![]() | This article may be too technical for most readers to understand.(August 2024) |
Part of the Politics and Economics series |
Electoral systems |
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In social choice theory and politics, the spoiler effect refers to a situation where a large group of like-minded voters split their votes among multiple candidates, which can affect the result of an election by allowing a candidate with a smaller base of support to win with a plurality. If a major candidate is perceived to have lost an election because a more minor candidate pulled votes away from them, the minor candidate is called a spolier candidate and the major candidate is said to have been spoiled. This phenomenon is also called vote splitting.[1] A voting system that is not affected by spoilers satisfies the condition independence of irrelevant alternatives.[2]
Arrow's impossibility theorem posits that all rank-based voting systems[note 1] are vulnerable to the spoiler effect. However, the frequency and severity of spoiler effects depends substantially on the voting method. Majority-rule methods are only rarely affected by spoilers, which are limited to rare[3][4] situations called cyclic ties.[5] Plurality is the most sensitive to spoilers, while ranked-choice voting (RCV-IRV) is still sensitive though less so in most scenarios.[6][7][8]
Spoiler effects exist but may be less common 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. Here, a new party entering an election can cause seats to shift from one unrelated party to another, even if the new party wins no seats; this is known as the new states paradox.
A spoiler effect occurs when a single party or a candidate entering an election changes the outcome to favor a different candidate.
In terms of the performance of the different election systems, we confirm the results of Merrill (1984, 1985, 1988) that in multicandidate elections run-off and sequential elimination systems perform far better than plurality elections, in that they are more likely to pick the Condorcet winner, and have a lower variance in their outcomes. This is true even if the distribution is skewed or bimodal. However, with skewed distributions, run-off and sequential elimination elections still have a bias away from the median in the direction of the mode, although this is typically smaller than that with plurality election.
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
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