Duverger's law

In political science, Duverger's law holds that plurality-rule elections (such as first past the post) structured within single-member districts tend to favor a two-party system, whereas "the double ballot majority system and proportional representation tend to favor multipartism".Maurice Duverger: Party Politics and Pressurce Groups. A comparative introduction. Crowell, New York 1972,pp 27–29. The law is attributed to French sociologist Maurice Duverger, who first observed it, and wrote several papers in the 1950s and 1960s. The idea is that someone taking a rational choice in a system with many parties will give his or her vote to a candidate who stands a chance of being elected, and not necessarily to the candidate of the party he or she supports most. This will mean that the votes will be for two candidates. One of them wlil be centre-left, the other cenre-right of the political spectrum. 

This approach has also been criticized: The effect only works if there are few lines of conflict (called cleavages) between the candidates. Examples for such line of conflict are rich voters vs. poor ones, Many societies also have other lines of conflict as well: Some people are more religious than others, ideas about idelogy and culture are different, often, there are ethnic minorities in parts of the country.

The classical example, where duverger's law worked was the United Kingdom: In 1974 and 2010, there was a third party (the Liberal Party, or the Liberal Democrats) that was able to get enough seats in parliament to change this.

 References 

Category:Voting
Category:Political science