wilson proved these to be unnecessary. [13] Older proofs have taken as axioms:
The IIA condition is an important assumption governing rational choice. The axiom says that adding irrelevant—i.e. rejected—options should not affect the outcome of a decision. From a practical point of view, the assumption prevents electoral outcomes from behaving erratically in response to the arrival and departure of candidates. [14]
Arrow defines IIA slightly differently, by stating that the social preference between alternatives and should only depend on the individual preferences between and ; that is, it should not be able to go from to by changing preferences over some irrelevant alternative, e.g. whether . This is equivalent to the above statement about independence of spoiler candidates, which can be proven by using the standard construction of a placement function. [16]
Arrow's requirement that the social preference only depend on individual preferences is extremely restrictive. May's theorem shows that the only "fair" way to aggregate ordinal preferences for pairs of preferences is simple majority; thus, assumptions only slightly stronger than Arrow's are already enough to lock us into the class of Condorcet methods. At this point, the existence of the voting paradox is enough to show the impossibility of rational behavior for ranked-choice voting.
While the above argument is intuitive, it is not rigorous, and it requires additional assumptions not used by Arrow.
Let A be a set of outcomes, N a number of voters or decision criteria. We denote the set of all total orderings of A by L(A).
An ordinal (ranked) social welfare function is a function:
which aggregates voters' preferences into a single preference order on A.
An N-tuple (R1, …, RN) ∈ L(A)N of voters' preferences is called a preference profile. We assume two conditions:
Then, this rule must violate independence of irrelevant alternatives:
Proof by decisive coalition |
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Arrow's proof used the concept of decisive coalitions. [14] Definition:
Our goal is to prove that the decisive coalition contains only one voter, who controls the outcome—in other words, a dictator. The following proof is a simplification taken from Amartya Sen [17] and Ariel Rubinstein. [18] The simplified proof uses an additional concept:
Thenceforth assume that the social choice system satisfies unrestricted domain, Pareto efficiency, and IIA. Also assume that there are at least 3 distinct outcomes. Field expansion lemma — if a coalition is weakly decisive over for some , then it is decisive. Proof Let be an outcome distinct from . Claim: is decisive over . Let everyone in vote over . By IIA, changing the votes on does not matter for . So change the votes such that in and and outside of . By Pareto, . By coalition weak-decisiveness over , . Thus . Similarly, is decisive over . By iterating the above two claims (note that decisiveness implies weak-decisiveness), we find that is decisive over all ordered pairs in . Then iterating that, we find that is decisive over all ordered pairs in . Group contraction lemma — If a coalition is decisive, and has size , then it has a proper subset that is also decisive. Proof Let be a coalition with size . Partition the coalition into nonempty subsets . Fix distinct . Design the following voting pattern (notice that it is the cyclic voting pattern which causes the Condorcet paradox): (Items other than are not relevant.) Since is decisive, we have . So at least one is true: or . If , then is weakly decisive over . If , then is weakly decisive over . Now apply the field expansion lemma. By Pareto, the entire set of voters is decisive. Thus by the group contraction lemma, there is a size-one decisive coalition—a dictator. |
Proof by pivotal voter |
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Proofs using the concept of the pivotal voter originated from Salvador Barberá in 1980. [19] The proof given here is a simplified version based on two proofs published in Economic Theory . [20] [21] We will prove that any social choice system respecting unrestricted domain, unanimity, and independence of irrelevant alternatives (IIA) is a dictatorship. The key idea is to identify a pivotal voter whose ballot swings the societal outcome. We then prove that this voter is a partial dictator (in a specific technical sense, described below). Finally we conclude by showing that all of the partial dictators are the same person, hence this voter is a dictator. For simplicity we have presented all rankings as if there are no ties. A complete proof taking possible ties into account is not essentially different from the one given here, except that one ought to say "not above" instead of "below" or "not below" instead of "above" in some cases. Full details are given in the original articles. Part one: There is a "pivotal" voter for B over ASay there are three choices for society, call them A, B, and C. Suppose first that everyone prefers option B the least: everyone prefers A to B, and everyone prefers C to B. By unanimity, society must also prefer both A and C to B. Call this situation profile 0. On the other hand, if everyone preferred B to everything else, then society would have to prefer B to everything else by unanimity. Now arrange all the voters in some arbitrary but fixed order, and for each i