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.

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. Citizens are actively encouraged to create, join and vote for new political parties if they are unhappy with current parties.