Instant-runoff voting

Instant-runoff voting (IRV; US/CA: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 system and party primary.

Instant-runoff voting has found some use in national elections in several countries, mostly in the former British Empire. 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). Other critics of instant-runoff voting have pointed to further mathematical pathologies, including the ability of certain implementations to have the potential 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 winner-take-all methods can leave a large portion of the electorate without representation. For example, in a race that tabulates votes using a simple majority, if a candidate wins with only 40% of the vote, this means that the majority (60%) of the electorate did not vote for the winning candidate. In countries without primaries or runoffs, like the United Kingdom, instant-runoff voting is seen as preventing 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. Some advocates also argue that voting methods should encourage candidates to appeal to their core support or political base rather than a broad coalition.