let profile i be the same as profile 0, but move B to the top of the ballots for voters 1 through i. So profile 1 has B at the top of the ballot for voter 1, but not for any of the others. Profile 2 has B at the top for voters 1 and 2, but no others, and so on. Since B eventually moves to the top of the societal preference as the profile number increases, there must be some profile, number k, for which Bfirst moves aboveA in the societal rank. We call the voter k whose ballot change causes this to happen the pivotal voter for B over A. Note that the pivotal voter for B over A is not, a priori, the same as the pivotal voter for A over B. In part three of the proof we will show that these do turn out to be the same. Also note that by IIA the same argument applies if profile 0 is any profile in which A is ranked above B by every voter, and the pivotal voter for B over A will still be voter k. We will use this observation below. Part two: The pivotal voter for B over A is a dictator for B over CIn this part of the argument we refer to voter k, the pivotal voter for B over A, as the pivotal voter for simplicity. We will show that the pivotal voter dictates society's decision for B over C. That is, we show that no matter how the rest of society votes, if pivotal voter ranks B over C, then that is the societal outcome. Note again that the dictator for B over C is not a priori the same as that for C over B. In part three of the proof we will see that these turn out to be the same too. In the following, we call voters 1 through k − 1, segment one, and voters k + 1 through N, segment two. To begin, suppose that the ballots are as follows:
Then by the argument in part one (and the last observation in that part), the societal outcome must rank A above B. This is because, except for a repositioning of C, this profile is the same as profile k − 1 from part one. Furthermore, by unanimity the societal outcome must rank B above C. Therefore, we know the outcome in this case completely. Now suppose that pivotal voter moves B above A, but keeps C in the same position and imagine that any number (even all!) of the other voters change their ballots to move B below C, without changing the position of A. Then aside from a repositioning of C this is the same as profile k from part one and hence the societal outcome ranks B above A. Furthermore, by IIA the societal outcome must rank A above C, as in the previous case. In particular, the societal outcome ranks B above C, even though Pivotal Voter may have been the only voter to rank B above C. By IIA, this conclusion holds independently of how A is positioned on the ballots, so pivotal voter is a dictator for B over C. Part three: There exists a dictatorIn this part of the argument we refer back to the original ordering of voters, and compare the positions of the different pivotal voters (identified by applying parts one and two to the other pairs of candidates). First, the pivotal voter for B over C must appear earlier (or at the same position) in the line than the dictator for B over C: As we consider the argument of part one applied to B and C, successively moving B to the top of voters' ballots, the pivot point where society ranks B above C must come at or before we reach the dictator for B over C. Likewise, reversing the roles of B and C, the pivotal voter for C over B must be at or later in line than the dictator for B over C. In short, if kX/Y denotes the position of the pivotal voter for X over Y (for any two candidates X and Y), then we have shown
Now repeating the entire argument above with B and C switched, we also have
Therefore, we have
and the same argument for other pairs shows that all the pivotal voters (and hence all the dictators) occur at the same position in the list of voters. This voter is the dictator for the whole election. |
Arrow's theorem establishes that no ranked voting rule can always satisfy independence of irrelevant alternatives, but it says nothing about the frequency of spoilers. This led Arrow to remark that "Most systems are not going to work badly all of the time. All I proved is that all can work badly at times." [2] [3]
Attempts at dealing with the effects of Arrow's theorem take one of two approaches: either accepting his rule and searching for the least spoiler-prone methods, or dropping his assumption of ranked voting to focus on studying rated voting rules.
The first set of methods economists have studied are the majority-rule methods, which limit spoilers to rare situations where majority rule is self-contradictory, and uniquely minimize the possibility of a spoiler effect among rated methods. [15] Arrow's theorem was preceded by the Marquis de Condorcet's discovery of cyclic social preferences, cases where majority votes are logically inconsistent. Condorcet believed voting rules should satisfy his majority rule principle, i.e. if most voters rank Alice ahead of Bob, Alice should defeat Bob in the election.
Unfortunately, as Condorcet proved, this rule can be self-contradictory (intransitive), because there can be a rock-paper-scissors cycle with three or more candidates defeating each other in a circle. Thus Condorcet proved a weaker form of Arrow's impossibility theorem long before Arrow himself, under the stronger assumption that a voting system in the two-candidate case will agree with a simple majority vote.
