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". 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.