Instant-runoff voting

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Instant-runoff voting (IRV), also known as ranked-choice voting (RCV), preferential voting (PV), or the alternative vote (AV), is a multi-round elimination rule where the loser of each round is determined by first-past-the-post voting. In academic contexts, the system is generally called instant-runoff voting to avoid conflating it with other methods of ranked voting in general.

IRV is counted in steps. In each step, IRV looks up every ballot’s top uneliminated candidate, then eliminates the candidate with the fewest top rankings. This continues until only one candidate is left, at which point they are declared the winner. IRV falls into the plurality-with-elimination family of voting rules, alongside first-past-the-post and two-round or primary systems (whether partisan or nonpartisan). It can be contrasted with the rated and majority-rule methods of holding elections.

IRV has found some use in national elections in several countries, predominantly in the Anglosphere, having been spread by British barristers such as Thomas Hare and Henry Richmond Droop in the 19th century. It is used to elect members of the Australian House of Representatives^[1] and the National Parliament of Papua New Guinea^[2] along with the figurehead presidents of India and of Ireland.^[3] It has recently gained substantial popularity in the United States and in Canada, after substantial promotion by advocacy groups like FairVote.

Despite this, the system has had a mixed reception among political scientists and social choice theorists.^[4][5] Political scientists have found that misunderstandings of ranked ballots cause ballots to be rejected roughly 10 times more often than in first-past-the-post races,^[6][6] meaning the system may not decrease the overall number of wasted votes relative to first-past-the-post;^[7][6][2] research has also identified high rates of exhausted ballots (i.e. incomplete, often partly-spoiled ballots), which occur often enough to potentially decide the outcome in most competitive elections.^[8] Research has found IRV causes lower confidence in and satisfaction with election results^[9][10][11] and does not substantially increase minority representation,^[12] voter turnout,^[4] or long-run electoral competition.^[4][12]

Research on the rule by social choice theorists dates back to the field's inception, with its inventor (the Marquis de Condorcet) being the first to show it could eliminate majority-preferred candidates. Since then, IRV has been shown to suffer from several mathematical pathologies not shared by other voting rules (see below), leading some to question its use.^[13][14] These include favoring extreme candidates^[15][16][17] and discouraging moderates from running;^[18] eliminating candidates who receive "too many" votes;^[19] violating the majority-rule principle;^[12][20] and spoiling election results at a relatively high rate.^[21][22]

Advocates have responded by arguing these properties are either intentional or unimportant, as voting rules should encourage candidates to focus on building a strong appeal to their core supporters, rather than compromising to build broad appeal with a majority of voters.^[23] They note that IRV eliminates some kinds of spoiler effects seen in first-past-the-post and argue that it is resistant to strategic voting. Others have claimed instant-runoff is simpler and easier to understand than majority-rule methods of voting, and its similarity to primary elections makes it familiar and easy to pass, although opponents have noted a high rate of repeals for the system.^[10]