section_path: Party systems/Two-party systems
section_name: Two-party systems
paragraph_text: Two-party systems
  
  In several countries, there are only two parties that have a realistic chance of competing to form government. One current example of a two-party system is the United States, where the national government has for much of the country's history exclusively been controlled by either the Democratic Party or the Republican Party. Other examples of countries which have had long periods of two-party dominance include Colombia, Uruguay, Malta, and Ghana. Two-party systems are not limited to democracies; they may be present in authoritarian regimes as well. Competition between two parties has occurred in historical autocratic regimes in countries including Brazil and Venezuela.
  
  A democracy's political institutions can shape the number of parties that it has. In the 1950s Maurice Duverger observed that single-member district single-vote plurality-rule elections tend to produce two-party systems, and this phenomenon came to be known as Duverger's law. Whether or not this pattern is true has been heavily debated over the last several decades. Some political scientists have broadened this idea to argue that more restrictive political institutions (of which first past the post is one example) tend to produce a smaller number of political parties, so that extremely small parties systems – like those with only two parties – tend to form in countries with very restrictive rules.
  
  Two-party systems have attracted heavy criticism for limiting the choices that electors have, and much of this criticism has centered around their association with restrictive political institutions. For example, some commentators argue that political institutions in prominent two-party systems like the United States have been specifically designed to ensure that no third party can become competitive. Criticisms also center around these systems' tendencies to encourage insincere voting and to facilitate the spoiler effect.