Duverger's law
Winner-takes-all voting systems tend to result in only two viable parties / From Wikipedia, the free encyclopedia
In political science, Duverger's law (/ˈduvərʒeɪ/ DOO-vər-zhay) holds that in political systems with single-member districts (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]