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Insocial choice theory andpolitics, aspoiler effect happens when a losing candidate affects the results of an election simply by participating.[1] Voting rules that are not affected by spoilers are said to bespoilerproof[2][3] and satisfyindependence of irrelevant alternatives.[1][3][4]
The frequency and severity of spoiler effects depends substantially on the voting method.First-past-the-post voting withoutwinnowing or primary elections[citation needed] is sensitive to spoilers. And so, to a degree, areInstant-runoff or ranked-choice voting (RCV) and thetwo-round system (TRS).[4][5][6][7]Majority-rule (or Condorcet) methods are only rarely affected by spoilers, which are limited to rare[8][9] situations calledcyclic ties.[8][9][10]Rated voting systems are not subject toArrow's theorem, allowing them to be spoilerproof so long as voters' ratings are consistent across elections.[2][11][12][13]
Spoiler effects can also occur in some methods ofproportional representation, such as thesingle transferable vote (STV or RCV-PR) and thelargest remainders method of party-list representation, where it is called thenew states 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.[14] This kind of spoiler effect is avoided bydivisor methods andproportional approval.[14]: Thm.8.3
Indecision theory,independence of irrelevant alternatives is a fundamental principle ofrational choice which says that a decision between two outcomes,A orB, should not depend on the quality of a third, unrelated outcomeC. A famous joke bySidney Morgenbesser illustrates this principle:[15]
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 andsocial choice theorists have long argued for the unfairness of spoiler effects. The mathematician and political economistNicolas de Condorcet was the first to study the spoiler effect, in the 1780s.[16]
Voting systems that violate independence of irrelevant alternatives are susceptible to being manipulated bystrategic nomination. Such systems may produce anincentive to entry, increasing a candidate's chances of winning if similar candidates join the race, or anincentive to exit, reducing the candidate's chances of winning.
Some systems are particularly infamous for their ease of manipulation, such as theBorda 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,"[17][18] and eventually led to its abandonment by theFrench Academy of Sciences.[18]
Other systems exhibit an exit incentive. The vote splitting effect inplurality 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.[19]
| Electoral system | Spoiler effect |
|---|---|
| Plurality voting | High |
| Runoffs orRCV | Medium |
| Condorcet methods | Low |
| Score orMedians | None |
Differentelectoral systems have different levels of vulnerability to spoilers. In general, spoilers are common withplurality voting, somewhat common inplurality-runoff methods, rare withmajoritarian methods, and impossible for mostrated voting methods.[note 1]
In cases where there are many similar candidates, spoiler effects occur most often infirst-preference plurality (FPP).[citation needed] For example, in the United States, vote splitting is common inprimaries, 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 thegeneral election by running only one candidate. In a two-party system, party primaries effectively turnFPP into atwo-round system.[20][21][22]
Vote splitting is the most common cause of spoiler effects inFPP. 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.[23][24] 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.[23][25]
Plurality-runoff methods like thetwo-round system andRCV still experience vote-splitting in each round. This produces a kind of spoiler effect called acenter 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.[24][26][27] As a result, instant-runoff voting still tends towardstwo-party rule through the process known asDuverger's law.[12][28] A notable example of this can be seen inAlaska's 2024 race, where party elites pressured candidateNancy Dahlstrom into dropping out to avoid a repeat of thespoiled 2022 election.[29][30][31]
Spoiler effects rarely occur when usingtournament 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.[10] This pairwise comparison means that spoilers can only occur when there is aCondorcet cycle, where there is no single candidate preferred to all others.[10][32][33]
Theoretical models suggest that somewhere between 90% and 99% of real-world elections have a Condorcet winner,[32][33] and the first Condorcet cycle in a ranked American election was found in 2021.[34] Some systems like theSchulze method andranked pairs have stronger spoiler resistance guarantees that limit which candidates can spoil an election without aCondorcet winner.[35]: 228–229
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 andscore (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.[36] 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.[37] Empirical results from panel data suggest that judgments are at least in part relative.[38][39] 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.