Condorcet methods avoid the spoiler effect in non-cyclic elections, where candidates can be chosen by majority rule. Political scientists have found such cycles to be empirically rare, suggesting they are of limited practical concern. Spatial voting models also suggest such paradoxes are likely to be infrequent [22] or even non-existent. [23]
Duncan Black showed his own remarkable result, the median voter theorem. The theorem proves that if voters and candidates are arranged on a left-right spectrum, Arrow's conditions are compatible, and all of them will be met by any rule satisfying Condorcet's principle. [23]
More formally, Black's theorem assumes preferences are single-peaked: a voter's happiness with a candidate goes up and then down as the candidate moves along some spectrum. For example, in a group of friends choosing a volume setting for music, each friend would likely have their own ideal volume; as volume gets progressively too loud or too quiet, they would be increasingly dissatisfied. [23]
If the domain is restricted to profiles where every individual has a single-peaked preference with respect to the linear ordering, then social preferences are acyclic. In this situation, Condorcet methods satisfy a wide variety of highly-desirable properties. [23]
Unfortunately, the rule does not generalize from the political spectrum to the political compass, a result called the McKelvey-Schofield Chaos Theorem. [24] However, a well-defined median and Condorcet winner do exist if the distribution of voters on the ideological spectrum is rotationally symmetric. [25] In realistic cases, when voters' opinions follow a roughly-symmetric distribution such as a normal distribution or can be accurately summarized in one or two dimensions, Condorcet cycles tend to be rare. [22] [26]
Campbell and Kelly (2000) showed that Condorcet methods are the most spoiler-resistant class of ranked voting systems: whenever it is possible for some ranked voting system to avoid a spoiler effect, a Condorcet method will do so. [15] In other words, replacing a ranked-voting method with its Condorcet variant (i.e. elect a Condorcet winner if they exist, and otherwise run the method) will sometimes prevent a spoiler effect, but never cause a new one. [15]
In 1977, Ehud Kalai and Eitan Muller gave a full characterization of domain restrictions admitting a nondictatorial and rational social welfare function. These correspond to preferences for which there is a Condorcet winner. [27]
Holliday and Pacuit devise a voting system that provably minimizes the potential for spoiler effects, albeit at the cost of other criteria, and find that it is a Condorcet method, albeit at the cost of occasional monotonicity failures (at a much lower rate than seen in instant-runoff voting). [26]
As shown above, the proof of Arrow's theorem relies crucially on the assumption of ranked voting, and is not applicable to rated voting systems. As a result, systems like score voting and graduated majority judgment pass independence of irrelevant alternatives. [2] These systems ask voters to rate candidates on a numerical scale (e.g. from 0–10), and then elect the candidate with the highest average (for score voting) or median (graduated majority judgment). [23]
While Arrow's theorem does not apply to graded systems, Gibbard's theorem still does: no electoral system can be strategy-free, [28] so the informal dictum that "no voting system is perfect" still has some mathematical basis. [29]
Arrow's framework assumed individual and social preferences are orderings or rankings, i.e. statements about which outcomes are better or worse than others. Taking inspiration from the strict behaviorism popular in psychology, some philosophers and economists rejected the idea of comparing internal human experiences of well-being. [30] Such philosophers claimed it was impossible to compare preferences of different people in situations where they disagree; Sen gives as an example that it is impossible to know whether the Great Fire of Rome was good or bad, because it allowed Nero to build himself a new palace. [31]
Arrow himself originally agreed with these positions and rejected cardinal utility, leading him to focus his theorem on preference rankings. [30] [32] However, he later reversed this opinion, admitting scoring methods can provide useful information that allows them to evade his theorem. [3] Similarly, Amartya Sen first claimed interpersonal comparability is necessary for IIA, but later argued it would only require "rather limited levels of partial comparability" to hold in practice. [33]
Balinski and Laraki dispute the necessity of any genuinely cardinal information for rated voting methods to pass IIA. They argue the availability of a "common language" and the use of verbal grades are sufficient to ensure IIA, because they allow voters to give consistent responses to questions about candidate quality regardless of which candidates run. [34]
John Harsanyi argued his theorem [35] and other utility representation theorems like the VNM theorem, which show that rational behavior implies consistent cardinal utilities. Harsanyi [35] and Vickrey [36] independently derived results showing such preferences could be rigorously defined using individual preferences over the lottery of birth. [37] [38]
These results have led to the rise of implicit utilitarian voting approaches, which model ranked-choice procedures as approximations of the utilitarian rule (i.e. score voting). [39]
Behavioral economists have shown human behavior can violate IIA (e.g. with decoy effects), [40] suggesting human behavior might cause IIA failures even if the voting method itself does not. However, such effects tend to be small. [41]
Strategic voting can also create pseudo-spoiler situations: if the winner with honest voting is A, but with strategy is B, then eliminating every candidate but A and B would change the strategic outcome from B to A since majority rule is strategy-proof.[ citation needed ]
In addition to the above practical resolutions, there exist unusual (less-than-practical) situations where Arrow's conditions can be satisfied.
