Duverger's law

Duverger's law is a hypothesis about how voting systems and political-party composition are related. It, along with related Duverger's hypotheses, were stated by French sociologist Maurice Duverger in Political Parties (Les partis politiques) in 1951, and extensively discussed by him and others in the decades since.
 * Duverger's law: First Past the Post produces a two-party system
 * Duverger's hypothesis #1: Runoff voting produces a multiparty system with coalitions
 * Duverger's hypothesis #2: Proportional representation produces a multiparty system

Duverger's law happens because many voters prefer to avoid wasting their votes on candidates who are unlikely to win, thus choosing one of the two biggest ones. This leads to a two-party system at a district level, even if not necessarily at a national level. In fact, some candidates have gotten a reputation as "spoilers", attracting votes from ideologically similar major candidates, thus letting their opposites win. Thus, in the 2000 US Presidential election, Ralph Nader attracted votes mainly from Al Gore, thus letting George W. Bush win.

By contrast, proportional systems allow multiparty systems because a party need not get much of the vote in order to get represented.