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The Additional Member System (AMS) is a branch of voting systems in which some representatives are elected from geographic constituencies and others are elected under proportional representation from a wider area, usually by party lists. Voters usually have two votes, one for the party and the second for the candidate in a constituency (sometimes these votes are combined). The constituency representatives are generally elected under the first-past-the-post voting system. The party representatives are elected by a party vote, where the electors vote for a political party, and usually not directly for an individual. The particular individuals selected come from lists drawn up by the political parties before the election, at a national or regional level.

Variations of the AMS have different ways of determining how many party representatives each party is entitled to. The main difference between systems is whether the constituency representatives are counted when party representatives are allocated.

  • Under the Mixed Member Proportional (MMP) or Top-Up (compensatory) system, the aim is either for the party's total number of representatives, including constituency representatives, to be proportional to its percentage of the party vote, or for the allocation of additional party seats to offset some or all of the disproportionate result in the constituencies. The party vote largely determines the number of representatives the party has in the assembly.
  • Under the Parallel Voting or Supplementary Member (SUP or SM) system, the party seats are allocated proportionally within themselves, without consideration of any constituency seats the party may have won.
  • Under the French proportional system designed to produce a strong majority, half the seats are given proportionally between party lists and the other half given to the list with a plurality, thus ensuring that a single list wins well over half the seats.

Parallel Voting is among the most common variation among voting systems of the world. Small parties will generally win more seats under MMP than SUP unless there is a threshold of exclusion, such as the 5% or 3 constituencies threshold in Germany, or the 5% or 1 constituency seat threshold in New Zealand.


Since smaller parties are likely, in compensatory systems, to win a larger number of proportional seats, such additional member systems hand additional political power to the leaders of these parties at the expense of regional directly elected representatives, unless the additional members are elected on an open regional list as in Bavaria or a closed regional list as in Scotland. With closed lists, "Party List" candidates may become puppets for the party leadership, or may add diversity to the party's elected members. The largest party in an election is likely to win a smaller number of proportional seats, so that governing parties are less likely to have such figureheads. However, the majority party may lose diversity, unless the members elected from the party list when it was in opposition then win local seats when the party gains enough support to form the government.

In Parallel systems even the largest party will elect members from the party list, so the top list positions are guaranteed seats. This system is found in emerging democracies like post-communist Russia, where new national parties were evolving, and the voting system was intended to foster them, while allowing local independent members to win local seats, many of whom then joined the winning party. It retains the plurality principle but has another paper to allow voting for a party rather than a candidate.

See also




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