Duverger's three laws of political party development

The following is a verbatim French→English translation of Duverger's own words (a 1980 statement by Maurice Duverger):

1. The plurality (1 winner) voting system tends to lead to a 2-party system.
2. The proportional representation (multiwinner) system tends to lead to many mutually independent parties.
3. The 2-ballot majority system [top-2 (delayed) runoff] tends to lead to multipartism moderated by alliances.

William H. Riker: [The Two-party System and Duverger's Law: An Essay on the History of Political Science, Amer. Polit. Sci. Review 76 (December 1982) 753-766] argued that actually, the theoretical link between the single-seat district plurality (winner-take-all) rule used in the US and UK, and a two-party system, really was first put forward by Henry Droop almost 90 years before Duverger did it.

More detailed discussion of Duverger law

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