Instant-runoff voting

Instant-runoff voting (IRV; US: ranked-choice voting (RCV), AU: preferential voting, UK/NZ: alternative vote) is a single-winner, multi-round elimination rule that uses ranked voting to simulate a series of runoff elections. In each round, the candidate with the fewest first-preferences (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,^[1] and is thus closely related to rules like the two-round runoff system.^[2]^[3]

IRV 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 rule was first studied by the Marquis de Condorcet, who observed it could eliminate the majority-preferred candidate (Condorcet winner).^[4]^[5]^[6]^[7] Since then, IRV has been criticized for other mathematical pathologies (discussed below), including its ability to eliminate candidates for having too much support or too many votes.^[8] Like plurality, IRV also exhibits a kind of spoiler effect called a center squeeze,^[9]^[10] which causes it to favor uncompromising alternatives over more-moderate ones.^[11]^[12]^[13]^[14]

Advocates of IRV often argue these properties are positive, as voting rules should encourage candidates to appeal to their core support or political base rather than a broad coalition.^[15] They also note that in countries like the UK without primaries or runoffs, IRV can prevent spoiler effects by eliminating minor-party candidates. Unlike a straight plurality vote, IRV also avoids some kinds of vote-splitting by near-identical (clone) candidates.

^ Nurmi, Hannu (June 2005). "Aggregation problems in policy evaluation: an overview". European Journal of Political Economy. 21 (2): 287–300. doi:10.1016/j.ejpoleco.2004.08.001. ISSN 0176-2680.
^ Aubin, Jean-Baptiste; Gannaz, Irène; Leoni-Aubin, Samuela; Rolland, Antoine (July 2024). A simulation-based study of proximity between voting rules.
^ Hyman, Ross; Otis, Deb; Allen, Seamus; Dennis, Greg (1 September 2024). "A majority rule philosophy for instant runoff voting". Constitutional Political Economy. 35 (3): 425–436. arXiv:2308.08430. doi:10.1007/s10602-024-09442-3. ISSN 1572-9966.
^ Condorcet, Jean-Antoine-Nicolas de Caritat (1788). "On the Constitution and the Functions of Provincial Assemblies". Complete Works of Condorcet (in French). Vol. 13 (published 1804). p. 243. En effet, lorsqu'il y a plus de trois concurrents, le véritable vœu de la pluralité peut être pour un candidat qui n'ait eu aucune des voix dans le premier scrutin.
^ Nanson, E. J. (1882). "Methods of election: Ware's Method". Transactions and Proceedings of the Royal Society of Victoria. 17: 206. The method was, however, mentioned by Condorcet, but only to be condemned.
^ Campbell, D.E.; Kelly, J.S. (2000). "A simple characterization of majority rule". Economic Theory. 15 (3): 689–700. doi:10.1007/s001990050318. JSTOR 25055296. S2CID 122290254.
^ McGann, Anthony J.; Koetzle, William; Grofman, Bernard (2002). "How an Ideologically Concentrated Minority Can Trump a Dispersed Majority: Nonmedian Voter Results for Plurality, Run-off, and Sequential Elimination Elections". American Journal of Political Science. 46 (1): 134–147. doi:10.2307/3088418. ISSN 0092-5853. JSTOR 3088418. As with simple plurality elections, it is apparent the outcome will be highly sensitive to the distribution of candidates.
^ Doron, Gideon; Kronick, Richard (1977). "Single Transferrable Vote: An Example of a Perverse Social Choice Function". American Journal of Political Science. 21 (2): 303–311. doi:10.2307/2110496. ISSN 0092-5853. JSTOR 2110496.
^ Nurmi, Hannu (December 1996). "It's not just the lack of monotonicity1". Representation. 34 (1): 48–52. doi:10.1080/00344899608522986. ISSN 0034-4893.
^ Atkinson, Nathan; Ganz, Scott C. (30 October 2022). "The flaw in ranked-choice voting: rewarding extremists". The Hill. Retrieved 14 May 2023. However, ranked-choice voting makes it more difficult to elect moderate candidates when the electorate is polarized. For example, in a three-person race, the moderate candidate may be preferred by a majority of voters to each of the more extreme candidates. However, voters with far-left and far-right views will rank the candidate in second place rather than in first place. Since ranked-choice voting counts only the number of first-choice votes (among the remaining candidates), the moderate candidate would be eliminated in the first round, leaving one of the extreme candidates to be declared the winner.
^ McGann, Anthony J.; Koetzle, William; Grofman, Bernard (2002). "How an Ideologically Concentrated Minority Can Trump a Dispersed Majority: Nonmedian Voter Results for Plurality, Run-off, and Sequential Elimination Elections". American Journal of Political Science. 46 (1): 134–147. doi:10.2307/3088418. ISSN 0092-5853. JSTOR 3088418. As with simple plurality elections, it is apparent the outcome will be highly sensitive to the distribution of candidates.
^ Merrill, Samuel (1985). "A statistical model for Condorcet efficiency based on simulation under spatial model assumptions". Public Choice. 47 (2): 389–403. doi:10.1007/bf00127534. ISSN 0048-5829. the 'squeeze effect' that tends to reduce Condorcet efficiency if the relative dispersion (RD) of candidates is low. This effect is particularly strong for the plurality, runoff, and Hare systems, for which the garnering of first-place votes in a large field is essential to winning
^ Merrill, Samuel (1984). "A Comparison of Efficiency of Multicandidate Electoral Systems". American Journal of Political Science. 28 (1): 23–48. doi:10.2307/2110786. ISSN 0092-5853. JSTOR 2110786. However, squeezed by surrounding opponents, a centrist candidate may receive few first-place votes and be eliminated under Hare.
^ Robinette, Robbie (1 September 2023). "Implications of strategic position choices by candidates". Constitutional Political Economy. 34 (3): 445–457. doi:10.1007/s10602-022-09378-6. ISSN 1572-9966.
^ Hyman, Ross; Otis, Deb; Allen, Seamus; Dennis, Greg (September 2024). "A Majority Rule Philosophy for Instant Runoff Voting". Constitutional Political Economy. 35 (3): 425–436. arXiv:2308.08430. doi:10.1007/s10602-024-09442-3. ISSN 1043-4062.

