A mixed-member proportional ("MMP") electoral system (known as an "additional member system" or "AMS" in the United Kingdom) is a mixed electoral system with one tier of single-member district representatives, and another tier of "additional members" elected through a partisan system and used to increase proportional representation.
Additional member system
The additional member system ("AMS") in the United Kingdom has constituency representatives which are generally elected under the single member plurality voting system. The particular individuals selected from the partisan system depends on the particular variant of the system.
Variations of the Additional Member System have different ways of determining how many party list representatives each party is entitled to. The main difference between systems is whether the constituency representatives are counted when list representatives are allocated to each party.
- Under the Mixed Member Proportional (MMP) or Top-Up system, the aim is for the party's total number of representatives, including constituency representatives, to be proportional to its percentage of the party vote.
- Under the Parallel Voting or Supplementary Member (SUP) system, the party list seats are allocated proportionally, and any constituency seats the party may have won are additional.
Parallel Voting is the more common variation among voting systems of the world, but MMP is the system described here. Small parties will generally win more seats under MMP than SUP.
Since this is a chimera of Single Member Plurality and Party List, it inherits all the issues of both systems. While some issues cancel each other, some do not. Since both votes are a plurality vote, issues like vote splitting remain. Due to the number of issues with this type of systems there are many variations which try to compensate for them.
The AMS is used to elect members to numerous representative bodies around the world.
- Bundestag (Federal lower house)
- New Zealand, where the system is known only as MMP (Mixed Member Proportional), not as AMS
- House of Representatives (unicameral Parliament)
- United Kingdom
- London Assembly
- Scottish Parliament
- Welsh Assembly
It would be used for the proposed Regional Assemblies in England.
AMS is also used in Bolivia, Italy, Lesotho, Mexico and Venezuela.
Hungary has a complex voting system that results in a less proportional representation than AMS but more proportional than Parallel voting.
Proposals for British Elections
In 1976, the Hansard Society recommended that the Additional Member System be used for UK parliamentary elections, but instead of using closed party lists, it proposed that seats allocated by proportional , and the remainder seats of would be filled using open party lists. There was not a referendum before the 2001 election and the statement was not repeated.
The voter makes two votes: one for a constituency representative and one for a party. In a lesser-used variant, which is used by some of the several States of Germany, both votes are combined into one, so that voting for a representative automatically means also voting for his party.
On the district or national level (i.e. above the constituency level), 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.
Because a party can gain less seats by the party vote than needed to justify the won constituency seats, overhang seats can occur. There are different ways of dealing with overhang seats. In the Scottish Parliament the number of overhang seats is taken from the number of proportional seats of the other parties, in Germany's Bundestag and the New Zealand House of Representatives the overhang seats remain.
In order to be eligible for list seats in the New Zealand, German and various United Kingdom systems, a party must either earn at least 5% of the party vote or must win at least one constituency seat (three constituency seats in Germany). This is extremely important to the minor parties. A party which wins no constituency seats and fails to meet the 5% threshold faces oblivion. Having a leader with a safe constituency seat is a tremendous asset to a minor party in such a system as it ensures survival.
Potential for Tactical Voting
In terms of tactical voting, the first vote for the district representative is often much less important than the second party list vote in determining the overall result of an election; in other cases a party may be so certain of winning seats in the district election 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.
Political parties can also abuse the system: 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.