Additional Member System

AMS Additional Member System
The Additional Member System
All about the Additional Member System and its pros and cons

Additional Member System

Additional Member System (AMS) can also be known as Mixed Member Proportional (MMP).

Where is AMS used?

The Scottish Parliament
The Welsh Assembly
The Greater London Assembly
The German Bundestag
New Zealand's House of Representatives
Mexico's Cámara de Diputados (lower house)
Bolivia's Cámara de Diputados (lower house)
Lesotho's National Assembly (lower house)

How does the Additional Member System work?

AMS is a hybrid voting system. It combines elements of First Past the Post where voters mark an X next to the candidate they want to represent them in their constituency, and proportional representation, where voters select from a list of candidates for each party who represent a larger regional constituency. This helps to overcome the disproportionally often associated with First Past the Post elections.

Under AMS, each voter typically gets two votes – one for a candidate and one for a party.

Each constituency returns a single candidate, in the style of First Past the Post. The votes for the party list candidates are then allocated on top of these constituency seats to ‘top up’ the number of seats won by each party to represent their share of the votes proportionally. These are the “additional members”.

Pros and cons of the Additional Member System

The case for

The arguments against

It is broadly proportional.

Many representatives are accountable to the party leadership rather than the voters.

Each voter has a directly accountable single constituency representative.

Having two different types of representative creates animosity between them. In Wales and Scotland, for example, AMs and MSPs elected via the regional lists have been seen as having 'got in via the backdoor' or as 'assisted place' or 'second class' members.

Every voter has at least one effective vote.

AMS sometimes gives rise to 'overhang' seats, where a party wins more seats via the constituency vote than it is entitled to according to their proportional vote. In Germany and New Zealand, but not in the UK, extra seats are allocated to the other parties to redress the balance. This can get complicated and lead to further bickering and animosity.

It allows a voter to express personal support for a candidate, without having to worry about going against their party.

It can be complicated, with people getting confused over exactly what they're supposed to do with their two votes.

Voting Systems

Proportional Representation Mixed Systems Majoritarian Systems
More representative as seats are distributed according to vote share. Combines the features of majoritarian-style systems and Proportional Representation. Systems that are highly disproportional.
Single Transferable Vote Additional Member System First Past the Post
Party List PR Alternative Vote Plus Alternative Vote
    Borda Count
    Block Vote
    Limited Vote
    Supplementary Vote
    Two-Round System