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Duverger's law

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Duverger's Law is a principle which asserts that a first-past-the-post election system naturally leads to a two-party system. The discovery of this principle is attributed to Maurice Duverger, a French sociologist who observed the effect and recorded it in several papers published in the 1950s and 1960s. In the course of further research, other political scientists began calling the effect a “law”.

While there are indeed many FPTP systems with two parties, there are significant counterexamples: Scotland has had until recently first-past-the-post and similar systems but has seen the development of several significant competing political parties. Many commentators regard the United Kingdom's Liberal Democrat party, since the 2005 General Election, as forming a 'third party' and creating a three-party system. Canada and India have multiple regional parties. Duverger himself did not regard his principle as absolute: instead he suggested that first-past-the-post would act to delay the emergence of a new political force, and would accelerate the elimination of a weakening force - proportional representation would have the opposite effect.

Additionally, William H. Riker noted that strong regional parties can distort matters, leading to more than two parties nationwide, even if there are only two parties competitive in any single district. He pointed to Canada's regional politics, as well as the U.S. presidential election of 1860, as examples of often temporary regional instability that occurs from time-to-time in otherwise stable two-party systems (Riker, 1982).

The converse of Duverger's Law doesn't necessarily follow, as other systems may also lead to stable two-party political systems. This is particularly true in the case of countries using systems that, while they are not strictly speaking "first past the post", do not incorporate proportional representation. For instance, Malta has a single transferable vote system and what seems to be stable two-party politics. Australia uses single transferable vote as well and, though not strictly a two-party system, is dominated by a major party (the Labor Party) and a major coalition (the Liberal/National coalition). Having preferential voting federally has encouraged it to spread into all the states, and to upper houses where STV gives minor parties a real chance of winning.

While some would argue that a two-party outcome is not necessarily harmful (see Two-party system), researchers and mathematicians have devoted considerable time to developing voting systems that do not appear to be subject to Duverger's Law.

Some systems are even more likely to lead to a two-party outcome: for example elections in Gibraltar use a partial block vote system in a single constituency, meaning that the third most popular party is unlikely to win any seats.

A frequent consequence of Duverger's Law is the spoiler effect, where a third-party candidate takes votes away from one of the two leading candidates.

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