Duverger's law
In political science, Duverger's law (/ˈduvərʒeɪ/ DOO-vər-zhay) holds that in political systems with only one winner (as in the U.S.), two main parties tend to emerge with minor parties typically splitting votes away from the most similar major party.[1][2] In contrast, systems with proportional representation usually have more representation of minor parties in government.[3]
Strength of effect[edit]
William Clark and Matt Golder (2006) find the effect largely holds up, noting that different methods of analyzing the data might lead to different conclusions. They emphasize other variables like the nuances of different electoral institutions and the importance that Duverger also placed on sociological factors.[14] Thomas R. Palfrey argued Duverger's law can be proven mathematically at the limit when the number of voters approaches infinity for one single-winner district and where the probability distribution of votes is known (perfect information).[15]
Duverger did not regard this principle as absolute, suggesting instead that plurality would act to delay the emergence of new political forces and would accelerate the elimination of weakening ones, whereas proportional representation would have the opposite effect.[7]
The U.S. system has two major parties which have won, on average, 98% of all state and federal seats.[1] There have only been a few rare elections where a minor party was competitive with the major parties, occasionally replacing one of the major parties in the 19th century.[2][10]
In Matt Golder's 2016 review of the empirical evidence to-date, he concluded that despite some contradicting cases, the law remains a valid generalization.[16]
Steven R. Reed argued in 2001 that Duverger's law could be observed in Italy, with 80% of electoral districts gradually but significantly shifting towards two major parties.[17] He finds a similar effect in Japan through a slow trial-and-error process that shifted the number of major parties towards the expected outcome.[18]
Eric Dickson and Kenneth Scheve argued in 2007 that Duverger's law is strongest when a society is homogenous or closely divided, but is weakened when multiple intermediate identities exist.[19] As evidence of this, Duhamel cites the case of India, where over 25 percent of voters vote for parties outside the two main alliances.[20]
Two-party politics may also emerge in systems that use a form of proportional representation, with Duverger and others arguing that Duverger's Law mostly represents a limiting factor (like a brake) on the number of major parties in other systems more than a prediction of equilibrium for governments with more proportional representation.[21][14]