Spoiler effect
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In social choice theory and politics, the spoiler effect or Arrow's paradox refers to a situation where 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.[3]
Arrow's impossibility theorem is a well-known theorem showing that all rank-based voting systems[note 1] are vulnerable to the spoiler effect. However, the frequency and severity of spoiler effects depends on the voting method. Plurality and ranked-choice runoffs (RCV) are both highly sensitive to spoilers,[4][5] and can manufacture spoiler effects even when doing so is not forced,[2][6][7][8] a situation known as a center squeeze. Majority-rule methods are usually not affected by spoilers, which are limited to rare[9][10] situations called cyclic ties.[11]
Spoiler effects also occur in some methods of proportional representation, such as the single transferable vote (STV-PR or RCV-PR) and the largest remainders method of party-list representation, where a new party entering the election can cause the number of seats for each party to change, even if the vote totals of every other party are unaffected.
Rated voting systems are not subject to Arrow's theorem; as a result, many satisfy independence of irrelevant alternatives (sometimes called spoilerproofness).[3][8][12]