Instant-runoff voting

For the proportional electoral system applying ranked ballots to multi-member constituencies, see Single transferable vote.
For ranked methods in general, see Ranked voting.

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 multiple runoff elections. In each round, the candidate with the fewest first-preference votes (among the remaining candidates) is eliminated. This continues until only one candidate is left. Instant runoff falls under the plurality-with-elimination family of voting methods, and is thus closely related to methods like the two-round runoff system and party primary systems.

Instant-runoff voting has found some use 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 system was first discussed by the Marquis de Condorcet, who showed the rule could eliminate the majority-preferred candidate (Condorcet winner). Since then, instant-runoff voting has been criticized for other mathematical pathologies, including its ability to eliminate candidates for having too much support or too many votes. Like first-past-the-post voting (FPTP), instant-runoff is vulnerable to a kind of spoiler effect called a center squeeze, which causes it to favor uncompromising alternatives over more-moderate ones, encouraging polarization.

Advocates of instant-runoff voting often argue these properties are positive, as voting methods 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, because it avoids some kinds of vote-splitting by nearly identical (clone) candidates. IRV has also been described as a natural extension of the two-round system or primary elections that avoids multiple rounds of voting.

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