Spoiler effect
Loser affecting an election's results / From Wikipedia, the free encyclopedia
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 (plurality-loser) are highly sensitive to spoilers,[4][5] and can manufacture spoiler effects even when doing so is not forced.[2][6][7][8] Majority-rule methods are usually not affected by spoilers, which are limited to rare[9][10] situations called cyclic ties.[11]
Rated voting systems are not subject to Arrow's theorem; as a result, many satisfy independence of irrelevant alternatives (sometimes called spoilerproofness).[3][12][8]