In political science, Duverger's law holds that single-ballot majoritarian elections with single-member districts (such as first past the post) tend to favor a two-party system. The discovery of this tendency is attributed to Maurice Duverger, a French sociologist who observed the effect and recorded it in several papers published in the 1950s and 1960s. In the course of further research, other political scientists began calling the effect a "law" or principle. As a corollary to the law, Duverger also asserted that proportional representation favors multi-partyism, as does the plurality system with runoff elections. Duverger's law draws from a model of causality from the electoral system to a party system. A proportional representation (PR) system creates electoral conditions that foster the development of many parties, whereas a plurality system marginalizes smaller political parties, generally resulting in a two-party system. Most countries with plurality voting have representation in their legislatures by more than two parties. While the United States is very much a two-party system, the United Kingdom, Canada and India have consistently had multiparty parliaments. However, only the two dominant parties of their times have formed governments in the United Kingdom and Canada. Eric Dickson and Kenneth Scheve argue that there is a counter force to Duverger's law, that on the national level a plurality system encourages two parties, but in the individual constituencies supermajorities will lead to the vote fracturing. Steven R. Reed has shown Duverger's law to work in Japan and Italy. In recent years the validity of this law has come under increasing quantitative scrutiny. Mechanism A two-party system often develops in a plurality voting system. In this system, voters have a single vote, which they can cast for a single candidate in their district, in which only one legislative seat is available. In plurality voting (also referred to as first past the post), in which the winner of the seat is determined purely by the candidate with the most votes, several characteristics can serve to discourage the development of third parties and reward the two major parties. Duverger argued that there were two mechanisms whereby plurality voting systems lead to fewer major parties: (i) small parties are disincentivized to form because they have great difficulty winning seats or representation, and (ii) voters are wary of voting for a smaller party whose policies they actually favor because they do not want to "waste" their votes (on a party