Instant-runoff voting
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Instant-runoff voting (IRV; US: ranked-choice voting (RCV), AU: preferential voting, UK/NZ: alternative vote) is a single-winner ranked voting election system where one or more eliminations are used to simulate runoff elections. When no candidate has a majority of the votes in the first round of counting, each following round eliminates the candidate with the fewest first-preferences (among the remaining candidates) and transfers their votes if possible. This continues until one candidate accumulates a majority of the votes still in play.
Instant-runoff voting falls under the plurality-based voting-rule family, in that under certain conditions the candidate with the least votes is eliminated, making use of secondary rankings as contingency votes. Thus it is related to the two-round runoff system and the exhaustive ballot. IRV could also be seen as a single-winner equivalent of single transferable voting.
Unlike first-past-the-post voting, IRV is a sequential procedure. Unlike contingent voting (AKA supplementary voting), which has just two rounds of counting at most, IRV may entail numerous rounds of counting.
Instant-runoff voting (IRV) has been used in national elections in several countries, predominantly in the Anglosphere. It is used to elect members of the Australian House of Representatives and the National Parliament of Papua New Guinea, and to elect the head of state in India, Ireland, and Sri Lanka.
The rule was first studied by the Marquis de Condorcet, who observed it could eliminate the majority-preferred candidate (Condorcet winner). Since then, instant-runoff voting has been criticized for failing other criteria, including its ability to eliminate candidates for having too much support or too many votes. Instant-runoff voting may exhibit a kind of Independence of irrelevant alternatives violation called a center squeeze, which causes it to favor uncompromising alternatives over more moderate ones, which may in turn hinder a recovery from increasing polarization between the candidates and limit free entry.
Some advocates of instant-runoff voting have argued these properties are positive, as voting rules should encourage candidates to appeal to their core support or political base rather than a broad coalition. They also note that in countries like the UK without primaries or runoffs, instant-runoff voting can prevent spoiler effects by eliminating minor-party candidates. Unlike a plurality vote system where votes are non-transferable, instant-runoff voting also avoids some kinds of vote-splitting by near-identical (clone) candidates. IRV has also been advocated for as a natural and practical generalization of the two-round system.