The Number of Political Parties: A Reexamination of Duverger's Law

The purpose of this essay is to investigate the theory that explains the variation in the number of candidates for office in single-member constituencies with plurality voting. In other words, I shall reexamine and amplify Maurice Duverger's law-the statement that "the simple majority, single ballot system favors the two-party system."'1 Many scholars have found this law attractive because it explains the structure of institutions as logically derived from rational choice by individuals.2 Macrostructure is thus made to depend on easily explainable and intuitively plausible microbehavior, a neatness of fit common enough in economic theory, but seldom arrived at in political theory. The argument is that supporters of a third party desert to one of the two larger parties because they recognize that continued support of their favorite results in "wasting" their votes, and may even contribute to the victory of their most despised. For example, suppose a plurality election in a single-member district at time t1 has produced the result: 10,000 votes for A, 9,000 votes for B, and 2,000 votes for C; and suppose that typically the supporters of C prefer C to B and B to A. For them to continue at timet to support C appears, ceteris paribus, to guarantee the success of A and to be a "waste" of their second choice. There is considerable indirect empirical support for this theory. The tendency toward two parties is especially strong in the United States, where the President is elected by a plurality of sorts in what is almost a single-member district. In Great Britain, most elections are by plurality in single-member districts; and, despite the persistence of third parties, most constituencies usually have serious candidates from only two parties. Strong third parties in England have typically been one of the two major parties in some geographic area, such as the Irish