Spoiler effect

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In social choice theory and politics, a spoiler effect happens when a losing candidate affects the results of an election simply by participating. Voting rules that are not affected by spoilers are said to be spoilerproof, and satisfy independence of irrelevant alternatives. The frequency and severity of spoiler effects depends substantially on the voting method. First-past-the-post voting, especially if without winnowing or primary elections, is sensitive to spoilers. And so, to a degree, are instant-runoff voting (often called ranked-choice voting (RCV) in the US) and the two-round system (TRS). Majority-rule (or Condorcet) methods are only rarely affected by spoilers, which are limited to rare situations, called cyclic ties. Rated voting systems are not subject to Arrow's theorem, allowing them to be spoilerproof so long as voters' ratings are consistent across elections. Spoiler effects can also occur in some methods of proportional representation, such as the single transferable vote...

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