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 without winnowing or primary elections is sensitive to spoilers. And so, to a degree, are instant-runoff or ranked-choice voting (RCV) 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 (STV or RCV-PR) and the largest remainders method of party-list representation, where it is called the new states paradox. A new party entering an election causes some seats to shift from one unrelated party to another, even if the new party wins no seats. This kind of spoiler effect is avoided by divisor methods and proportional approval.