Duverger’s Law (Political Science)
Named after Maurice Duverger who postulated that winner-take-all and/or first-past-the-post elections tend to favor two party systems, while plurality voting tends to favor multi-party systems.
This is based on the idea that voters tend to be strategic with their votes, wishing to place them with a candidate or party who 1) is close to their political views and 2) has the best chance to influence public policy. Nationwide first-past-the-post (and elections to a legislature that works at the national level) elections will shut out any candidate who does not receive a plurality of votes. Voters knowing this have no incentive to vote for “minor party ” candidates who have little chance of being seated. On the other hand in plurality voting systems even parties who have no chance of winning a plurality of votes may still gain representation in a legislature and so will be able to influence public policy. In fact small parties in plurality systems may be very powerful when they play role of a tie breaker.
Further reading: The Two-Party System and Duverger’s Law: An Essay on the History of Political Science (paywall or Library acces may be required)
Duverger’s law of 2-party domination