The UK uses a wide range of voting systems to elect MPs; MEPs in the European Parliament; members of the devolved parliaments or assemblies in Scotland, Wales and London; councillors in local authorities; and the London Mayor, other city mayors and police commissioners in England. Here Patrick Dunleavy, Tony Travers, and Chris Gilson offer the definitive simple guide to all you need to know about the five voting systems used across the British Isles.

1. First Past the Post (or ‘FPTP’ for short) – the current system in use in the UK since mediaeval times to elect MPs and since the late nineteenth century to choose local councillors. The country is divided into local voting districts, called constituencies for Westminster or wards inside local authorities, and the election takes place within each of these.  Voters mark a single X on the ballot paper against their preferred candidate. The person who gets the largest number of votes in each constituency (or ward) becomes the MP (or councillor). They do not need to gain majority support (that is, more than half of all votes cast), just more votes than anyone else in that contest.  At Westminster, one party has usually got a clear majority of MPs and formed the government in the post-war period.  But ‘hung Parliaments’ where no party has a majority have occurred in 1974 and 1977-79, and now in 2010.  If there is no majority, it is necessary for two or more parties to work together, or there is a minority government. In England and Wales the system is also still used to elect local councillors and here ‘no overall control’ situations frequently occur, where either a coalition of parties runs the council, or the largest party assumes control without a majority.

Incidentally, the ‘first past the post’ label is completely misleading because there is no fixed winning post. What you need to win a local seat is just a ‘plurality’ of votes, that is, more votes than anyone else. So the more parties compete in each seat, the lower the winning ‘post’ gets. Consequently political scientists call this system ‘plurality rule’, a much more accurate label.

2. The Supplementary Vote (SV) – this system is employed to elect single office-holders for a whole regional or local authority area (spanning many constituencies or wards, and sometimes called ‘at large’ elections). It has been very successfully used to elect the powerful London Mayor since 2000, and all the other elected mayors in England. In 2012 it was employed for the first time to elect Police Commissioners in England and Wales. The Supplementary Vote keeps the tradition of X voting in local areas. But now people have a ballot paper with two columns on it, one for their first choice and one for their second choice. They put an X vote against their chosen candidate in the first preference column, and then (if they wish) an X vote in the second preference column.

The key difference in the SV system from FPTP is in what candidates need to do to get elected. We count the first preference votes and if anyone has more than 50% of the votes cast then they are elected straightaway, and the counting ends there. However, if no one has majority support then the top two candidates go into a runoff stage, and the candidates placed third, fourth, fifth etc are all eliminated at the same time. We then look at the second preference votes of people who voted for one of the eliminated candidates. If any of these voters cast a second choice vote for either of the two candidates still in the race then these votes are added to their piles. Whichever of the two top candidates now has the most votes then wins.

This process of knocking out low ranked candidates and redistributing their voters’ second choices ensures that the largest feasible number of votes count in deciding who is elected as the mayor or police commissioner. It does not always completely guarantee that the person elected has a majority of votes cast. But in repeated London elections the winning mayor has had nearly three fifths support amongst votes counted – a very clear result that greatly enhance the legitimacy of the office-holder.

3.  List proportional representation (PR) – is the system used for electing the UK’s Members of the European Parliament (MEPs). The country is divided into 13 large regions (actually the Government’s Standard Regions), ranging in size from the South East (10 seats) and London (8 seats) down to the North East and Northern Ireland (3 seats each). The main parties all select enough candidates to contest all of a region’s seats (while smaller parties may only contest some of the available seats). The parties arrange their candidates in an order, to form their List, where candidates are ranked from the top in the order that the party will win seats if get enough support. The ballot paper shows each party’s List of candidates and voters choose just one party to support using a single X vote.

We then count up all the votes in each region and for each party we give seats to candidates from its list in proportion to the party’s vote share. So, suppose we have a region with 10 seats where party A gets 40 per cent of the vote – they should end up with 4 of the available seats.  This system is very proportional but it may tend to favour larger parties somewhat if many votes are heavily fragmented across many smaller parties. List PR is used widely across Europe for electing national parliaments, as well as the European Parliament.

