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”. It is important to realise Duverger's law suggests a nexus or synthesis between a party system and an electoral system - with a proportional representation (PR) system creating the electoral conditions necessary to foster party development and a FPTP system marginalising many smaller - single issue - political parties.
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How and why it occurs
Two-party systems often develop spontaneously when the voting system used for elections discriminates against third or smaller parties, because the number of votes received for a party in a whole country is not directly related to the proportion of seats it receives in the country's assembly/assemblies. While there is sometimes a coincidental relationship between votes cast and seats received in these systems, voters are not assured that their one vote will directly count toward an additional seat. The most widely-used system to have this effect, the simple plurality system (first past the post) often appears to pull systems into encouraging the survival of only two major parties: a third force can break in on the scene (the Labour Party in 20th-century Britain, or the Republican Party in the 19th-century United States, for example) but only at the ultimate expense of a former major party (the Liberal Party in Britain, the Whigs in the USA respectively). The overall system re-stabilizes into two-party mode after a three-party interlude.
Some representation systems - such as those involving a single elected president or a mayor dominating the government - may encourage two-party systems, since ultimately the contest will pit the two most popular candidates against each other.
When constituencies (districts) vote for candidates on the basis of a geographical constituency, all votes for candidates other than the winner count for nothing. This reflects another factor that encourages a two party system: smaller parties often cannot win enough votes in a constituency because they have smaller support and sometimes more scattered support than larger parties. Often a first-past-the-post electoral system and the election of candidates from geographical constituencies (districts) appear together in a single political system: this means that some smaller parties can garner a significant proportional of votes nationally, but receive few constituency seats and thus cannot realistically expect to compete overall on an equal footing with larger country-wide parties.
In countries that use proportional representation (PR), especially where the whole country forms a single constituency (like Israel), the electoral rules discourage a two-party system; the number of votes received for a party relates directly and proportionally to the number of representative seats won, and new parties can thus develop an immediate electoral niche. Duverger identified that the use of proportional representation would make a two party system less likely. However, other systems do not guarantee new parties access to the system: Malta provides an example of a stable two-party system using the single transferable vote.
Often, two-party systems result from various factors, mostly the use of a first-past-the-post voting system, rather than from deliberate electoral/political engineering.
Counterexamples
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. The New Democratic Party in Canada has had a constant presence in Parliament since its founding in 1961. India has 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 receiving seats in the national legislature, 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).
Converse does not apply
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.
Spoiler effect
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. The common desire among voters (including a third-party candidate oneself) to avoid this problem frequently leads to the distortion of tactical voting, in which a race can be won by a candidate who is not comparatively (and proportionally) the most popular.
References
- Maurice Duverger, "Factors in a Two-Party and Multiparty System," in Party Politics and Pressure Groups (New York: Thomas Y. Crowell, 1972), pp. 23-32.
- William H. Riker, The Two-party System and Duverger's Law: An Essay on the History of Political Science American Political Science Review, 76 (December, 1982), pp. 753-766. (This link is only accessible to individuals with access to JSTOR through a university, a corporation or another organization.)