Additional member (Scottish Parliament)

The additional member system (AMS), also known as mixed-member proportional representation (MMP) outside the United Kingdom, is a mixed electoral system with one tier of single-member district representatives, and another tier of "additional members" elected to make the overall election results more proportional.

The term additional member system, introduced by the Hansard Society, has been largely replaced in the literature by the term mixed member proportional coined by New Zealand's Royal Commission on the Electoral System (1984-1986). This article focuses primarily on semi-proportional implementations of MMP designed to yield moderately proportional election results, similar to the mixed systems used in the UK and referred to locally as AMS. In Scotland, Wales, and the London Assembly, the fixed numbers of additional members, elected in relatively small regions, are not always sufficient to fully compensate for the disproportionality caused by the single-member district (first-past-the-post voting) tier.

In an election using the additional member system, each voter casts two votes: a vote for a candidate standing in their constituency (with or without an affiliated party), and a vote for a party list standing in a wider region made up of multiple constituencies. The constituency vote is used to elect a single representative in the voter's constituency using the traditional first past the post system: the candidate with the most votes (not necessarily a majority of the electorate) wins. The regional vote is used to elect representatives from party lists to stand in regional seats, taking into account how many seats were gained by that party in the constituency vote, using a system of proportional representation: the number of seats a party receives will roughly reflect its percentage of the vote.

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