Duverger's Law
A principle which states that plurality-rule elections (such as first-past-the-post) structured within single-member districts tend to favor a two-party system, and that the double ballot majority system and proportional representation tend to favor multipartism.
Origin
French political scientist Maurice Duverger proposed his law in 1951, publishing it in Political Parties (English edition 1954). The law states that "the simple majority, single ballot system favours the two‐party system" in countries like the United States and United Kingdom, while proportional representation and two-round elections favor multipartism. The mechanism: rational politicians and voters realize it's hopeless to have more than two parties competing nationally—citizens fear splitting votes away from major parties. In countries with proportional representation (France, Sweden, New Zealand, Spain), there's no two-party duopoly. While Matt Golder's 2016 review concluded the law remains a valid generalization despite exceptions, critics argue about numerous exceptions and reversed causality.