Additional Member Systems
Many variants of Additional Member Systems (AMS) are possible. All involve two "layers" of MP - the first being elected to represent a constituency, in the same way as First Past the Post, and the second elected on a regional basis. Constituency MPs are nominated by their local parties, as under FPTP, and regional MPs come from ranked lists submitted by regional parties. Voters typically have two votes – the first to vote for a constituency MP, and the second to vote for their party of choice. They vote for both on the same day. Alternatively, voters can have just one vote, with the additional members elected from the highest-scoring runners up.
The constituency votes are counted first; ‘regional seat MPs’ are awarded as ‘additional’ or ‘top-up’ seats to compensate for the disproportionality of the FPTP election.
Three of the UK devolved assemblies (London, Wales and the Scottish Parliament) use an electoral system that is an Additional-Member, 2-vote, Two-Tier representation system that applies the little-known ‘Modified D’Hondt’ formula for counting the votes.
When processing using the 'Modified D'Hondt' formula, parties are allocated seats in the regions in such a way as to ensure that they receive a fair proportion of seats that largely matches their proportion of votes. This in practice prevents the leading party in the constituency tier of representation from gaining further seats in the regional tier if this would give it seats over and above the proportion (%) it is entitled to.
In other words, a party that gains over-representation in the constituency tier will be prevented from gaining further seats in the regional tier.
The Regional tier of representation is often seen as ‘unfriendly’ to the leading parties of the Constituency tier and friendly to parties that did not stand - or did not gain many seats – in the constituencies. The effect of the Modified D’Hondt formula is to prevent excessive seat gain by a leading or runner-up party and to boost the presence of smaller or non-traditional parties that experience difficulties gaining constituency seats.
Overall, AMS systems are very flexible. In New Zealand and Germany, half the seats are awarded on a regional basis, which produces a very proportional system. By contrast, the Plant Commission proposed a much more minimal version of AMS for the UK, with less than 1/4 of the seats awarded on a regional basis; this would be much less proportional, although still better than FPTP. Additionally, it is possible to impose a threshold, so that parties need to win a certain percentage of the popular vote, or at least one constituency, in order to get any regional seats.
The Additional Member System is used in New Zealand (where it is known as Mixed Member Proportional (MMP), in Germany (where it was imposed by the Allies after the end of WWII), and in a number of other countries. A form of AMS was supported by members of the Plant Commission (MMS), the Jenkins Commission (AV+), and the Hansard Society, the Institute for Public Policy Research and the New Zealand Constitutional Convention.
Explainer on how AMS works: