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A single-member district or single-member constituency is an electoral district that returns one officeholder to a body with multiple members such as a legislature. This is also sometimes called single-winner voting.
Elections for single-member districts are held under a number of voting systems, including plurality (first past the post), runoffs, instant-runoff voting (IRV), approval voting, range voting, Borda count, and Condorcet methods (such as the Minimax Condorcet, Schulze method, and Ranked Pairs). Of these, plurality and runoff voting are the most common.
A small constituency with a single member, as opposed to a large, multiple-member one, encourages a stronger connection between representative and constituent and increases accountability. In multi-member district countries such as Israel, where the whole country is treated as a single constituency and representatives are selected by party-lists, the constituency link is lost altogether.
It is often claimed that because each electoral district votes for its own representative, the elected candidate is held accountable to their own voters, thereby helping to prevent incompetent, fraudulent or corrupt behavior by elected candidates. The voters in the electoral district can easily replace him since they have full power over who they want to represent them.
On the other hand, in a constituency system, a candidate who is popular nationally may be removed if he is unpopular in his own district. Also, a candidate may have no incentive to be accountable to any constituents who are inclined to vote against him anyway.
Each representative must be a winner
Sometimes voters are in favor of a political party but do not like specific candidates. For example, voters in Canada re-elected the Alberta government in 1989 but, because of dissatisfaction with its leadership, the premier and leader of the governing party, Don Getty, lost his seat.
Fewer minority parties
Single-member districts tend to promote two-party systems (with some regional parties). Called Duverger's Law, this principle has also been empirically supported by the cube rule which shows how the winning party in a first-past-the-post system is mathematically over represented in the final chamber of representatives. Supporters view this as beneficial, as parliamentary governments are typically more stable in two-party systems, and thus small minorities are not given undue power to break a coalition. First-past-the-post minimizes the influence of third parties and thus arguably keeps out extremists. Critics of two-party systems believe that two-party systems offer less choice to voters, create an exaggerated emphasis on issues that dominate more marginal seats, and does not completely remove the possibility of a balanced chamber (or hung parliament), which can also give undue power to independents and lead to even greater instability.
A safe seat is one in which a plurality or majority of voters, depending on the voting system, support a particular candidate or party so strongly that the candidate's election is practically guaranteed in advance of the vote. This means all other voters in the constituency make no difference to the result. This results in feelings of disenfranchisement and to abstentionism among voters.
Without some sort of proportional representation, which generally requires either multi-seat elections or at-large elections, the aggregate representation gained by some political interest may be out of proportion to their popular support. For example, in 2012, candidates from the Republican Party won more seats in the U.S. House of Representatives than candidates from the Democratic Party, but in aggregate, candidates from the Democratic Party received more votes than candidates from the Republican Party in elections to the House of Representatives. Later analyses demonstrate that this could have occurred without any intentional gerrymandering, and that Republicans will likely continue to hold a majority in the House even if as much as 55% of voters prefer Democrats in 2014.
Single-seat districts allow those who decide the boundaries of those districts to control election outcomes. For example, those drawing district maps may pack voters who tend to favor one political party into a small number of districts that will heavily favor that party while breaking up the remaining supporters into a large number of districts such that the majority of districts favor one political party even if the majority of voters favor another. This is especially common where elected representatives control redistricting and can draw districts specifically to ensure their own re-elections.