Spoiler effect

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The "spoiler effect" describes the effect a minor party candidate with little chance of winning has upon a close election, when that candidate's presence in the election draws votes from a candidate similar to them, thereby causing a candidate dissimilar to them to win the election. The minor candidate causing this effect is often referred to as a spoiler.

The spoiler effect is one of the components contributing to Duverger's law, the political-science principle that a first-past-the-post election system creates and preserves a two-party system.

If preferential ballots are not used, the spoiler candidate takes votes away from the viable similar candidate (an effect called vote splitting). In some cases, even though the spoiler candidate cannot win themselves, their influence upon the voters may allow them to deliberately determine which of the viable candidates wins the election—a situation known as a kingmaker scenario.

In a preferential voting system, a voter can vote for a minor party candidate as their first choice, and in addition, they can record a preference between the remaining candidates, whether they are in a "major party" or not. For example, voters for a very left-wing candidate might select a moderately left-wing candidate as their second choice, thus minimizing the chances that their vote will result in the election of a right-wing candidate. Approval voting can also reduce the impact of the "spoiler effect".

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Mathematical definitions

Possible mathematical definitions for the spoiler effect include failure of the independence of irrelevant alternatives (IIA) axiom, and vote splitting.

Arrow's impossibility theorem states that rank-voting systems are unable to satisfy the independence of irrelevant alternatives criterion without exhibiting other undesirable properties as a consequence. However, different voting systems are affected to a greater or lesser extent by IIA failure. For example, instant runoff voting is considered to have less frequent IIA failure than First Past the Post. The independence of Smith-dominated alternatives (ISDA) criterion is similar to IIA; unlike IIA, some ranked-ballot voting methods can pass ISDA.

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