Duverger's law
Encyclopedia
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.”
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.
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.
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.
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.
s:
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 — 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.
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. 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
.
Political science
Political Science is a social science discipline concerned with the study of the state, government and politics. Aristotle defined it as the study of the state. It deals extensively with the theory and practice of politics, and the analysis of political systems and political behavior...
, Duverger's law is a principle which asserts that a plurality rule
Plurality voting system
The plurality voting system is a single-winner voting system often used to elect executive officers or to elect members of a legislative assembly which is based on single-member constituencies...
election system tends to favor a two-party system
Two-party system
A two-party system is a system where two major political parties dominate voting in nearly all elections at every level of government and, as a result, all or nearly all elected offices are members of one of the two major parties...
. This is one of two hypotheses proposed by Duverger, the second stating that “the double ballot majority system and proportional representation tend to multipartism.”
The discovery of this tendency is attributed to Maurice Duverger
Maurice Duverger
Maurice Duverger is a French jurist, sociologist and politician. He was born in Angoulême, Charente.Starting his career as a jurist at the University of Bordeaux, Duverger became more and more involved in political science and in 1948 founded one of the first faculties for political science in...
, 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
Law
Law is a system of rules and guidelines which are enforced through social institutions to govern behavior, wherever possible. It shapes politics, economics and society in numerous ways and serves as a social mediator of relations between people. Contract law regulates everything from buying a bus...
” or principle
Principle
A principle is a law or rule that has to be, or usually is to be followed, or can be desirably followed, or is an inevitable consequence of something, such as the laws observed in nature or the way that a system is constructed...
. Duverger's law suggests a nexus or synthesis between a party system
Party system
A party system is a concept in comparative political science concerning the system of government by political parties in a democratic country. The idea is that political parties have basic similarities: they control the government, have a stable base of mass popular support, and create internal...
and an electoral system: a proportional representation
Proportional representation
Proportional representation is a concept in voting systems used to elect an assembly or council. PR means that the number of seats won by a party or group of candidates is proportionate to the number of votes received. For example, under a PR voting system if 30% of voters support a particular...
(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 systemTwo-party system
A two-party system is a system where two major political parties dominate voting in nearly all elections at every level of government and, as a result, all or nearly all elected offices are members of one of the two major parties...
often develops from the single-member district plurality
Plurality voting system
The plurality voting system is a single-winner voting system often used to elect executive officers or to elect members of a legislative assembly which is based on single-member constituencies...
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
Third party (politics)
In a two-party system of politics, the term third party is sometimes applied to a party other than the two dominant ones. While technically the term is limited to the third largest party or third oldest party, it is common, though innumerate, shorthand for any smaller party.For instance, in the...
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.
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
Liberal Democrats
The Liberal Democrats are a social liberal political party in the United Kingdom which supports constitutional and electoral reform, progressive taxation, wealth taxation, human rights laws, cultural liberalism, banking reform and civil liberties .The party was formed in 1988 by a merger of the...
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
Green Party of Canada
The Green Party of Canada is a Canadian federal political party founded in 1983 with 10,000–12,000 registered members as of October 2008. The Greens advance a broad multi-issue political platform that reflects its core values of ecological wisdom, social justice, grassroots democracy and...
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
Gerrymandering
In the process of setting electoral districts, gerrymandering is a practice that attempts to establish a political advantage for a particular party or group by manipulating geographic boundaries to create partisan, incumbent-protected districts...
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.
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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
immediately preceding the Civil War
Origins of the American Civil War
The main explanation for the origins of the American Civil War is slavery, especially Southern anger at the attempts by Northern antislavery political forces to block the expansion of slavery into the western territories...
allowed the Republican Party
Republican Party (United States)
The Republican Party is one of the two major contemporary political parties in the United States, along with the Democratic Party. Founded by anti-slavery expansion activists in 1854, it is often called the GOP . The party's platform generally reflects American conservatism in the U.S...
to replace the Whig Party
Whig Party (United States)
The Whig Party was a political party of the United States during the era of Jacksonian democracy. Considered integral to the Second Party System and operating from the early 1830s to the mid-1850s, the party was formed in opposition to the policies of President Andrew Jackson and his Democratic...
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
Mason-Dixon line
The Mason–Dixon Line was surveyed between 1763 and 1767 by Charles Mason and Jeremiah Dixon in the resolution of a border dispute between British colonies in Colonial America. It forms a demarcation line among four U.S. states, forming part of the borders of Pennsylvania, Maryland, Delaware, and...
