Single-member district

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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.

Results

Constituency link

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.

Safe seats

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.

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
Approval	Yes	No	No	Rated	?	No	No	No	Yes	Yes	Yes	Yes	No
Range (Score) Voting	Yes	No	No	No	?	No	No	No	Yes	Yes	Yes	Yes	No
Schulze	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	No
Ranked pairs	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	No
Kemeny-Young	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes	No	No	No
Nanson	No	Yes	Yes	Yes	Yes	Yes	Yes	No	No	Yes	Yes	No	No
Baldwin	No	Yes	Yes	Yes	Yes	Yes	Yes	No	No	No	Yes	No	No
AV/IRV	No	No	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	No	Yes
Borda	Yes	No	Yes	No	Yes	No	No	No	No	Yes	Yes	Yes	No
Bucklin	Yes	No	No	Yes	Yes	Yes	No	No	No	No	Yes	No	No
Coombs	No	No	Yes	Yes	Yes	Yes	No	No	No	No	Yes	No	?[1]
MiniMax	Yes	Yes	No	Yes	No	No	No	No	No	No	Yes	No	No
Plurality	Yes	No	No	Yes	No	No	No	No	No	No	Yes	Yes	Yes
Anti-plurality	Yes	No	No	No	Yes	No	No	No	No	No	Yes	Yes	?[1]
Contingent voting	No	No	Yes	Yes	Yes	No	No	No	No	No	Yes	No	Yes
Sri Lankan contingent voting	No	No	No	Yes	No	No	No	No	No	No	Yes	No	Yes
Supplementary voting	No	No	No	Yes	No	No	No	No	No	No	Yes	No	Yes

References

↑ 1.0 1.1 Coombs' method and anti-plurality voting are defined only for situations where each voter casts a complete ranking of all candidates.