What is the Additional Member System?

The Additional Member System (AMS) is designed to balance fair party representation with strong local accountability. Voters cast two votes: one for the constituency candidate they back to be their local MP, and one for their preferred party. Versions of this system are in use in Scotland, Germany and New Zealand.

How does the Additional Member System work?

Constituency vote

This first part works exactly like general elections now under First Past the Post. Voters choose an individual to represent them in their constituency, and the candidate with the most votes wins.

Proportional top-up

But in addition to electing their local MP, voters also have an opportunity to vote for their preferred party. Based on this second vote, parties are allocated “top-up” MPs to ensure that the overall make-up of parliament more accurately reflects the way people voted.

Example

For example, if Party A wins two-thirds of the single-winner constituencies, but receives only 40% of the party preference vote, it will receive fewer top-up seats than Party B that wins few constituencies, but gets 30% on the second vote.

Table with columns: Party, Share of party preference votes, Constituency seats, Top-up seats, Total seats. A: 40%, 10, 1, 11 B: 30%, 4, 4, 8 C: 20%, 1, 4, 5 D: 10%, 0, 2, 2

Why is the Additional Member System better than First Past the Post?

Unlike First Past the Post, AMS helps ensure that parties’ share of seats more closely reflects how people vote. This makes it less likely that a party will win a large majority in Parliament on a small share of the votes.

Voters retain a strong, direct link to their local representatives, as under First Past the Post.

Meanwhile, the top-up system helps ensure that their party preference is reflected in the overall result, meaning wasted votes - a major problem with straight First Past the Post elections - are kept to a minimum.

What is Unlock Democracy’s view on the Additional Member System?

Unlock Democracy believes that the Additional Member System is one of several Proportional Representation systems that would make elections fairer and ensure that Parliament better reflects how people vote.

We welcome the National Commission on Electoral Reform which will recommend a voting system fit for the future. It may well consider the Additional Member System among other options.