Spoiler effects can also occur in some methods ofproportional representation, such as thesingle transferable vote (STV or RCV-PR) and thelargest remainders method of party-list representation, where it is called anew 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.[14] This kind of spoiler effect is avoided bydivisor methods andproportional approval.[14]: Thm.8.3
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.[40] The two major parties in the United States, theRepublican Party andDemocratic Party, have regularly won 98% of all state and federal seats.[41] The US presidential elections most consistently cited as having been spoiled by third-party candidates are1844[42] and2000.[43][44][45][42] The2016 election is more disputed as to whether it contained spoiler candidates or not.[46][47][48] For the2024 presidential election, Republican lawyers and operatives have fought to keep right-leaning third-parties like theConstitution Party off swing state ballots[49] while working to getCornel West on battleground ballots.[50] Democrats have helped some right-leaning third-parties gain ballot access while challenging ballot access of left-leaning third-parties like theGreen Party.[51] According to theAssociated Press, the GOP effort to prop up possible spoiler candidates in 2024 appears more far-reaching than the Democratic effort.[52]Barry Burden argues that they have almost no chance of winning the 2024 election but are often motivated by particular issues.[53]
Third party candidates are always controversial because almost anyone could play spoiler.[54][55] This is especially true in close elections where the chances of a spoiler effect increase.[56]Strategic voting, especially prevalent during high stakes elections with highpolitical 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.[41][57][58] Third-party campaigns are more likely to result in the candidate a third party voter least wants in the White House.[55] Third-party candidates prefer to focus on their platform than on their impact on the frontrunners.[55]
An unintentional spoiler is one that has a realistic chance of winning but falls short and affects the outcome of the election. Some third-party candidates express ambivalence about which major party they prefer and their possible role as spoiler[59][60] or deny the possibility.[61]
InBurlington, Vermont's second IRV election, spoilerKurt Wright knocked out DemocratAndy Montroll in the second round, leading to the election ofBob Kiss, despite the election results showing most voters preferred Montroll to Kiss.[62] The results of every possible one-on-one election can be completed as follows:
 | Andy Montroll (D) | 6262 (Montroll) – 591 (Simpson) | 4570 (Montroll) – 2997 (Smith) | 4597 (Montroll) – 3664 (Wright) | 4064 (Montroll) – 3476 (Kiss) | 4/4 Wins |
|---|---|---|---|---|---|---|
 | Bob Kiss (P) | 5514 (Kiss) – 844 (Simpson) | 3944 (Kiss) – 3576 (Smith) | 4313 (Kiss) – 4061 (Wright) | 3/4 Wins | RCV winner |
 | 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 | ||||
 | James Simpson (G) | 0/4 Wins | ||||
This leads to an overall preference ranking of:
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.[62][63]
Because all ballots were fully released, it is possible to reconstruct the winners under other voting methods. While Wright would have won underplurality, Kiss won underIRV, and would have won under atwo-round vote or a traditionalnonpartisan blanket primary. Montroll, being themajority-preferred candidate, would have won if the ballots were counted usingranked pairs (or any otherCondorcet method).[64]
InAlaska's first-ever IRV election,Nick Begich was eliminated in the first round to advanceMary Peltola andSarah Palin. However, the pairwise comparison shows that Begich was theCondorcet winner while Palin was both theCondorcet loser and a spoiler:[65]
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In the wake of the election, a poll found 54% of Alaskans, including a third of Peltola voters, supported a repeal of RCV.[67][68][69] Observers noted such pathologies would have occurred under Alaska's previous primary system as well, leading several to suggest Alaska adopt any one ofseveral alternatives without this behavior.[70]
A spoiler effect occurs when a single party or a candidate entering an election changes the outcome to favor a different candidate.
the 'squeeze effect' that tends to reduce Condorcet efficiency if the relative dispersion (RD) of candidates is low. This effect is particularly strong for the plurality, runoff, and Hare systems, for which the garnering of first-place votes in a large field is essential to winning
As with simple plurality elections, it is apparent the outcome will be highly sensitive to the distribution of candidates.
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 ... doesnot 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
One important finding was that having cardinal utilities is not by itself enough to avoid an impossibility result. ... Intuitively speaking, to put information about preference strengths to good use it has to be possible to compare the strengths of different individuals' preferences.
