In political science, Duverger’s law is a principle which asserts that a plurality rule election system (voters vote for one candidate, and the candidate with the most votes wins) tends to favor a two-party system. This is one of two hypotheses proposed by French sociologist and politician Maurice Duverger, the second stating that ‘the double ballot majority system and proportional representation tend to multipartism.’
Duverger 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 system (a political party that wins x% of the vote, will win x% of the seats in parliament) 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.
In a plurality voting 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. If the winner of the seat is determined purely by the candidate with the most votes. The plurality system has several characteristics that can serve to discourage the development of third parties and reward the two major parties. Duverger suggests two reasons this: one is the result of the ‘fusion’ (or an alliance very much like fusion) of the weak parties; and the other is the ‘elimination’ of weak parties by the voters, by which he means that voters gradually desert the weak parties on the grounds that they have no chance of winning.
Because the system gives only 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 legislature is significantly less than their proportion of the national vote. The Green Party of Canada is also a good example. The party received approximately 5% of the popular vote from 2004-2011 but had only won one seat in the House of Commons in the same span of time.
Another example was seen in the 1992 U.S. presidential election, when Ross Perot’s candidacy received zero electoral votes despite getting 19% of the popular vote. Gerrymandering (redrawing the district borders) is sometimes used to counteract such geographic difficulties in local politics but is 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.
Regarding the second of his problems, Duverger suggested an election in which 100,000 moderate voters and 80,000 radical voters are voting for a single official. If two moderate parties ran candidates and one radical candidate were to run, the radical candidate would win unless one of the moderate candidates gathered fewer than 20,000 votes. Observing this, moderate voters would be more likely to vote for the candidate most likely to gain more votes, with the goal of defeating the radical candidate. Either the two parties must merge, or one moderate party must fail, as the voters gravitate to the two strong parties, a trend Duverger called ‘polarization.’
A third party can enter the arena only if it can exploit the mistakes of a pre-existing major party, ultimately at that party’s expense. For example, the political chaos in the United States immediately preceding the Civil War allowed the Republican Party to replace the Whig Party as the progressive half of the American political landscape. Loosely united on a platform of country-wide economic reform and federally funded industrialization, the decentralized Whig leadership failed to take a decisive stance on the slavery issue, effectively splitting the party along the Mason-Dixon Line. Southern rural planters, initially lured by the prospect of federal infrastructure and schools, quickly aligned themselves with the pro-slavery Democrats, while urban laborers and professionals in the northern states, threatened by the sudden shift in political and economic power and losing faith in the failing Whig candidates, flocked to the increasingly vocal anti-slavery Republican Party.
In countries that use proportional representation (PR), and especially in countries such as Israel where the whole country forms a single constituency, the electoral rules discourage a two-party system. The number of votes received for a party determines the number of seats won, and new parties can thus develop an immediate electoral niche. Duverger identified that the use of PR 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, although it is worth noting that its presidential elections are won by a plurality, which may put a greater two-party bias in the system than in a purely proportional system.
While there are indeed many plurality systems with two parties, there are counterexamples: In Canada, there are five political parties represented in Parliament. As in the United Kingdom the election system is plurality, usually resulting in a majority government of one party. Duverger himself did not regard his principle as absolute. Instead he suggested that plurality would act to delay the emergence of a new political force and would accelerate the elimination of a weakening force— PR would have the opposite effect. In India, there are thirty eight political parties represented in Parliament and like UK and Canada have winner takes all system.
These counterexamples are partly due to the effect of smaller parties that have the majority of their support concentrated in a small number of electorates rather than diluted across many electorates. 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.
In the case of Canada, the highly regionalized parties are evident in province-by-province examination: while the multiparty system can be seen in the Canadian House of Commons, many of the provinces’ elections are dominated by two-party systems. Quebec, for instance, is driven mainly by the separatist Parti Quebecois and the centr-left Liberal Party, while in Saskatchewan, it is the left-wing New Democratic Party and the centr-right Saskatchewan Party (a coalition of those affiliated with the Conservative and Liberal Parties). Unlike in the United States, where the two major parties are organized and unified at the federal, state, and local level, Canada’s federal and provincial parties generally operate as separate organizations.
The converse of Duverger’s Law is not always valid; two-party politics may emerge even when the plurality vote is not used. This is particularly true in the case of countries using systems that even if they do not use the plurality vote, do not fully incorporate PR either. In the Australian Senate, there is proportional voting but even though smaller parties have been able to win seats, there is still a trend towards the major parties, whose dominance in the lower house effectively promotes their upper house candidates.
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, so the third most popular party is unlikely to win any seats. In recent years some researchers have modified Duverger’s Law by suggesting that electoral systems are an effect of party systems rather than a cause. It has been shown that changes from a plurality system to a proportional system are typically preceded by the emergence of more than two effective parties, and are typically not followed by a substantial increase in the effective number of parties.
The Daily Omnivore 
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