Spoiler effect

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In social choice theory and politics, a spoiler is a losing candidate who affects the results of an election simply by participating, [1] [2] a situation that is called a spoiler effect. If a major candidate is perceived to have lost an election because of a minor candidate, the minor candidate is called a spoiler candidate and the major candidate is said to have been spoiled. Often times the term spoiler will be applied to candidates or situations which do not meet the full definition, typically in real-world scenarios where the introduction of a new candidate can cause voters to change their opinions, either through their campaign or merely by existing. A voting system that is not affected by spoilers is called independent of irrelevant alternatives or spoilerproof. [3]

Contents

The frequency and severity of spoiler effects depends substantially on the voting method. Instant-runoff or ranked-choice voting (RCV), the two-round system (TRS), and especially first-past-the-post (FPP) without winnowing or primary elections [4] are highly sensitive to spoilers (though RCV and TRS less so in some circumstances), and all three rules are affected by center-squeeze and vote splitting. [5] [6] [7] [8] Majority-rule (or Condorcet) methods are only rarely affected by spoilers, which are limited to rare [9] [10] situations called cyclic ties. [11] [12] [13] Rated voting systems are not subject to Arrow's theorem. Whether such methods are spoilerproof depends on the nature of the rating scales the voters use to express their opinions. [14] [15] [16] [17]

Spoiler effects can also occur in some methods of proportional representation, such as the single transferable vote (STV or RCV-PR) and the largest remainders method of party-list representation, where it is called a new party paradox. A new party entering an election causes some seats to shift from one unrelated party to another, even if the new party wins no seats. [18] This kind of spoiler effect is avoided by divisor methods and proportional approval. [18] :Thm.8.3

Motivation

In decision theory, independence of irrelevant alternatives is a fundamental principle of rational choice which says that a decision between two outcomes, A or B, should not depend on the quality of a third, unrelated outcome C. A famous joke by Sidney Morgenbesser illustrates this principle: [19]

A man is deciding whether to order apple, blueberry, or cherry pie before settling on apple. The waitress informs him that the cherry pie is very good and a favorite of most customers. The man replies "in that case, I'll have the blueberry."

Politicians and social choice theorists have long argued for the unfairness of spoiler effects. The mathematician and political economist Nicolas de Condorcet was the first to study the spoiler effect, in the 1780s. [20]

Manipulation by politicians

Voting systems that violate independence of irrelevant alternatives are susceptible to being manipulated by strategic nomination. Such systems may produce an incentive to entry, increasing a candidate's chances of winning if similar candidates join the race, or an incentive to exit, reducing the candidate's chances of winning.

Some systems are particularly infamous for their ease of manipulation, such as the Borda count, which exhibits a particularly severe entry incentive, letting any party "clone their way to victory" by running a large number of candidates. This famously forced de Borda to concede that "my system is meant only for honest men," [21] [22] and eventually led to its abandonment by the French Academy of Sciences. [22]

Other systems exhibit an exit incentive. The vote splitting effect in plurality voting demonstrates this method's strong exit incentive: if multiple candidates with similar views run in an election, their supporters' votes will be diluted, which may cause a unified opposition candidate to win despite having less support. This effect encourages groups of similar candidates to form an organization to make sure they don't step on each other's toes. [23]

By electoral system

Susceptibility to spoilers
Electoral systemSpoiler effect
Plurality voting High
Runoffs or RCV Medium
Condorcet methods Low
Score or Medians Depends

Different electoral systems have different levels of vulnerability to spoilers. In general, spoilers are common with plurality voting, somewhat common in plurality-runoff methods, rare with majoritarian methods, and with a varying level of spoiler vulnerability with most rated voting methods. [note 1]

First-preference plurality

In cases where there are many similar candidates, spoiler effects occur most often in first-preference plurality (FPP).[ citation needed ] For example, in the United States, vote splitting is common in primaries, where many similar candidates run against each other. The purpose of a primary election is to eliminate vote splitting among candidates from the same party in the general election by running only one candidate. In a two-party system, party primaries effectively turn FPP into a two-round system. [24] [25] [26]

Vote splitting is the most common cause of spoiler effects in FPP. In these systems, the presence of many ideologically-similar candidates causes their vote total to be split between them, placing these candidates at a disadvantage. [27] [28] This is most visible in elections where a minor candidate draws votes away from a major candidate with similar politics, thereby causing a strong opponent of both to win. [27] [29]

