Spoiler effect
Loser affecting an election's results / From Wikipedia, the free encyclopedia
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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]