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Mixed member proportional representation, also termed mixed-member proportional voting and commonly abbreviated to MMP, is a voting system used to elect representatives to numerous legislatures around the world. MMP is similar to other forms of proportional representation (PR) in that the overall total of party members in the elected body is intended to mirror the overall proportion of votes received; it differs by including a set of members elected by geographic constituency who are deducted from the party totals so as to maintain overall proportionality. In the United Kingdom, the different, only partly proportional form of MMP in use for several bodies is known as the additional member system (AMS), although the term additional member system can also be more broadly applied to include parallel voting, a semi-proportional system. In Germany it is called "personalized proportional representation" as distinct from the PR system used before MMP superseded it.

MMP is currently in use in:

• Bolivia
  • Chamber of Deputies (Cámara de Diputados), the lower house
• Germany
  • Bundestag, the federal lower house
  • all state parliaments except Saarland, Bremen and Hamburg, but it is being introduced in Hamburg, see Hamburg Voting System.
• Italy, with the new Electoral System introduced in 2005^[1]
  • Camera dei Deputati, the lower house
  • Senato della Repubblica, the upper house
• Lesotho
  • National Assembly, the lower house
• New Zealand
  • House of Representatives, unicameral (see Electoral system of New Zealand)
• South Africa
  • Municipal elections
• United Kingdom
  • Scottish Parliament, unicameral
  • National Assembly for Wales, unicameral
  • London Assembly
• Venezuela
  • National Assembly of Venezuela (Asamblea Nacional)

The unicameral National Assembly of Hungary has a complex voting system that results in a less proportional representation than MMP but more proportional than Parallel voting.

In 1976, the Hansard Society recommended that MMP in a form different from the German be used for UK parliamentary elections, but instead of using closed party lists, it proposed that seats be filled by defeated candidates, on the 'best runner-up' basis used by the German state of Baden-Wurttemberg. The system was eventually adopted without that provision in parts of Britain.

A similar system was proposed by the Independent Commission in 1999, known as Alternative Vote plus (AV+). This would have involved the use of the Alternative Vote for electing members from single-member constituencies, and regional party lists. However, contrary to the Labour Party's earlier manifesto promises, there was not a referendum before the 2001 general election and the statement was not repeated.

The MMP system in use in the London Assembly would have been used for the other proposed regional assemblies of England, but this process has stalled since the No vote in the Northern England referendums in 2004.

In March 2004 the Law Commission of Canada proposed a system of MMP for the Canadian House of Commons [1] but that proposal has not been adopted.

A proposal to adopt MMP for elections to the Legislative Assembly of Prince Edward Island was defeated in a plebiscite there on November 28, 2005.

In 2007 the Citizens' Assembly on Electoral Reform in Ontario, Canada, also recommended the use of MMP in future elections to the Legislative Assembly of Ontario, with a ballot similar to New Zealand's. A binding referendum on the proposal, held in conjunction with the provincial election on 10 October 2007, saw it heavily defeated.

Ballot for electoral district 252, Würzburg, for the 2005 German federal election. Constituency vote on left, party list vote on right.

The voter makes two votes: one for a constituency representative and one for a party (or, with open-list in Bavaria, for one of the party's list candidates). In the original variant used at first in Germany, still used by two States of Germany, both votes were combined into one, so that voting for a representative automatically means also voting for the representative's party. Most of Germany changed to the two-vote variant to make local MPs more personally accountable. For example, in New Zealand's last election 20% of local MPs were elected from electorates which gave a different party a plurality of votes.

In each constituency, the representative is chosen using a single winner method, typically first-past-the-post (that is, the candidate with the most votes, by plurality, wins).

On the regional or national level (i.e. above the constituency level) three different methods have been used:

• The total number of seats in the assembly are allocated to parties proportionally to the number of votes the party received in the party portion of the ballot. Subtracted from each party's allocation is the number of constituency seats that party won. The number of seats remaining allocated to that party are filled using the party's list. If a candidate is on the party list, but wins a constituency seat, they do not receive two seats; they are instead crossed off the party list and replaced with the next candidate down (this is common to all forms of MMP using closed lists).
• Alternatively, in the British model, a highest averages method is used to allocate list seats only, but the number of seats already won in the constituencies is taken into account in determining the quotients used in the calculations for the list seats.
• In the Italian model of MMP (used 1993-2005), like the British only partly proportional with PR applied to list seats only, for every constituency seat won by a party that party's vote total is reduced by the number of votes received by the second-place candidate in the constituency, subject to the condition that the deduction cannot be less than 25% of the total vote cast in the constituency, unless this would make the deduction larger than the number of votes received by the winning candidate, in which case that candidate's total vote is subtracted.

Because a party can gain fewer seats by the party vote than needed to justify the won constituency seats, overhang seats can occur, though not in the British and Italian models because proportional allocation is applied only to list seats. There are different ways of dealing with overhang seats.

In Germany's Bundestag and the New Zealand House of Representatives the overhang seats remain. In most German states the other parties also receive extra seats ("balance seats") to restore proportionality. Both cases raise the total number of seats.

