Duverger's law

This was originally asserted by Maurice Duverger, a French sociologist who observed this effect in several papers published in the 1950s and 1960s. It eventually became referred to as a "law" in works by other political scientists, who applied further research to the proposition, and devised alternative voting systems which do not appear to be subject to the same characteristics as first-past-the-post.

A frequent consequence of Duverger's law is the spoiler effect, where a third-party candidate takes votes away from one of the two leading candiates.

External links:

• Maurice Duverger, "Factors in a Two-Party and Multiparty System (http://www.janda.org/c24/Readings/Duverger/Duverger.htm)," in Party Politics and Pressure Groups (New York: Thomas Y. Crowell, 1972), pp. 23-32.
• William H. Riker, The Two-party System and Duverger's Law: An Essay on the History of Political Science (http://www.jstor.org/cgi-bin/jstor/printpage/00030554/di961007/96p0002q/0.pdf?userID=9803f96d@duke.edu/01cc99331a00503f125f&backcontext=results&config=jstor&dowhat=Acrobat&0.pdf) American Political Science Review, 76 (December, 1982), pp. 753-766. (This link is only accessible on universities with JSTOR access).