When equal treatment of candidates is not a necessity, Condorcet's majority-rule criterion can be modified to require a supermajority. Such situations become more practical if there is a clear default (such as doing nothing, or allowing an incumbent to complete their term in a recall election). In this situation, setting a threshold that requires a majority to select between 3 outcomes, for 4, etc. does not cause paradoxes; this result is related to the Nakamura number of voting mechanisms. [42]
In spatial (n-dimensional ideology) models of voting, this can be relaxed to require only (roughly 64%) of the vote to prevent cycles, so long as the distribution of voters is well-behaved (quasiconcave). [43]
Fishburn shows all of Arrow's conditions can be satisfied for uncountable sets of voters given the axiom of choice; [44] however, Kirman and Sondermann showed this requires disenfranchising almost all members of a society (eligible voters form a set of measure 0), leading them to refer to such societies as "invisible dictatorships". [45]
Maximal lotteries satisfy a probabilistic version of Arrow's criteria in fractional social choice models, where candidates can be elected by lottery or engage in power-sharing agreements (e.g. where each holds office for a specified period of time). [46]
Arrow's theorem does not deal with strategic voting, which does not appear in his framework. The Arrovian framework of social welfare assumes all voter preferences are known and the only issue is in aggregating them. [47]
Contrary to a common misconception, Arrow's theorem deals with the limited class of ranked-choice voting systems, rather than voting systems as a whole. [48] [49]
Approval voting is an electoral system in which voters can select any number of candidates instead of selecting only one.
Score voting, sometimes called range voting, is an electoral system for single-seat elections. Voters give each candidate a numerical score, and the candidate with the highest average score is elected. Score voting includes the well-known approval voting, but also lets voters give partial (in-between) approval ratings to candidates.
In social choice theory, a Condorcet paradox is a situation where majority rule behaves in a way that is self-contradictory. In such a situation, every possible choice is rejected by the electorate in favor of another, because there is always some other outcome that a majority of voters consider to be better.
In social choice theory and politics, the spoiler effect refers to a situation where the entry of a losing candidate affects the results of an election. A voting system that is not affected by spoilers satisfies independence of irrelevant alternatives or independence of spoilers.
A Condorcet method is an election method that elects the candidate who wins a majority of the vote in every head-to-head election against each of the other candidates, whenever there is such a candidate. A candidate with this property, the pairwise champion or beats-all winner, is formally called the Condorcet winner. The head-to-head elections need not be done separately; a voter's choice within any given pair can be determined from the ranking.
Independence of irrelevant alternatives (IIA), also known as binary independence, the independence axiom, is an axiom of decision theory and economics describing a necessary condition for rational behavior. The axiom says that a choice between and should not depend on the quality of a third, unrelated outcome .
The Gibbard–Satterthwaite theorem is a theorem in voting theory. It was first conjectured by the philosopher Michael Dummett and the mathematician Robin Farquharson in 1961 and then proved independently by the philosopher Allan Gibbard in 1973 and economist Mark Satterthwaite in 1975. It deals with deterministic ordinal electoral systems that choose a single winner, and states that for every voting rule of this form, at least one of the following three things must hold:
Strategic nomination refers to the entry of a candidate into an election with the intention of changing the ranking of other candidates. The name is an echo of ‘tactical voting’ and is intended to imply that it is the candidates rather than the voters who are seeking to manipulate the result in a manner unfaithful to voters’ true preferences.
In an election, a candidate is called a Condorcet, beats-all, or majority-rule winner if a majority of voters would support them in a race against any other candidate. Such a candidate is also called an undefeated or tournament champion. Voting systems where a majority-rule winner will always win the election are said to satisfy the majority-rule principle, also known as the Condorcet criterion. Condorcet voting methods extend majority rule to elections with more than one candidate.