... if two or more candidates with similar views run in the same plurality election, then the voters who support those views will be divided among them, giving an advantage to other candidates with opposed views. Therefore, it is helpful for groups of fairly like-minded people to form some kind of association – that is, a political party – which fields only one candidate per election, and which provides some kind of process for deciding whom this one candidate should be – that is, a primary.
Finally, we should not discount the role of primaries. When we look at the range of countries withfirst-past-the-post (FPTP) elections (given no primaries), none with an assembly larger than Jamaica's (63) has a strict two-party system. These countries include theUnited Kingdom andCanada (where multiparty competition is in fact nationwide). Whether the U.S. should be called 'FPTP' itself is dubious, and not only because some states (e.g.Georgia) hold runoffs or use thealternative vote (e.g.Maine).Rather, the U.S. has an unusual two-round system in which the first round winnows the field. This usually is at the intraparty level, although sometimes it is without regard to party (e.g. in Alaska and California).
American elections become a two-round run-off system with a delay of several months between the rounds.
In effect, the primary system means that the USA has a two-round runoff system of elections.
Those votes that are cast for minor party candidates are perceived as taking away pivotal votes from major party candidates. ... This phenomenon is known as the 'spoiler effect'.
plurality-rule voting is seriously vulnerable to vote-splitting ... runoff voting ... as French history shows, it too is highly subject to vote-splitting. ... [Condorcet] majority rule avoids such vote-splitting debacles because it allows voters to rank the candidates and candidates are compared pairwise
a spoiler effect occurs when entry by a third-party candidate causes party A to defeat party B even though Party B would have won in a two-candidate race.
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 ... doesnot do away with the spoiler problem entirely, although it ... makes it less likely
IRV is excellent for preventing classic spoilers-minor candidates who tip the election from one major candidate to another. It is not so good when the 'spoiler' has a real chance of winning
The 2021 Minneapolis election for city council seat in Ward 2 contained three candidates, each of whom has a legitimate claim to be the winner, the first known example of an American political election without a Condorcet winner ...
The Smith criterion and Smith-IIA (where IIA means "independence of irrelevant alternatives") say that weak alternatives should have no impact on the result of the elections ... the Schulze method, as defined in section 2.2, satisfies Smith-IIA.
If f satisfies U, I, P, and CNC then there exists a dictator.
At best, it is contended that, for an individual, his utility function is uniquely determined up to a linear transformation ... the value of the aggregate (say a sum) are dependent on how the choice is made for each individual.
Perot was running what is commonly referred to as a "spoiler campaign," a campaign that cannot win the election but still impacts its outcome.
The perception that Johnson and Stein 'stole' the 2016 presidential election from Clinton is widespread...Our analysis indicates that Johnson and Stein did not deprive Clinton of an Electoral College majority, nor Trump the legitimacy of winning the national popular vote.
Four years ago, the Green Party candidate played a significant role in several crucial battleground states, drawing a vote total in three of them — Wisconsin, Michigan and Pennsylvania — that exceeded the margin between Donald J. Trump and Hillary Clinton.
Republicans and Democrats view third-party candidates as a threat to siphon critical support from their nominees, especially considering that Pennsylvania was decided by margins of tens of thousands of votes both in 2020 for Democrat Joe Biden and in 2016 for Trump.
there are signs across the country that groups are trying to affect the outcome by using deceptive means — and in most cases in ways that would benefit Republican Donald Trump. Their aim is to whittle away President Joe Biden's standing with the Democratic Party's base by offering left-leaning, third-party alternatives who could siphon off a few thousand protest votes in close swing state contests.
And despite the contenders' claims that the nation deserves an alternative to two unpopular major party choices, the reality, experts say, is that these back-of-the-pack candidates may well cement the election of the candidate they least want in the White House.
There is a Condorcet ranking according to distance from the center, but Condorcet winner M, the most central candidate, was squeezed between the two others, got the smallest primary support, and was eliminated.
Since Begich wins both … he is the Condorcet winner of the election … AK election also contains a Condorcet loser: Sarah Palin. … she is also a spoiler candidate
