Duverger's law is a principle which asserts that a plurality rule election system tends to a stable two-party system.

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.

How and why it occurs

A two-party system often develops spontaneously from the single-member district plurality voting system (SMDP), in which legislative seats are awarded to the candidate with a plurality of the total votes within his or her constituency, rather than apportioning seats to each party based on the total votes gained in the entire set of constituencies. This trend develops out of the inherent qualities of the SMDP system that discourage the development of third parties and reward the two major parties.

The most obvious inhibiting feature unique to the SMDP voting system is purely statistical. A small third party cannot gain legislative power if it is based in a populous area. Similarly, a statistically significant third party can be too geographically scattered to muster enough votes to win seats, although technically its numbers would be sufficient to overtake a major party in an urban zone. Gerrymandering is sometimes used to counteract such geographic difficulties in local politics, but is impractical and 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 psychological. Maurice Duverger [1] 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 less 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-slave Republican Party.

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 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.

Counterexamples

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

• India, the world's largest democracy, has multiple regional parties.
• Scotland has had until recently SMDP and similar systems, but has seen the development of several significant competing political parties.
• 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.
• In Canada, the New Democratic Party and its predecessor the Co-operative Commonwealth Federation have had a constant presence in Parliament since the CCF's first election in 1935, and at least four political parties have been represented in the Canadian parliament at any given time since the 1993 election. In addition, the now-defunct Social Credit Party of Canada also maintained itself in Parliament nearly consistently from 1935 to 1979, often resulting in Parliaments with four national parties represented.

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).

Duverger's Law's converse

The converse of Duverger's Law is not 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 incorporate PR either. For instance, Malta has a single transferable vote (STV) system and what seem 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), though the fact that this phenomenon occurs within the context of the use of the alternative vote for Lower House elections must be taken into account. 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 holding the balance of power.

While some would argue that a two-party system is not necessarily harmful, 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.

See also

• Plurality voting
• Two-party system
• Range voting
• Observations named after people
• This entry is related to, but not included in the elections and voting series. Other related articles can be found at the .

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.)