Duverger's law holds that plurality-rule elections (such as first past the post) structured within single-member districts tend to favor a two-party system, whereas "the double ballot majority system and proportional representation tend to favor multipartism". The discovery of this tendency is attributed to Maurice Duverger, a French sociologist who observed the effect and recorded it in several papers published in the 1950s and 1960s.
Duverger's law draws from a model of causality from the electoral system to a party system. A proportional representation (PR) system creates electoral conditions that foster the development of many parties, whereas a plurality system marginalizes smaller political parties, generally resulting in a two-party system.
Most countries with plurality voting have representation in their legislatures by more than two parties. While the United States is very much a two-party system, the United Kingdom, Canada and India have consistently had multiparty parliaments.. However, only the two dominant parties of their times have formed governments in the United Kingdom and Canada. Eric Dickson and Ken Scheve argue that there is a counter force to Duverger's law, that on the national level a plurality system encourages two parties, but in the individual constituencies supermajorities will lead to the vote fracturing. Steven R. Reed has shown Duverger's law to work in Japan and Italy.
- Grzymala-Busse, Anna (31 December 2014). "Remembering Duverger". Mischiefs of Faction. Archived from the original on 4 May 2016. Retrieved 28 April 2016.
- Sartori, Giovanni (1994). Comparative Constitutional Engineering: An Inquiry into Structures, Incentives and Outcomes. Macmillan.
- Dunleavy, Patrick (18 June 2012). "Duverger's Law is a dead parrot. Outside the USA, first-past-the-post voting has no tendency at all to produce two party politics". blogs.lse.ac.uk.
- Dunleavy, Patrick; Diwakar, Rekha (2013). "Analysing multiparty competition in plurality rule elections" (PDF). Party Politics. 19 (6): 855–886. doi:10.1177/1354068811411026.
- Dickson, Eric S.; Scheve, Kenneth (2010). "Social Identity, Electoral Institutions and the Number of Candidates" (PDF). British Journal of Political Science. 40 (2): 349–375. CiteSeerX 10.1.1.75.155. doi:10.1017/s0007123409990354. JSTOR 40649446. Archived from the original (PDF) on 2018-07-21. Retrieved 2017-10-26.
- Reed, Steven R. (1990). "Structure and Behaviour: Extending Duverger's Law to the Japanese Case". British Journal of Political Science. 20 (3): 335–356. doi:10.1017/S0007123400005871. JSTOR 193914.
- Reed, Steven R. (2010). "Duverger's Law is Working in Italy". Comparative Political Studies. 34 (3): 312–327. doi:10.1177/0010414001034003004.
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