Duverger's law

In political science, Duverger's law holds that single-ballot majoritarian elections with single-member districts (such as first past the post) tend to favor a two-party system. The theory is attributed to Maurice Duverger, a French sociologist who published several papers on it in the 1950s and 1960s. 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-partyism, as does the plurality system with runoff elections. 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. Steven R. Reed argues Duverger's law is present in Japan and Italy. The law, however, is subject to debate, especially for systems outside the United States like the United Kingdom, Canada and India with multiparty parliaments. However, only the two dominant parties of their times have formed governments in the United Kingdom and Canada. 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. A two-party system is more common in a plurality voting system. In this system, voters have a single vote, which they can cast for a single candidate in their district, in which only one legislative seat is available. In plurality voting (also referred to as first past the post), in which the winner of the seat is determined purely by the candidate with the most votes, several characteristics can serve to discourage the development of third parties and reward the two major parties.