Single-Member Districts

A single-member district is an electoral district that sends one representative to a legislature. The winner is determined by plurality vote. This system, used throughout the United States, tends to produce two dominant parties.

Most American legislative elections are conducted in single-member districts decided by plurality vote. A state is divided into districts of roughly equal population. Each district elects one representative. The candidate with the most votes wins, even if that share is less than fifty percent. The system is sometimes called first-past-the-post. The Constitution does not require single-member districts. Article I, Section 2 simply provides that representatives shall be chosen by the people of the several states. For the first half of American history, some states elected representatives at large or in multi-member districts. Congress required single-member districts in 1842, repealed the requirement, and reimposed it permanently in 1967. Most state legislative districts are also single-member, though some states use multi-member districts for parts of their lower chambers. Single-member plurality systems tend toward two-party competition. This pattern is so consistent across countries using the system that political scientists call it Duverger's Law. A third party in such a system rarely wins seats commensurate with its support, because its votes are spread thinly across many districts. The result is consolidation into two large coalitions that compete for the median voter in each district. The alternative is some form of proportional representation, in which seats are allocated to parties in proportion to their share of the vote. Most European democracies use some version of proportional representation. The United States has rejected it at the federal level and in nearly every state. The two-party system, the structure of American legislatures, and the practice of gerrymandering all flow from the choice of single-member districts.