Duverger's law

In political science, Duverger's law (/ˈduːvərʒeɪ/ DOO-vər-zhay) holds that in political systems with single-member districts and the first-past-the-post voting system, as in, for example, the United States and Britain, only 2 powerful political parties tend to control power. Citizens do not vote for small parties because they fear splitting votes away from the major party.^[1][2]

By contrast, in countries with proportional representation or two-round elections, such as France, Sweden, New Zealand or Spain, there is no two-party duopoly on power. There is usually a significant number of political parties.^[3] Citizens are actively encouraged to create, join and vote for new political parties if they are unhappy with current parties.^[3]