Additional Member System

The Additional Member System (AMS) is a mixed electoral system in which voters typically cast two ballots: one for a candidate in a single-member constituency (usually decided by first-past-the-post) and one for a political party. Constituency seats are filled by the plurality winners, while a second tier of "additional" or "list" seats is distributed among parties to correct disproportionality between vote share and seat share.

The list seats are allocated using a proportional formula — most commonly the d'Hondt or Sainte-Laguë method — applied to regional or national party-vote totals, with already-won constituency seats subtracted so that under-represented parties receive a "top-up." Because of this compensatory mechanism, AMS is often classified as a form of Mixed-Member Proportional (MMP) representation, though the two terms are not always used interchangeably: "AMS" is the label preferred in the United Kingdom, while "MMP" is used in Germany, New Zealand, and academic literature.

AMS is used to elect the Scottish Parliament and the Senedd (Welsh Parliament), both established in 1999, and was used for the London Assembly from 2000. Germany's Bundestag uses a closely related compensatory system, as does New Zealand's House of Representatives following its 1993 referendum (first used in 1996).

Key design features that vary across jurisdictions:

• Ratio of constituency to list seats (e.g., Scotland 73:56; Wales 40:20 prior to reforms).
• Threshold for list-seat eligibility (Germany applies a 5% national threshold or three constituency wins).
• Tier of aggregation — regional lists (Scotland, Wales) vs. national pooling (Germany).
• Treatment of overhang seats, where a party wins more constituencies than its proportional share.

Critics note that AMS can create "two classes" of legislators and that small regional top-up tiers limit proportionality. Supporters argue it preserves a local constituency link while reducing the distortions of pure plurality voting.