In political science, Duverger's law holds that single-ballot plurality-rule elections (such as first past the post) structured within single-member districts tend to favor a two-party system. 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.[1] In the course of further research, other political scientists began calling the effect a "law" or principle.
As a corollary to the law, Duverger also asserted that proportional representation favors multi-partism, as does the plurality system with runoff elections.[2]
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.[3][4] However, only the two dominant parties of their times have formed governments in the United Kingdom and Canada.[dubious – discuss][citation needed] Eric Dickson and Kenneth 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.[5] Steven R. Reed has shown Duverger's law to work in Japan[6] and Italy.[7]