Runoff systems

Plurality-runoff methods like the two-round system and RCV still experience vote-splitting in each round. This produces a kind of spoiler effect called a center squeeze. Compared to plurality without primaries, the elimination of weak candidates in earlier rounds reduces their effect on the final results; however, spoiled elections remain common compared to other systems. [28] [30] [31] As a result, instant-runoff voting still tends towards two-party rule through the process known as Duverger's law. [16] [32] A notable example of this can be seen in Alaska's 2024 race, where party elites pressured candidate Nancy Dahlstrom into dropping out to avoid a repeat of the spoiled 2022 election. [33] [34] [35]

Tournament (Condorcet) voting

Spoiler effects rarely occur when using tournament solutions, where candidates are compared in one-on-one matchups to determine relative preference. For each pair of candidates, there is a count for how many voters prefer the first candidate in the pair to the second candidate The resulting table of pairwise counts eliminates the step-by-step redistribution of votes, which is usually the cause for spoilers in other methods. [36] This pairwise comparison means that spoilers can only occur when there is a Condorcet cycle, where there is no single candidate preferred to all others. [36] [37] [38]

Theoretical models suggest that somewhere between 90% and 99% of real-world elections have a Condorcet winner, [37] [38] and the first Condorcet cycle in a ranked American election was found in 2021. [39] Some systems like the Schulze method and ranked pairs have stronger spoiler resistance guarantees that limit which candidates can spoil an election without a Condorcet winner. [40] :228–229

Rated voting

Rated voting methods ask voters to assign each candidate a score on a scale (e.g. rating them from 0 to 10), instead of listing them from first to last. Highest median and score (highest mean) voting are the two most prominent examples of rated voting rules. Whenever voters rate candidates independently, the rating given to one candidate does not affect the ratings given to the other candidates. Any new candidate cannot change the winner of the race without becoming the winner themselves, which would disqualify them from the definition of a spoiler. For this to hold, in some elections, some voters must use less than their full voting power despite having meaningful preferences among viable candidates.

The outcome of rated voting depends on the scale used by the voter or assumed by the mechanism. [41] If the voters use relative scales, i.e. scales that depend on what candidates are running, then the outcome can change if candidates who don't win drop out. [42] Empirical results from panel data suggest that judgments are at least in part relative. [43] [44] Thus, rated methods, as used in practice, may exhibit a spoiler effect caused by the interaction between the voters and the system, even if the system itself passes IIA given an absolute scale.

Proportional representation

Spoiler effects can also occur in some methods of proportional representation, such as the single transferable vote (STV or RCV-PR) and the largest remainders method of party-list representation, where it is called a new party paradox. A new party entering an election causes some seats to shift from one unrelated party to another, even if the new party wins no seats. [18] This kind of spoiler effect is avoided by divisor methods and proportional approval. [18] :Thm.8.3

Spoiler campaign

United States

A spoiler campaign in the United States is often one that cannot realistically win but can still determine the outcome by pulling support from a more competitive candidate. [45] The two major parties in the United States, the Republican Party and Democratic Party, have regularly won 98% of all state and federal seats. [46] The US presidential elections most consistently cited as having been spoiled by third-party candidates are 1844 [47] and 2000. [48] [49] [50] [47] The 2016 election is more disputed as to whether it contained spoiler candidates or not. [51] [52] [53] For the 2024 presidential election, Republican lawyers and operatives have fought to keep right-leaning third-parties like the Constitution Party off of swing state ballots [54] while working to get Cornel West on battleground ballots. [55] Democrats have helped some right-leaning third-parties gain ballot access while challenging ballot access of left-leaning third-parties like the Green Party. [56] According to the Associated Press , the GOP effort to prop up possible spoiler candidates in 2024 appears more far-reaching than the Democratic effort. [57] Barry Burden argues that they have almost no chance of winning the 2024 election but are often motivated by particular issues. [58]

Third party candidates are always controversial because almost anyone could play spoiler. [59] [60] This is especially true in close elections where the chances of a spoiler effect increase. [61] Strategic voting, especially prevalent during high stakes elections with high political polarization, often leads to a third-party that underperforms its poll numbers with voters wanting to make sure their least favorite candidate is not in power. [46] [62] [63] Third-party campaigns are more likely to result in the candidate a third party voter least wants in the White House. [60] Third-party candidates prefer to focus on their platform than on their impact on the frontrunners. [60]

Notable unintentional spoilers

An unintentional spoiler is one that has a realistic chance of winning but falls short and affects the outcome of the election.

