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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 or winner takes all. The alternative are multi-member districts, or the election of a body by the whole electorate voting as one constituency.
A number of voting systems use single-member districts, 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.
In some countries, such as Australia and India, members of the lower house of parliament are elected from single-member districts; and members of the upper house are elected from multi-member districts.
It has been argued by proponents of single-member constituencies that it encourages a stronger connection between the representative and constituents and increases accountability and is a check on incompetence and corruption. In countries that have multi-member constituencies, it is argued that the constituency link is lost. For example, in Israel the whole country is a single constituency and representatives are selected by party-lists.
On the other hand, today most voters tend to vote for a candidate because they are endorsed by a particular political party or because they are in favour of who would become or remain the leader of the government, more than their feelings for or against the actual candidate standing. 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
It has been argued that 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 minorities do not have 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 nonparticipation by some voters, both supporters of the dominant candidate as well as his or her detractors.
Comparison of single-member district election methods
|Monotonic||Condorcet||Condorcet loser||Majority||Majority loser||Mutual majority||Smith||ISDA||Clone independence||Reversal symmetry||Polynomial time||Participation, Consistency||Later no harm|
|Range (Score) Voting||Yes||No||No||No||?||No||No||No||Yes||Yes||Yes||Yes||No|
|Sri Lankan contingent voting||No||No||No||Yes||No||No||No||No||No||No||Yes||No||Yes|