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In social choice theory and politics, the spoiler effect refers to a situation where the entry of a losing (that is, irrelevant) candidate affects the results of an election.[1][2] A voting system that is not affected by spoilers satisfies independence of irrelevant alternatives or independence of spoilers.
By Arrow's theorem, all ranked-choice voting systems[note 1] are vulnerable to spoiler effects. However, the susceptibility of different ranked systems varies greatly. Plurality and instant-runoff voting suffer from spoilers more often than other methods.[2][3][4] Majority-rule systems are usually not affected by spoilers, which are limited to rare[5][6] situations known as cyclic ties.[4]
Rated voting systems are not subject to Arrow's theorem; as a result, many satisfy independence of irrelevant alternatives (sometimes called spoilerproofness).[7][8]
A spoiler effect occurs when a single party or a candidate entering an election changes the outcome to favor a different candidate.
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
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