In political science, Duverger’s law holds that single-ballot plurality-rule elections (such as first past the post) structured within single-member districts tend to favor a two-party system.
Duverger’s law is the reason why U.S. voters select one of the two leading candidates when voting tactically, even if a third-party candidate would better represent their interests. This behavior arises out to fear of vote splitting
One consequence of Duverger’s law is that you can improve the viability of third-party candidates by changing the voting system. For example, Approval voting is more resistant to vote splitting and therefore more conducive to third-party candidates.
Text for the lazy.