A theory, formulated by the French political scientist Maurice Duverger in his 1951 book Les partis politiques, that connects a country's electoral system to the number of parties that come to dominate it: single-member-district plurality systems like first past the post tend toward two dominant parties, while proportional representation and other systems requiring more than a plurality to win tend toward multiple parties. Duverger proposed two mechanisms behind the pattern, a mechanical effect, a plurality formula wastes votes cast for any candidate who does not finish first, so smaller parties win far fewer seats than their vote share, and a psychological effect, voters and donors, anticipating the mechanical effect, gravitate toward the two candidates seen as able to win rather than backing a party unlikely to place first. Duverger himself treated the pattern as a strong tendency rather than an absolute rule, and later scholars, among them Kenneth Benoit and Josep Colomer, have argued the influence runs in both directions, an established party system shapes the electoral rules that get chosen at least as much as the rules shape the party system, while researchers including Eric Dickson and Kenneth Scheve have found the two-party tendency weakens where strong regional or social divisions cut across the national party contest, as in India, Canada and the United Kingdom, all first past the post systems that nonetheless sustain more than two significant parties.
Facts
Origin YearOriginal French publication, Les partis politiques, Maurice Duverger, 1951. Secondary accounts give varying years for the English translation (1954 and 1964 both appear across sources checked); this atlas records the original formulation year, which is uncontested across every source checked. Founding ContextMaurice Duverger proposed the generalization after comparing electoral systems across many countries, observing that single member plurality voting tends to squeeze a political system toward two major parties because voters and party strategists learn that a vote for a third place candidate is usually wasted. He set this against proportional systems, which he found tended to sustain more parties instead. 1 Connections
Associated With
This source names India directly: "the national party contest, as in India, Canada and the United Kingdom, all first past the post systems that nonetheless sustain more than two significant parties."
Source Duverger's Law (Britannica)
This source names United Kingdom directly: "contest, as in India, Canada and the United Kingdom, all first past the post systems that nonetheless sustain more than two significant parties."
Source Duverger's Law (Britannica)
Influenced By
The observed tendency of first-past-the-post systems toward two dominant parties is the empirical pattern Maurice Duverger generalized into the law.
Source Duverger's Law (Britannica)
Why this is disputed. Whether mixed-member proportional representation's compensatory list tier produces the same multiparty pull as full proportional representation, or whether the plurality-won district tier still exerts some of the two-party pressure Duverger's law predicts, is disputed in the comparative electoral systems literature; Germany itself has sustained more than two significant parties under the system, which both sides of the debate cite for different reasons.
The observed tendency of proportional systems toward multiple viable parties is the other half of the empirical pattern Duverger's law generalizes.
Source Duverger's Law (Britannica)
Why this is disputed. Ranked-choice voting is, like first past the post, a single-winner method, so whether it escapes Duverger's law's two-party prediction or merely delays it is genuinely disputed: some scholars find its elimination-and-transfer process weakens the psychological pressure toward two dominant candidates that plurality voting creates, others find the underlying single-winner mechanics reassert themselves once a race narrows to two credible contenders.
Practiced By
Source Duverger's law (Wikipedia)
Sources
1. Duverger's Law (Britannica)
Encyclopaedia BritannicaReference excerpt under political science: Post-World War II trends and debatesQuote, Reference excerpt under political science: Post-World War II trends and debates
generalization was later called "Duverger's law."
Duverger's law (Wikipedia)
Practiced By: United States, Article text
in political systems with single-member districts and the first-past-the-post voting system, as in, for example, the United States and United Kingdom, only two powerful political parties tend to control power every election cycle.
In Group: Franchise, Districting and Electoral Effects, Lead paragraph
In political science, Duverger's law ( DOO-vər-zhay) holds that in political systems with single-member districts and the first-past-the-post voting system, as in, for example, the United States and United Kingdom, only two powerful political parties tend to control power every election cycle.
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