Duverger's law

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{government, party, election}
{theory, work, human}
{rate, high, increase}
{area, part, region}
{black, white, people}
{system, computer, user}
{mi², represent, 1st}

In political science, Duverger's law is a principle which asserts that a plurality rule election system tends to favor a two-party system. This is one of two hypotheses proposed by Duverger, the second stating that “the double ballot majority system and proportional representation tend to multipartism.”[1]

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. In the course of further research, other political scientists began calling the effect a “law” or principle. Duverger's law suggests a nexus or synthesis between a party system and an electoral system: a proportional representation (PR) system creates the electoral conditions necessary to foster party development while a plurality system marginalizes many smaller political parties, resulting in what is known as a two-party system.

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How and why it occurs

A two-party system often develops from the single-member district plurality voting system (SMDP). In an SMDP 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. The winner of the seat is determined by the candidate with the most votes. This means that the SMDP system has several qualities that can serve to discourage the development of third parties and reward the two major parties.

Duverger suggests two reasons why single-member district plurality voting systems favor a two party system. One is the result of the "fusion" (or an alliance very like fusion) of the weak parties, and the other is the "elimination" of weak parties by the voters, by which he means that the voters gradually desert the weak parties on the grounds that they have no chance of winning.[2][3]

A prominent restrictive feature unique to the SMDP voting system is purely statistical. Because the SMDP system only gives the winner in each district a seat, a party which consistently comes third in every district will not gain any seats in the legislature, even if it receives a significant proportion of the vote. This puts geographically thinly spread parties at a significant disadvantage. An example of this is the Liberal Democrats in the UK, whose proportion of seats in the legislative is significantly less than their proportion of the national vote. The Green Party of Canada is also a good example. The party receives approximately 5% of the popular vote, but has not yet won a seat in the House of Commons. Gerrymandering is sometimes used to counteract such geographic difficulties in local politics, but controversial on a large scale. These numerical disadvantages can create an artificial limit on the level at which a third party can engage in the political process.

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