In political science and social choice theory, the median voter theorem states that if voters and candidates are distributed along a one-dimensional spectrum and voters have single peaked preferences, any voting method satisfying the Condorcet criterion will elect the candidate preferred by the median voter.
Social choice theory or social choice is a branch of welfare economics that analyzes mechanisms and procedures for collective decision-making. Social choice incorporates insights from economics, mathematics, and game theory to find the best ways to combine individual opinions, preferences, or beliefs into a single coherent measure of the quality of different outcomes, called a social welfare function.
Kenneth Arrow's monograph Social Choice and Individual Values and a theorem within it created modern social choice theory, a rigorous melding of social ethics and voting theory with an economic flavor. Somewhat formally, the "social choice" in the title refers to Arrow's representation of how social values from the set of individual orderings would be implemented under the constitution. Less formally, each social choice corresponds to the feasible set of laws passed by a "vote" under the constitution even if not every individual voted in favor of all the laws.
The Borda count is a family of positional voting rules which gives each candidate, for each ballot, a number of points corresponding to the number of candidates ranked lower. In the original variant, the lowest-ranked candidate gets 0 points, the next-lowest gets 1 point, etc., and the highest-ranked candidate gets n − 1 points, where n is the number of candidates. Once all votes have been counted, the option or candidate with the most points is the winner. The Borda count is intended to elect broadly acceptable options or candidates, rather than those preferred by a majority, and so is often described as a consensus-based voting system rather than a majoritarian one.
Rated voting refers to any electoral system which allows the voter to give each candidate an independent evaluation, typically a rating or grade. These are also referred to as cardinal, evaluative, or graded voting systems. Cardinal methods and ordinal methods are the two modern categories of voting systems.
The term ranked voting, also known as preferential voting or ranked-choice voting, pertains to any voting system where voters indicate a rank to order candidates or options—in a sequence from first, second, third, and onwards—on their ballots. Ranked voting systems vary based on the ballot marking process, how preferences are tabulated and counted, the number of seats available for election, and whether voters are allowed to rank candidates equally.
Maximal lotteries are a probabilistic voting method and tournament solution, first proposed by the French mathematician and social scientist Germain Kreweras in 1965, and popularized by Peter Fishburn. The method uses ranked ballots and returns the probability distribution of candidates that is preferred by a majority of voters to any other.
The McKelvey–Schofield chaos theorem is a result in social choice theory. It states that if preferences are defined over a multidimensional policy space, then majority rule is in general unstable: there is no Condorcet winner. Furthermore, any point in the space can be reached from any other point by a sequence of majority votes.
Computational social choice is a field at the intersection of social choice theory, theoretical computer science, and the analysis of multi-agent systems. It consists of the analysis of problems arising from the aggregation of preferences of a group of agents from a computational perspective. In particular, computational social choice is concerned with the efficient computation of outcomes of voting rules, with the computational complexity of various forms of manipulation, and issues arising from the problem of representing and eliciting preferences in combinatorial settings.
A major branch of social choice theory is devoted to the comparison of electoral systems, otherwise known as social choice functions. Viewed from the perspective of political science, electoral systems are rules for conducting elections and determining winners from the ballots cast. From the perspective of economics, mathematics, and philosophy, a social choice function is a mathematical function that determines how a society should make choices, given a collection of individual preferences.
The highest median voting rules are a class of graded voting rules where the candidate with the highest median rating is elected.
Now there's another possible way of thinking about it, which is not included in my theorem. But we have some idea how strongly people feel. In other words, you might do something like saying each voter does not just give a ranking. But says, this is good. And this is not good[...] So this gives more information than simply what I have asked for.
Candidates C and D spoiled the election for B ... With them in the running, A won, whereas without them in the running, B would have won. ... Instant runoff voting ... does not do away with the spoiler problem entirely, although it ... makes it less likely
IRV is subject to something called the "center squeeze." A popular moderate can receive relatively few first-place votes through no fault of her own but because of vote splitting from candidates to the right and left. [...] Approval voting thus appears to solve the problem of vote splitting simply and elegantly. [...] Range voting solves the problems of spoilers and vote splitting
Is there such a thing as a perfect voting system? The respondents were unanimous in their insistence that there is not.
...the fictitious notion of 'original position' [was] developed by Vickery (1945), Harsanyi (1955), and Rawls (1971).