What is the ideal number of MPs per constituency in proportional representation?

Dylan Difford, guest contributor. Opinions and research are solely the author's and do not necessarily reflect the opinions of the ERS.

Posted on the 26th November 2021

When choosing a proportional voting system, one of the most important decisions is the number of MPs each constituency elects – its ‘district magnitude’. For example, rather than having three constituencies in Lewisham as we do now (Lewisham Deptford, Lewisham East and Lewisham West and Penge) each electing one MP each, under a proportional system you might have one constituency that covers all of Lewisham, but elects 3 MPs.

Whether you decide to make a Lewisham seat that elects 3 MPs or an Inner South London seat that elects 12, can affect everything from the overall proportionality of the elections to the relationship between MPs and their constituents.

But it is a game of trade-offs – larger constituencies mean more proportional results, but necessarily cover a larger geographic area and have been accused of being more distant from voters. So, what is the ideal constituency size and is there a perfect balance between proportionality and localness?

What is the ideal number of MPs per constituency?

Within European countries that use proportional representation, there is a fair bit of variation in terms of what size a constituency is. At the smaller end are Austria (median constituency = 4 seats), Spain (5), Switzerland (5.5) and the countries that use STV – Ireland (4) and Malta (5). Those with larger constituencies include Belgium (15), Finland (16) and Latvia (14).

Then there is the Netherlands that eschews constituencies entirely in favour of a single nationwide allocation of seats and the countries that use mixed-member systems, like Germany and Italy, which combine single-member constituencies with overlapping multi-member regions. But overall, most European countries have an average constituency size of between 4 and 15 seats.

The impact of constituency size on proportionality

The general rule with proportionality and constituency size is ‘the smaller the constituency, the less proportional the results’. This is largely due to smaller constituencies raising the effective threshold at which a party can win a seat, leading to more seats going to larger parties. You need a smaller share of the vote to win one of ten seats, than one of three.

But the relationship isn’t as simple as one extra MP means one extra unit of proportionality. To demonstrate this, I’ve projected every general election since 1997 onto several schemes of Party List PR – Micro (2-3), STV-size (3-6), Sub-county (3-12), County (6-20), Sub-regions (16-30) and Regions or Nations (18-84) – and averaged the results.

What’s most apparent is how non-linear the trade-off is between proportionality and constituency size. Even relatively small STV-size constituencies yield a significant level of proportionality for fairly little loss of localness. And once you go beyond an average constituency size of around 10, the improvements in proportionality become increasingly limited compared to a constituency’s continually increasing geographic area.

As the chart shows, the relationship between proportionality and constituency size is also intertwined with different electoral formulae. Choosing Sainte-Laguë over D’Hondt, for instance, allows you to have much smaller constituencies for the same level of proportionality.

Impact of variation of constituency size

But it isn’t just average constituency size that matters, it is also the range in size. And range varies from uniformity in Malta (all 5 member) and Croatia (all 14), through small variances in Ireland (3-5) or Iceland (7-11), to significant variation in Finland (7-36) and Portugal (2-48). The problem with high levels of variation is that it creates inequality between different areas, as it is easier to win seats in larger constituencies than in smaller ones.

This is compounded by the fact that larger constituencies are almost always big cities – meaning that small parties with an urban support base can be at an advantage compared to a similarly supported party with a largely rural votership.

But uniformity isn’t necessarily the answer either. Variably sized constituencies are able to coincide with ‘natural’ political communities (cities, counties, states, etc.) and are usually permanent (with the number of MPs they elect being refreshed every few years).

Constituencies of equal numbers of MPs are much less flexible – with boundaries having to be regularly redrawn, often with ‘natural’ political communities being split or unrelated ones merged to stay within population quotas. Generally, it is preferable to have at least some level of variation in constituency size, within certain parameters.

Strengthening the constituency link

The opposite end of the pole to proportionality is the MP-constituency link and there is always the concern by some that any increase to the size of constituencies will weaken that link. It is undoubtedly true that constituencies under any PR system will be larger in area and have a greater population than single-member districts, although the ratio of MPs to voters stays the same.

It is always worth noting that under most versions of PR proposed for Britain, every constituency outside of the Scottish Highlands would be smaller in area than the largest current constituency (Ross, Skye and Lochaber).

Internationally, the link between an MP and their constituents is hard to compare because it depends so much on national political culture. However, the evidence generally suggests that constituency work is highest in countries that have a combination of small constituencies and voting systems that emphasise individuals, such as Open List PR or the Single Transferable Vote. This has been particularly true with the latter – one study even finding that backbench Irish TDs did a bit more constituency work than their British counterparts.

Having a group of MPs that actually represent the voters can produce a stronger constituency link, than having a single MP only a minority voted for.

A 2011 paper by John Carey and Simon Hix that found that the ‘electoral sweet spot’ is an average district magnitude of between four and eight. There you achieve a relatively high level of proportionality without sacrificing much in the way of local representation.

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