A view in favor of unicameral legislatures is that if an upper house is democratic, it simply mirrors the equally democratic lower house, and is therefore duplicative. A theory in favor of this view is that the functions of a second chamber, such as reviewing or revising legislation, can be performed by parliamentary committees, while further constitutional safeguards can be provided by a written Constitution.
In many instances, the governments that now have unicameral legislatures were once bicameral and subsequently eliminated the upper chamber. One reason for some such a changes is because an elected upper house had overlapped the Lower house and obstructed passage of legislation, an example being the case of the Landsting in Denmark (abolished in 1953). Another reason is because an appointed chamber had proven ineffectual, one example being the case of the Legislative Council in New Zealand (abolished in 1951).
Supporters of unicameralism noted the need to control government spending and the elimination of redundant work done by both chambers. Critics of unicameralism pointed out the double checks and balances that a bicameral system affords, forcing a greater level of consensus on legislative issues. Another disadvantage of unicameralism is that urban areas with large populations have more influence than sparsely populated rural ones. In many cases the only way to get the smaller population areas on board for unified governments is to implement a bicameral system (such as the early United States).
Some of the subnational entities with unicameral legislatures include Nebraska in the United States, Queensland in Australia, all of the provinces and territories in Canada, and all of the German Bundesländer (Bavaria having abolished its Senate in 1999).
In the United Kingdom, the devolved Scottish Parliament, National Assembly for Wales and Northern Ireland Assembly are also unicameral.
Virtually all city legislatures are also unicameral in the sense that the city councils are not divided into two chambers. Until the turn of the 20th century, bicameral city councils were common in the United States.
The American Commonwealth of Puerto Rico currently has a bicameral legislature composed of a Senate (Senado) and a House of Representatives (Camara de Representantes). In a referendum was held on July 10, 2005, Puerto Rican voters approved the change to a unicameral legislature by 456,267 votes in favor versus 88,720 against. However, another referendum will be held in that commonwealth during 2007 to approve the specific amendments to the Puerto Rican Constitution that are required for the change. If those constitutional changes are likewise approved, Puerto Rico will switch to a unicameral legislature from 2009.