Tag Archives: additional member system

The Additional Member System (AMS), otherwise known as Mixed Member Proportional (MMP), is one of the leading contenders as a Proportional Representation system for UK General Elections. However, as this piece describes, it can turn alarmingly disproportional when the number of parties in contention increases to the levels we are seeing today: five parties in England, and six in Scotland, are now polling at over 10%.

AMS/MMP has been used for the Scottish and Welsh parliaments, and for the London Assembly, since devolution came in in 1999 – though the Welsh Parliament (Senedd) has decided to use a closed List system from next year’s election onwards.

Recent polls for next year’s Scottish Parliament election were widely reported as suggesting that the SNP are predicted to win 62 of the 129 seats (48%).

What has been less reported is that this is despite their predicted proportional entitlement being only 43 seats (33.3%). How can this happen in a supposedly proportional system?

The answer lies in what are known as ‘overhangs’, where a party wins more of the constituency seats, elected using FPTP, than their proportion of the list vote justifies.

For example, in a Scottish electoral region with 9 constituencies and 7 list seats, if a party gets 33% of the list vote, its proportional entitlement will usually work out at 6 seats (out of 16). So if it wins 8 of the 9 constituencies, which is quite likely if there are many other parties splitting the votes, it has 2 more seats than its proportional share. The ‘top-up’ component of the system will give it no list seats, but it already has too many for proportionality. What is or can be done about this?

In Scotland, as in the London Assembly and formerly in Wales, a party is allowed to keep its overhang seats, and other parties’ shares are reduced. According to analysis by Ballotbox Scotland current polling data suggest that in next May’s Scottish Parliament election the SNP would have 19 overhang seats; these would come at the expense of Labour (19 instead of 25), Reform (17 instead of 23), Conservatives (11 instead of 14), Liberal Democrats (10 instead of 13) and the Greens (10 instead of 11). Note that one implication is that pro-independence parties (SNP/Greens) would have a majority in the parliament of 72 seats (56%), against their proportional entitlement of 54 (42%).

Germany, which was the first country to adopt an AMS/MMP system, has had a similar problem of overhangs in recent years. But it has dealt with it differently, prioritising proportionality above constituency entitlements. In both 2017 and 2021 it allowed parties to keep overhang seats, but added extra list seats to maintain strict proportionality at national level. This required adding 111 and 133 seats respectively to the German parliament in those two elections.

Following those results, Germany has now adopted a different way of dealing with overhangs, so as to keep the Parliament at a fixed size (630): if a party wins too many constituency seats in a region, some are disallowed, using the percentage achieved in their constituency vote as criterion. In this year’s election 23 constituency winners were disallowed, with those constituencies left without an FPTP elected representative. The alternative under the previous arrangement would have again required adding well over 100 seats to the parliament.