. 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
Proportional representation
Proportional representation is a concept in voting systems used to elect an assembly or council. PR means that the number of seats won by a party or group of candidates is proportionate to the number of votes received. For example, under a PR voting system if 30% of voters support a particular...
, especially where the whole country forms a single constituency (like Israel
Israel
The State of Israel is a parliamentary republic located in the Middle East, along the eastern shore of the Mediterranean Sea...
), 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
Maurice Duverger
Maurice Duverger is a French jurist, sociologist and politician. He was born in Angoulême, Charente.Starting his career as a jurist at the University of Bordeaux, Duverger became more and more involved in political science and in 1948 founded one of the first faculties for political science in...
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
Malta
Malta , officially known as the Republic of Malta , is a Southern European country consisting of an archipelago situated in the centre of the Mediterranean, south of Sicily, east of Tunisia and north of Libya, with Gibraltar to the west and Alexandria to the east.Malta covers just over in...
provides an example of a stable two-party system using the single transferable vote
Single transferable vote
The single transferable vote is a voting system designed to achieve proportional representation through preferential voting. Under STV, an elector's vote is initially allocated to his or her most preferred candidate, and then, after candidates have been either elected or eliminated, any surplus or...
, 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 counterexampleCounterexample
In logic, and especially in its applications to mathematics and philosophy, a counterexample is an exception to a proposed general rule. For example, consider the proposition "all students are lazy"....
s:
- In the United KingdomUnited KingdomThe United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, the Liberal party/Alliance/Liberal DemocratsLiberal DemocratsThe Liberal Democrats are a social liberal political party in the United Kingdom which supports constitutional and electoral reform, progressive taxation, wealth taxation, human rights laws, cultural liberalism, banking reform and civil liberties .The party was formed in 1988 by a merger of the...
have, since the February 1974 General ElectionUnited Kingdom general election, February 1974The United Kingdom's general election of February 1974 was held on the 28th of that month. It was the first of two United Kingdom general elections held that year, and the first election since the Second World War not to produce an overall majority in the House of Commons for the winning party,...
, usually obtained between 15% and 25% of the vote forming a "third party" and creating a three party system. 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.
- In CanadaCanadaCanada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, 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 — PR would have the opposite effect.
Additionally, William H. Riker
William H. Riker
William Harrison Riker was an American political scientist who applied game theory and mathematics to political science....
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
United States presidential election, 1860
The United States presidential election of 1860 was a quadrennial election, held on November 6, 1860, for the office of President of the United States and the immediate impetus for the outbreak of the American Civil War. The nation had been divided throughout the 1850s on questions surrounding the...
, 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.
Duverger's Law's converse
The converse of Duverger's Law is not always valid; 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, MaltaMalta
Malta , officially known as the Republic of Malta , is a Southern European country consisting of an archipelago situated in the centre of the Mediterranean, south of Sicily, east of Tunisia and north of Libya, with Gibraltar to the west and Alexandria to the east.Malta covers just over in...
has a single transferable vote
Single transferable vote
The single transferable vote is a voting system designed to achieve proportional representation through preferential voting. Under STV, an elector's vote is initially allocated to his or her most preferred candidate, and then, after candidates have been either elected or eliminated, any surplus or...
(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
Elections in Gibraltar
Elections in Gibraltar gives information on election and election results in Gibraltar.Gibraltar elects on national level a legislature. The Gibraltar Parliament has 17 members, all elected for a four year term in one constituency with each voter getting to vote for their selection of eight...
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. 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
Effective number of parties
The effective number of parties is a concept which provides for an adjusted number of political parties in a country's party system. The idea behind this measure is to count parties and, at the same time, to weight the count by their relative strength...
.
See also
- Plurality votingPlurality voting systemThe plurality voting system is a single-winner voting system often used to elect executive officers or to elect members of a legislative assembly which is based on single-member constituencies...
- Two-party systemTwo-party systemA two-party system is a system where two major political parties dominate voting in nearly all elections at every level of government and, as a result, all or nearly all elected offices are members of one of the two major parties...
- First Past the Post
- Range votingRange votingRange voting is a voting system for one-seat elections under which voters score each candidate, the scores are added up, and the candidate with the highest score wins.A form of range voting was apparently used in...
- Observations named after people
- MouselandMouselandThe Story of Mouseland was a story told first by Clarence Gillis, and later and most famously by Tommy Douglas, leader of the Saskatchewan Co-operative Commonwealth Federation and, later, the New Democratic Party of Canada, both social democratic parties...