Duverger's law

Duverger's law holds that single-member plurality elections tend to produce two-party competition. Strategic voting and parties’ incentives to coordinate help explain this tendency.

Duverger's mechanical effect: The tendency of plurality rules to underrepresent parties that lose in individual districts when seats are allocated. Small parties can win substantial votes yet few seats, weakening their electoral prospects.
United States two-party system: The long-standing dominance of the Democratic and Republican parties in United States national elections. Single-member plurality elections are a prominent setting for Duverger's predicted two-party competition.
Proportional representation: Electoral systems that allocate seats roughly in proportion to parties' shares of the vote. Proportional seat allocation reduces the penalty that plurality rules impose on smaller parties.
Maurice Duverger: A French political scientist whose comparative work shaped the study of electoral systems and party systems. His analysis gave the law its name and established its central electoral-system claim.
Effective number of parties: A measure of party-system fragmentation that weights parties by their vote or seat shares. It captures degrees of two-party dominance without treating every system as simply two-party or multiparty.
Duverger's psychological effect: The strategic responses of voters and political elites to the mechanical effects of electoral rules. Voters and politicians anticipate wasted votes and coordinate around viable contenders.
United Kingdom two-party system: The historical dominance of two major parties in British parliamentary elections, alongside persistent smaller parties. Plurality elections often favor the leading parties, though regional support sustains exceptions.
Single transferable vote: A ranked-choice electoral system that elects multiple representatives per district through transferable votes. Multimember districts and transferable preferences can support more than two viable parties.
Political parties: Organizations that seek political power by nominating candidates, contesting elections, and coordinating political action. Duverger's law concerns how electoral rules shape the number and structure of parties.
Electoral threshold: The minimum vote share or other requirement a party must meet to qualify for representation. Formal thresholds can reinforce or counteract the pressures Duverger associates with electoral rules.

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