[1] Nurmi, Hannu (June 2005). "Aggregation problems in policy evaluation: an overview". European Journal of Political Economy. 21 (2): 287–300. doi:10.1016/j.ejpoleco.2004.08.001. ISSN 0176-2680.

[2] Aubin, Jean-Baptiste; Gannaz, Irène; Leoni-Aubin, Samuela; Rolland, Antoine (July 2024). A simulation-based study of proximity between voting rules.

[3] Hyman, Ross; Otis, Deb; Allen, Seamus; Dennis, Greg (1 September 2024). "A majority rule philosophy for instant runoff voting". Constitutional Political Economy. 35 (3): 425–436. arXiv:2308.08430. doi:10.1007/s10602-024-09442-3. ISSN 1572-9966.

[:03-4] Condorcet, Jean-Antoine-Nicolas de Caritat (1788). "On the Constitution and the Functions of Provincial Assemblies". Complete Works of Condorcet (in French). Vol. 13 (published 1804). p. 243. En effet, lorsqu'il y a plus de trois concurrents, le véritable vœu de la pluralité peut être pour un candidat qui n'ait eu aucune des voix dans le premier scrutin.

[:03223-5] Nanson, E. J. (1882). "Methods of election: Ware's Method". Transactions and Proceedings of the Royal Society of Victoria. 17: 206. The method was, however, mentioned by Condorcet, but only to be condemned.

[:223223-6] Campbell, D.E.; Kelly, J.S. (2000). "A simple characterization of majority rule". Economic Theory. 15 (3): 689–700. doi:10.1007/s001990050318. JSTOR 25055296. S2CID 122290254.

[:85223-7] McGann, Anthony J.; Koetzle, William; Grofman, Bernard (2002). "How an Ideologically Concentrated Minority Can Trump a Dispersed Majority: Nonmedian Voter Results for Plurality, Run-off, and Sequential Elimination Elections". American Journal of Political Science. 46 (1): 134–147. doi:10.2307/3088418. ISSN 0092-5853. JSTOR 3088418. As with simple plurality elections, it is apparent the outcome will be highly sensitive to the distribution of candidates.

[8] Doron, Gideon; Kronick, Richard (1977). "Single Transferrable Vote: An Example of a Perverse Social Choice Function". American Journal of Political Science. 21 (2): 303–311. doi:10.2307/2110496. ISSN 0092-5853. JSTOR 2110496.

[:733-9] Nurmi, Hannu (December 1996). "It's not just the lack of monotonicity1". Representation. 34 (1): 48–52. doi:10.1080/00344899608522986. ISSN 0034-4893.

[:03244-10] Atkinson, Nathan; Ganz, Scott C. (30 October 2022). "The flaw in ranked-choice voting: rewarding extremists". The Hill. Retrieved 14 May 2023. However, ranked-choice voting makes it more difficult to elect moderate candidates when the electorate is polarized. For example, in a three-person race, the moderate candidate may be preferred by a majority of voters to each of the more extreme candidates. However, voters with far-left and far-right views will rank the candidate in second place rather than in first place. Since ranked-choice voting counts only the number of first-choice votes (among the remaining candidates), the moderate candidate would be eliminated in the first round, leaving one of the extreme candidates to be declared the winner.

[:8524-11] McGann, Anthony J.; Koetzle, William; Grofman, Bernard (2002). "How an Ideologically Concentrated Minority Can Trump a Dispersed Majority: Nonmedian Voter Results for Plurality, Run-off, and Sequential Elimination Elections". American Journal of Political Science. 46 (1): 134–147. doi:10.2307/3088418. ISSN 0092-5853. JSTOR 3088418. As with simple plurality elections, it is apparent the outcome will be highly sensitive to the distribution of candidates.

[Merril_1985224223-12] Merrill, Samuel (1985). "A statistical model for Condorcet efficiency based on simulation under spatial model assumptions". Public Choice. 47 (2): 389–403. doi:10.1007/bf00127534. ISSN 0048-5829. the 'squeeze effect' that tends to reduce Condorcet efficiency if the relative dispersion (RD) of candidates is low. This effect is particularly strong for the plurality, runoff, and Hare systems, for which the garnering of first-place votes in a large field is essential to winning

[Merrill_1984223-13] Merrill, Samuel (1984). "A Comparison of Efficiency of Multicandidate Electoral Systems". American Journal of Political Science. 28 (1): 23–48. doi:10.2307/2110786. ISSN 0092-5853. JSTOR 2110786. However, squeezed by surrounding opponents, a centrist candidate may receive few first-place votes and be eliminated under Hare.

[:2423-14] Robinette, Robbie (1 September 2023). "Implications of strategic position choices by candidates". Constitutional Political Economy. 34 (3): 445–457. doi:10.1007/s10602-022-09378-6. ISSN 1572-9966.

[15] Hyman, Ross; Otis, Deb; Allen, Seamus; Dennis, Greg (September 2024). "A Majority Rule Philosophy for Instant Runoff Voting". Constitutional Political Economy. 35 (3): 425–436. arXiv:2308.08430. doi:10.1007/s10602-024-09442-3. ISSN 1043-4062.

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Instant-runoff voting

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