Duverger's law

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.

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 (see first past the post). 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 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 a single seat in the House of Commons in the same span of time. 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.

The second unique problem is both statistical and tactical. Duverger suggested an election in which 100,000 moderate voters and 80,000 radical voters are voting for a single official. If two moderate 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.^[4]

A third party can only enter the arena 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, 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 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.

Counterexamples

While there are indeed many SMDP systems with two parties, there are counterexamples:

• In the United Kingdom, the Liberal party/Alliance/Liberal Democrats have, since the February 1974 General Election, usually obtained between 15% and 25% of the vote forming a "third party" and creating a three party system.^[5] In the 2010 election, as well as the three major parties, 8 other parties gained seats in Parliament. However, with the First Past The Post system and constituency areas used in general elections for the UK, despite gathering around a fifth of votes consistently for over twenty years, their share of seats in parliament has not been more than a tenth in that time.^[6]

• In Canada, there are five political parties represented in Parliament. Just like in the United States the election system is "first past the post" with, usually, a majority government of one party.

Duverger himself did not regard his principle as absolute. Instead he suggested that SMDP would act to delay the emergence of a new political force, and would accelerate the elimination of a weakening force^{[citation needed]} — PR 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). Another example for multiple regional parties in an SMDP system is India.^{[further explanation needed][citation needed]}

Duverger's Law's converse

The converse of Duverger's Law is not always valid;^{[citation needed]} two-party politics are not necessarily the result of SMDP. This is particularly true in the case of countries using systems that, while not SMDP, do not fully incorporate PR either. For instance, Malta has a single transferable vote (STV) system and what seems to be stable two-party politics.

In the Australian upper house there is proportional voting but there is still a trend towards the major parties, though smaller parties have been able to win seats. (Also, this phenomenon occurs within the context of the use of the alternative vote for Lower House elections.)

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.

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.^[7] 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.^[8]

See also

• Plurality voting
• Two-party system
• First Past the Post
• Range voting
• Observations named after people
• Mouseland

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References

1. ^ Sartori, Giovanni, Comparative Constitutional Engineering, An Inquiry into Structures, Incentives and Outcomes.
2. ^ http://www.pratiquesciencessociales.net/exposes/S12.%20Maurice%20Duverger%20and%20the%20Study%20of%20Political%20Parties%20%28Schlesinger%202006%29.pdf
3. ^ http://books.google.com/books?id=fecTeDjAZZ8C&pg=PA12&dq=Duverger%27s+law&hl=en&ei=w9nBTPCDMsrOswbQ1b3OCA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CCwQ6AEwAQ#v=onepage&q=Duverger%27s%20law&f=false
4. ^ 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. http://janda.org/c24/Readings/Duverger/Duverger.htm
5. ^ See references in United Kingdom general elections, 1974 to 2010.
6. ^ http://en.wikipedia.org/wiki/Liberal_Democrats#Electoral_results
7. ^ Benoit, Kenneth (June 2007). "Electoral Laws as Political Consequences: Explaining the Origins and Change of Electoral Institutions". Annual Review of Political Science 10 (1): 363–390. doi:10.1146/annurev.polisci.10.072805.101608. 
8. ^ Colomer, Josep M. (March 2005) (PDF). It's Parties that Choose Electoral Systems (or Duverger's Law Upside Down). http://www.politicalstudies.org/pdf/edsfavourites/colomer.pdf. Retrieved 2009-05-31. 

Bibliography

• 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.)