I learned something today...

Duverger's Law, Duverger's Hypothesis

In 1951, the French political scientist, Maurice Duverger, formulated what he called "a true sociological law" concerning the effects of different electoral systems on the structure of politics. This "law" was that systems in which office is awarded to a candidate who receives the most votes (with two candidates, a majority, but with more than two, a plurality) in a single ballot election will produce a two-party political system, rather than a multi-party one. Maurice Duverger, Political Parties: Their Organization and Activity in the Modern State (1954). This plurality, winner-take-all system is used for nearly all American elections as it is for British elections. Other political scientists had reached similar conclusions: "single-member district-system- plus- plurality-elections... discriminate moderately against the second party; but against the third, fourth, and fifth parties the force of this tendency is multiplied to the point of extinguishing their chances of winning seats altogether." Elmer E. Schattschneider, Party Government 75 (1942).

-- From page 1091 of my Election Law casebook, The Law of Democracy: Legal Structure of the Political Process, Issacharoff, Karlan and Pildes (2002).