2009 Burlington mayoral election

In Burlington, Vermont's second IRV election, spoiler Kurt Wright knocked out Democrat Andy Montroll in the second round, leading to the election of Bob Kiss, despite the election results showing most voters preferred Montroll to Kiss. [64] The results of every possible one-on-one election can be completed as follows:

Democratic Disc.svg Andy Montroll (D)6262 (Montroll) –

591 (Simpson)

4570 (Montroll) –

2997 (Smith)

4597 (Montroll) –

3664 (Wright)

4064 (Montroll) –

3476 (Kiss)

4/4 Wins
Progressive Disk.png Bob Kiss (P)5514 (Kiss) –

844 (Simpson)

3944 (Kiss) –

3576 (Smith)

4313 (Kiss) –

4061 (Wright)

3/4 Wins RCV winner
Republican Disc.svg Kurt Wright (R)5270 (Wright) –

1310 (Simpson)

3971 (Wright) –

3793 (Smith)

2/4 Wins Spoiler for Montroll
Dan Smith (I)5570 (Smith) –

721 (Simpson)

1/4 Wins
Green Disc.svg James Simpson (G) 0/4 Wins

This leads to an overall preference ranking of:

  1. Montroll – defeats all candidates below, including Kiss (4,064 to 3,476)
  2. Kiss – defeats all candidates below, including Wright (4,313 to 4,061)
  3. Wright – defeats all candidates below, including Smith (3,971 to 3,793)
  4. Smith – defeats Simpson (5,570 to 721) and the write-in candidates

Montroll was therefore preferred over Kiss by 54% of voters, over Wright by 56%, and over Smith by 60%. Had Wright not run, Montroll would have won instead of Kiss. [64] [65]

Because all ballots were fully released, it is possible to reconstruct the winners under other voting methods. While Wright would have won under plurality, Kiss won under IRV, and would have won under a two-round vote or a traditional nonpartisan blanket primary. Montroll, being the majority-preferred candidate, would have won if the ballots were counted using ranked pairs (or any other Condorcet method). [66]

2022 Alaska's at-large congressional district special election

In Alaska's first-ever IRV election, Nick Begich was eliminated in the first round to advance Mary Peltola and Sarah Palin. However, the pairwise comparison shows that Begich was the Condorcet winner while Palin was both the Condorcet loser and a spoiler: [67]

Pairwise comparison matrix by vote totals [68] [67]
BegichPeltolaPalin
Begich-88,126101,438
Peltola79,486-91,375
Palin63,66686,197-
Pairwise as a percentage
WinnerLoserWinnerLoser
Begichvs.Peltola52.6%vs. 47.4%
Begichvs.Palin61.4%vs. 38.6%
Peltolavs.Palin51.5%vs. 48.5%

In the wake of the election, a poll found 54% of Alaskans, including a third of Peltola voters, supported a repeal of RCV. [69] [70] [71] Observers noted such pathologies would have occurred under Alaska's previous primary system as well, leading several to suggest Alaska adopt any one of several alternatives without this behavior. [72]

See also

Notes

  1. Strategic voting can sometimes create additional spoiler-like behavior. However, this does not substantially affect the general order described here.

Related Research Articles

<span class="mw-page-title-main">Plurality voting</span> Type of electoral system

Plurality voting refers to electoral systems in which the candidates in an electoral district who poll more than any other are elected.

<span class="mw-page-title-main">Two-round system</span> Voting system

The two-round system, also called ballotage, top-two runoff, or two-round plurality, is a single winner voting method. It is sometimes called plurality-runoff, although this term can also be used for other, closely-related systems such as instant-runoff voting or the exhaustive ballot. It falls under the class of plurality-based voting rules, together with instant-runoff and first-past-the-post (FPP). In a two-round system, if no candidate receives a majority of the vote in the first round, the two candidates with the most votes in the first round proceed to a second round where all other candidates are excluded. Both rounds are held under choose-one voting, where the voter marks a single favored candidate.

<span class="mw-page-title-main">Condorcet method</span> Pairwise-comparison electoral system

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 or Pairwise Majority Rule Winner (PMRW). The head-to-head elections need not be done separately; a voter's choice within any given pair can be determined from the ranking.

<span class="mw-page-title-main">Arrow's impossibility theorem</span> Proof all ranked voting rules have spoilers

Arrow's impossibility theorem is a key result in social choice theory, showing that no ranking-based decision rule can satisfy the requirements of rational choice theory. Most notably, Arrow showed that no such rule can satisfy independence of irrelevant alternatives, the principle that a choice between two alternatives A and B should not depend on the quality of some third, unrelated option C.

<span class="mw-page-title-main">Negative responsiveness paradox</span> Property of electoral systems

In social choice, the negative responsiveness, perversity, or additional support paradox is a pathological behavior of some voting rules, where a candidate loses as a result of having "too much support" from some voters, or wins because they had "too much opposition". In other words, increasing (decreasing) a candidate's ranking or rating causes that candidate to lose (win). Electoral systems that do not exhibit perversity are said to satisfy the positive response or monotonicitycriterion.