For example, in New Zealand's 2005 General Election the Māori Party won 2.1% of the Party Vote, entitling them to 3 seats in the House, but won 4 "electorates" (constituencies), leaving an overhang of 1 seat, which results in a 121-member house. If the party vote for the Māori Party had been more in proportion with the constituency seats won, there would have more likely been a normal 120-member house.

In order to be eligible for list seats in the New Zealand and German systems, a party must either earn at least 5% of the total party vote or must win at least one constituency seat (three constituency seats in Germany). If neither of the two conditions are met, no candidates from the party list are chosen. Candidates having won a constituency will still have won their seat. In Bolivia the threshold is 3%. Having a member with a 'safe' constituency seat is therefore a tremendous asset to a minor party in New Zealand. In almost all elections in the UK there are no thresholds except the "effective threshold" inherent in the regional structure. However the elections for the London Assembly have a threshold of 5% which has at times denied seats to the Christian People's Alliance (in the 2000 election), the British National Party and the Respect - The Unity Coalition (both in the 2004 election).

In terms of tactical voting, the vote for the constituency representative is normally much less important (than the party vote) in determining the overall result of an election; but in some cases a party may be so certain of winning seats in the constituency elections that it expects no extra seats in the proportional top-up. Some voters may therefore seek to get a double representation by voting tactically and splitting their votes, though this runs the risk of unintended consequences.

In systems with a threshold, tactical voting for a minor party that is predicted to poll slightly below the threshold is relatively common, especially by voters who are afraid that the minor party missing the threshold would weaken the larger political camp that the minor party belongs to. For example the German moderate-right Free Democratic Party (FDP) has often received votes from voters who preferred the larger Christian Democratic Union (CDU) party, because they feared that if the FDP received less than five percent of the votes, the conservative camp would be weakened so much that the CDU wouldn't be able to form a government. Conversely, in New Zealand, some voters who preferred a large party have voted for the minor party's local candidate to ensure it qualifies for seats. In the latter case the tactic does not distort the outcome but makes it more proportional by by-passing the 5% threshold. In the former it also ensures that votes are not wasted, but at the cost of giving the FDP more seats than CDU voters would really have preferred. The FDP and other smaller parties campaigned to lower the threshold to the 4% used in Scandinavia and Austria, which might have succeeded except for the fear of resurgence of neo-nazi parties.

Political parties can also abuse the system. They can separate their party in two. One contests the electorates seats, the other contests for the list seats. This will produce an overhang. They can co-ordinate their campaign and work together within the legislature, while remaining legally separate entities. This can also give other advantages in areas such as party funding.

For instance in the 2001 Italian elections, the two main coalitions (the House of Freedoms and the Olive Tree) linked many of their constituency candidates to decoy lists (liste civetta) in the proportional parts, under the names Abolizione Scorporo and Paese Nuovo respectively, so that if they won constituencies then they would not reduce the number of proportional seats received by the coalitions. Between them, the two decoy lists won 360 of the 475 constituency seats, more than half of the 630 total number of seats, despite winning a combined total of less than 0.2% of the national proportional part of the vote. In the case of Forza Italia (part of the House of Freedoms), the tactic was so successful that it did not have enough candidates in the proportional part to receive as many seats as it in fact won, missing out on 12 seats.

Decoy lists are not used in most countries using MMP, where most voters vote for candidates from parties with long-standing names.

There is some evidence that many Scottish voters did not understand the implications of the system. In the first election for Scotland's new Parliament, the majority of voters surveyed misunderstood some key aspects of the difference there between the "first" (constituency) vote and the "second" (regional list) vote; indeed in some ways the understanding worsened in the second election. The Arbuthnott Commission found references to first and second votes fuelled a misperception that the constituency vote should be a first preference and the regional vote a second one. That misperception was not helped by the Green Party's tactic of running only regional candidates and appealing for "second votes."

In New Zealand the party vote is the first vote. In Scotland, to deal with the misunderstanding between "first" and "second" votes, the ballot for the latest Scottish Parliament election was changed as recommended by the Arbuthnott Commission. The British government announced on 22 November 2006 that the two separate ballot papers used in previous Scottish Parliament elections would be replaced for the elections in May 2007 by a single paper — with the left side listing the parties standing for election as regional MSPs and the right side the candidates standing as constituency MSPs.

However, the detailed results of the Scottish Social Attitudes Survey, 2003 (shown in the table above) show the confusion was about "first" and "second" votes, creating an average of 28% wrong answers:

• List of democracy and elections-related topics
• Voting system
• Leveling seat
• [2] Mudambi, R. and Navarra, P. 2004. Electoral Strategies in Mixed Systems of Representation. European Journal of Political Economy, Vol.20, No.1, pp. 227-253.

• ACE Project: Germany, the cradle of MMPR
• A Handbook of Electoral System Design from International IDEA
• Electoral Design Reference Materials from the ACE Project
• Scottish Social Attitudes Survey, 2003.