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The two-round system (TRS or 2RS), also called ballotage, top-two runoff, or two-round plurality (as originally termed in French[1]), is a single winner voting method. It is sometimes called plurality-runoff,[2] although this term can also be used for other, closely-related systems such as instant-runoff (or ranked-choice) voting or the exhaustive ballot (which typically produce similar results). It falls under the class of plurality-based voting rules, together with instant-runoff (or ranked-choice) and first-past-the-post (FPP).[3] In a two-round system, if no candidate receives a majority of the vote in the first round, the two candidates with the most votes in the first round proceed to a second round where all other candidates are excluded.[note 1] Both rounds are held under choose-one voting, where the voter marks a single favored candidate.
The two-round system first emerged in France, and has since become the most common single-winner electoral system worldwide.[1][4] The two-round system is widely used in the election of legislative bodies and directly elected presidents. Despite this, the rule has received substantial criticism from social choice theorists, leading to the rise of electoral reform movements seeking to abolish it in France and elsewhere.
In the United States, the system is used to elect most public officials in Louisiana (though parties do not put forward just one candidate, which allows multiple candidates from the same party to run in the first round) and in Mississippi and Georgia, though these two states first hold a partisan primary to select each parties' nominees. The states of California, Washington, and Alaska use a similar system known as a nonpartisan blanket primary, where the second round takes place whether or not a candidate receives a majority of the vote in the first round. Alaska's system also differs by advancing four candidates with a ranked-choice runoff between them in the second round. In the rest of the country, the use of partisan primaries paired with the two-party system is structurally similar and is often described as a de facto two-round system.[5][6][7]
Although advocates hoped the two-round method would elect more moderates and encourage turnout among independents, research has shown the method has little to no effect when compared to partisan primaries,[8][9] or with systems that only require a single round such as ranked-choice voting.[3][10][11] Research by social choice theorists has long identified all three rules as vulnerable to center squeeze, a kind of spoiler effect favoring extremists in crowded elections.[5][12][13]
- ^ a b Sabsay, Daniel Alberto (1995). "El sistema de doble vuelta o ballotage" (PDF). Lecciones y Ensayos (62). Facultad de Derecho de la Universidad de Buenos Aires. ISSN 0024-0079. Archived (PDF) from the original on 2022-11-07. Retrieved 2024-07-18.
- ^ Martinelli, César (2002-10-01). "Simple plurality versus plurality runoff with privately informed voters". Social Choice and Welfare. 19 (4): 901–919. doi:10.1007/s003550200167. ISSN 1432-217X.
- ^ a b Aubin, Jean-Baptiste; Gannaz, Irène; Leoni-Aubin, Samuela; Rolland, Antoine (July 2024). A simulation-based study of proximity between voting rules.
- ^ "El ballotage, con raíces históricas en Francia". El Día. 1 November 2015. Archived from the original on 2023-10-23. Retrieved 2024-07-18.
- ^ a b Santucci, Jack; Shugart, Matthew; Latner, Michael S. (2023-10-16). "Toward a Different Kind of Party Government". Protect Democracy. Archived from the original on 2024-07-16. Retrieved 2024-07-16.
Finally, we should not discount the role of primaries. When we look at the range of countries with first-past-the-post (FPTP) elections (given no primaries), none with an assembly larger than Jamaica's (63) has a strict two-party system. These countries include the United Kingdom and Canada (where multiparty competition is in fact nationwide). Whether the U.S. should be called 'FPTP' itself is dubious, and not only because some states (e.g. Georgia) hold runoffs or use the alternative vote (e.g. Maine). Rather, the U.S. has an unusual two-round system in which the first round winnows the field. This usually is at the intraparty level, although sometimes it is without regard to party (e.g. in Alaska and California).
- ^ Gallagher, Michael; Mitchell, Paul (2005-09-15). "The American Electoral System". The Politics of Electoral Systems. OUP Oxford. p. 192. ISBN 978-0-19-153151-4.
American elections become a two-round run-off system with a delay of several months between the rounds.
- ^ Bowler, Shaun; Grofman, Bernard; Blais, André (2009), "The United States: A Case of Duvergerian Equilibrium", Duverger's Law of Plurality Voting: The Logic of Party Competition in Canada, India, the United Kingdom and the United States, New York, NY: Springer, pp. 135–146, doi:10.1007/978-0-387-09720-6_9, ISBN 978-0-387-09720-6, retrieved 2024-08-31,
In effect, the primary system means that the USA has a two-round runoff system of elections.
- ^ Kousser, Thad. "California's jungle primary sets up polarized governor's race for November". The Conversation. Retrieved 2018-06-23.
The idea was that by opening up primaries to all voters, regardless of party, a flood of new centrist voters would arrive. That would give moderate candidates a route to victory .. Candidates did not represent voters any better after the reforms, taking positions just as polarized as they did before the top two. We detected no shift toward the ideological middle.
- ^ Hill, Seth J.; Kousser, Thad (2015-10-17). "Turning Out Unlikely Voters? A Field Experiment in the Top-Two Primary". Political Behavior. 38 (2): 413–432. doi:10.1007/s11109-015-9319-3. ISSN 0190-9320. S2CID 54541384.
Two groups that were predicted by advocates to increase their participation in response to this reform—those registered with third parties or no-party-preference registrants (independents) who were not guaranteed a vote in any party's primary before the move to the top-two—also show declines in turnout
- ^ McGhee, Eric; Masket, Seth; Shor, Boris; Rogers, Steven; McCarty, Nolan (April 2014). "A Primary Cause of Partisanship? Nomination Systems and Legislator Ideology". American Journal of Political Science. 58 (2): 337–351. doi:10.1111/ajps.12070. ISSN 0092-5853.
- ^ Patterson, Shawn (2020-12-01). "Estimating the unintended participation penalty under top-two primaries with a discontinuity design". Electoral Studies. 68: 102231. doi:10.1016/j.electstud.2020.102231. ISSN 0261-3794.
- ^ 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.
- ^ 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
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