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<span class="mw-page-title-main">Condorcet winner criterion</span> Property of electoral systems

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<span class="mw-page-title-main">No-show paradox</span> When voting for a candidate makes them lose

In social choice, a no-show paradox is a surprising behavior in some voting rules, where a candidate loses an election as a result of having too many supporters. More formally, a no-show paradox occurs when adding voters who prefer Alice to Bob causes Alice to lose the election to Bob. Voting systems without the no-show paradox are said to satisfy the participation criterion.

<span class="mw-page-title-main">Nanson's method</span> Single-winner electoral system

The Borda count electoral system can be combined with an instant-runoff procedure to create hybrid election methods that are called Nanson method and Baldwin method. Both methods are designed to satisfy the Condorcet criterion, and allow for incomplete ballots and equal rankings.

Instant-runoff voting (IRV) is a ranked voting method used in single-winner elections. IRV is also known outside the US as the alternative vote (AV). Today it is in use at a national level to elect the Australian House of Representatives, the National Parliament of Papua New Guinea, the President of Ireland and President of India. In Australia it is also used for elections to the legislative assemblies of all states and territories except Tasmania and the Australian Capital Territory, and for the Tasmanian Legislative Council.

Later-no-harm is a property of some ranked-choice voting systems, first described by Douglas Woodall. In later-no-harm systems, increasing the rating or rank of a candidate ranked below the winner of an election cannot cause a higher-ranked candidate to lose. It is a common property in the plurality-rule family of voting systems.

<span class="mw-page-title-main">Ranked-choice voting in the United States</span> Electoral system used in some cities and states

Ranked-choice voting (RCV) can refer to one of several ranked voting methods used in some cities and states in the United States. The term is not strictly defined, but most often refers to instant-runoff voting (IRV) or single transferable vote (STV), the main difference being whether only one winner or multiple winners are elected.

<span class="mw-page-title-main">Instant-runoff voting</span> Single-winner ranked-choice electoral system

Instant-runoff voting (IRV), is a single-winner, multi-round elimination rule that uses ranked voting to simulate a series of runoff elections. In each round, the last-place finisher according to a plurality vote is eliminated, and the votes supporting the eliminated choice are transferred to their next available preference until one of the options reaches a majority of the remaining votes. Instant runoff falls under the plurality-with-elimination family of voting methods, and is thus closely related to rules like the exhaustive ballot and two-round runoff system.

<span class="mw-page-title-main">2009 Burlington mayoral election</span> American municipal election in Vermont

The 2009 Burlington mayoral election was the second mayoral election since the city's 2005 change to instant-runoff voting (IRV), also known as ranked-choice voting (RCV), after the 2006 mayoral election. In the 2009 election, incumbent Burlington mayor won reelection as a member of the Vermont Progressive Party, defeating Kurt Wright in the final round with 48% of the vote.

<span class="mw-page-title-main">Electoral system</span> Method by which voters make a choice between options

An electoral or voting system is a set of rules used to determine the results of an election. Electoral systems are used in politics to elect governments, while non-political elections may take place in business, non-profit organisations and informal organisations. These rules govern all aspects of the voting process: when elections occur, who is allowed to vote, who can stand as a candidate, how ballots are marked and cast, how the ballots are counted, how votes translate into the election outcome, limits on campaign spending, and other factors that can affect the result. Political electoral systems are defined by constitutions and electoral laws, are typically conducted by election commissions, and can use multiple types of elections for different offices.

<span class="mw-page-title-main">Ranked voting</span> Voting systems that use ranked ballots

Ranked voting is any voting system that uses voters' rankings of candidates to choose a single winner or multiple winners. More formally, a ranked system is one that depends only on which of two candidates is preferred by a voter, and as such does not incorporate any information about intensity of preferences. Ranked voting systems vary dramatically in how preferences are tabulated and counted, which gives them very different properties. In instant-runoff voting (IRV) and the single transferable vote system (STV), lower preferences are used as contingencies and are only applied when all higher-ranked preferences on a ballot have been eliminated.

There are a number of different criteria which can be used for voting systems in an election, including the following

<span class="mw-page-title-main">Top-four primary</span> Nonpartisan blanket primary

A final-four or final-five primary is an electoral system using a nonpartisan primary by multi-winner plurality in the first step.

<span class="mw-page-title-main">Center squeeze</span> Type of independence of irrelevant alternatives violation

Center squeeze is a kind of independence of irrelevant alternatives violation seen in a number of election rules, such as two-round and instant runoff, for example. In a center squeeze, the Condorcet winner is eliminated before they have the chance to face any of the other candidates in a one-on-one race. The term can also refer to tendency of such rules to encourage polarization among elected officials.

References

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