Join | Donate | Sign up to our bulletins | Contact us |

IN THIS
SECTION

Stay informed with ERS email updates.

Sign up...

Systems Guide

Additional Member System

What is the Additional Member System?

AMS is a hybrid voting system. It is part First-Past-the-Post (FPTP) and part closed party list. The party list element is added on to make the result more proportional, overcoming (to a greater or lesser extent) the distortion inherent in FPTP. Supporters of AMS claim that it combines the best of both; its detractors say it combines the worst of both.

The exact proportion of constituency representatives and list representatives varies from country to country; the constituency element usually makes up between 50 and 80 per cent.

Under AMS, each voter typically gets two votes – one for a real person, and one for a party. Under certain varieties of AMS, the two votes are merged into one, with the vote for the candidate counting as a vote for their party as well. This doesn't happen very often, however.

When all the votes are in, each constituency returns a winner, in the traditional FPTP style. If a candidate was standing in a constituency as well as on a party list, their name is taken off the list, with everyone below them moving up a place.

The additional members are then allocated with the aim of tallying the number of seats won by each party to their share of the vote. Some systems do this solely on the basis of the party vote, others include the constituency vote too.

Finally, some variants of AMS include a 'threshold', such that a party must gain, say, 4 per cent of the vote if they are to have any seats.

Also known as:

Outside of the UK, AMS is more commonly referred to as Mixed Member Proportional (MMP).

Real-world application of AMS

The Scottish Parliament (73 FPTP, 56 top-up)
The Welsh Assembly (40 FPTP, 20 top-up)
The Greater London Assembly (14 FPTP, 11 top-up)
Italy's Senate and Chamber of Deputies were both elected with a 75:25 Additional Member System between 1993 and 2005. This was changed in the run-up to the 2005 general election.
The German Bundestag (299 FPTP, 299 top-up), as well as some German state parliaments.
New Zealand's House of Representatives (since 1993, 60 FPTP, 60 top-up)
Mexico's Cámara de Diputados (lower house) (300 FPTP, 200 top-up)
Bolivia's Cámara de Diputados (lower house) (68 FPTP, 62 top-up)
Lesotho's National Assembly (lower house) (80 FPTP, 40 top-up)

Arguments used in support of AMS

It is broadly proportional.
Each voter has a directly accountable single constituency representative.
Every voter has at least one effective vote.
It allows a voter to express personal support for a candidate, without having to worry about going against their party.

Arguments used against AMS

Many representatives are accountable to the party leadership rather than the voters.
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. This is especially marked in Wales, where Labour has no list AMs.
AMS sometimes gives rise to 'overhang' seats, where a party wins more seats via the constituency vote than it is entitled to, proportionally speaking. 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.
AMS can lead to the problem of 'decoy lists'.
Some people get confused over exactly what they're supposed to do with their two votes.