Additional Member System

The Additional Member System uses a mix of first past the post constituencies and party lists.

The Additional Member System (AMS) is used to elect the Scottish and Welsh devolved institutions and to elect the London Assembly. It is also used for elections in Germany, New Zealand and many other states, although it is often called Mixed Member Proportional (MMP) when used abroad.

How to vote

Voter are given two ballots. On one is a list of candidates from different parties or who are independents. The voter marks their preferred candidate to be their constituency representative. On the other is a list of parties. Each party will have published a list of candidates in advance, a vote for a party on the second ballot is a vote for their list of candidates.

How it’s counted

An Additional Member System parliament has both First Past the Post constituency seats elected from the first ballot paper, and seats filled proportionally from party’s lists according to their share of the vote on the second ballot paper. In Scotland, for instance, 73 seats are elected from the constituencies, while 56 are filled by parties based on their share of the vote. The goal is to provide a proportional parliament but also retain a single local MP.

 

"This proportion of constituency to list seats can be different for each institution, but the more first past the post seats, the less proportional AMS will be"

Electoral Reform Society

For the constituency ballot paper, the candidate with the most votes wins regardless of whether they have won over half the votes.

For the party ballot paper, seats are assigned proportionally. These seats are compensatory, which means that they take into account how many constituency seats a party already has. So, for instance, if a party has won 5 constituencies and its fair share is 8 seats then it will win 3 party list seats. It may be that seats are assigned proportionally according to the vote in a region, as in Scotland or Wales, or assigned nationally.

Effects and Features

The Additional Member System has become popular due to its position as a compromise solution in terms of electoral system design.

But, as a compromise, it retains the safe seat constituencies of first past the post, and also uses closed-lists. Hence it is argued that the Additional Member System gives control to parties over who is elected, rather than people.

While it does include single-member constituencies their size has to increase to cover a larger area. Although, the list MPs provide a second layer of representation allowing for alternatives should the voter feel their MP does not represent them for any reason.

Some also argue that the Additional Member System creates two classes of MPs, and that this can create tension. For instance constituency MPs receive local casework, whilst the party list MPs do not.