4. ‘Additional Member’ System (AMS) – this is a proportional representation system used to elect members of the Scottish Parliament (MSPs), the Welsh Assembly (AMs) and Greater London Assembly. Somewhat over half of the members of these bodies are locally elected in constituencies using FPTP, as above. The remaining 40 to 45 per cent of representatives for each body (the ‘additional members’ ) are elected in large regional areas using a proportional representation system, so as to match every party’s share of winning candidates to their votes share.

The basic idea of electing two types of representatives is to ensure a close fit between what voters want and the make-up of the Assembly. Voters get two ballot papers, one for their local constituency and one for the wider regional contest, and they mark one X vote on each paper. Over half of representatives are locally elected in constituencies using FPTP: here whoever gets the largest vote in each local area is the winner. But in AMS voters also have a second vote for their regional top-up members. Candidates are put forward on regional lists by each party (as under List PR above). To decide who should get these additional members we look at how many local seats a party already has within a region from the local contests, and at what share of the list votes it has in this region. If a party already has its full share of seats, it gets none of the top-up members. But if the party it does not have enough seats already we assign it additional members, taken from its regional list of candidates, so as to bring each party up to having equal numbers of seats and vote shares. There’s a formula for doing this that works near perfectly, but again may slightly over-represent larger parties if a lot of the list vote is split across multiple smaller parties.

The one detailed point to notice here is that in some AMS elections (like the Greater London Assembly and in the German Bundestag elections) parties have to get a minimum share of the vote to qualify for getting any representatives at all: usually the requirement is 5 per cent of votes, as in London. In other AMS elections (such as Scotland and Wales) the regions used for top-MPs are small enough already, so that this extra rule is not needed.

One for the nerds – AMS is usually called ‘Mixed Member Proportional’ (MMP) in academic discussions, and this label is also widely used in New Zealand where the system has also operated since the late 1990s.

5. Single Transferable Vote (STV) – elects all members in local areas, but uses much bigger, multi-member constituencies (electing 3 to 5 representatives each) than FPTP. The aim is to allocate seats to different parties in relation to their vote shares. This is again a fully proportional system that for a country or locality as a whole will match how many representatives a party wins closely to its votes share. In the UK the system is used for all local government elections in Scotland, and in Northern Ireland for local and Assembly elections. Elsewhere in the world the system is used to elect parliaments in Ireland and Malta.

The number of constituencies is less (around a third or a fifth of the number under FPTP) and their size is increased, so that we can elect 3 to 5 representatives at a time in each local contest. Voters mark their preference using numbers, so putting 1 for top choice, 2 for their second choice, 3 for their third, and so on. If they want to, voters can choose to support candidates from across different parties, so as to exactly match their personal preferences. A complex counting process then operates that allocates seats in an order to the candidates that have most votes, so as to get the best fit possible between party vote shares and their number of local MPs. Looking across the country or local authority area as a whole the results should be proportional.

Want to know more about how this magic counting system works? These next two paragraphs are for you. We look at the votes and divide them by the number of seats being contested +1. This gives a ‘quota’, a vote share that guarantees a party one seat. (E.g. if we count 100,000 votes and have 4 seats to elect in a constituency, then the quota would be 100,000 divided by (4+1) = 20,000 votes). Any party with more than a quota gets a seat straightaway; a party that has two quotas, gets two seats, etc. Every time we give the party a seat, we deduct one quota share of votes from its total.

When we’ve done this, there will normally be about half of the seats still unfilled. Here we shift into a different method, by beginning to knock out candidates from the bottom. We take the least popular candidate and eliminate them from the race, and then redistribute their voters’ second preferences across the candidates still in the race. We keep doing this until one of the parties still in the race has enough votes for a quota and so wins the next seat. We then deduct this quota from that party’s votes (as above) and carry on with the ‘knocking out the bottom candidate’ process until all the seats are allocated.

What difference do the systems make? Who benefits?

The first past the post system has historically benefited the top two parties in every local area, generally the Conservatives and Labour, although in some areas the Liberal Democrats and the Scottish or Welsh Nationalists have done well enough to become top two contenders. The proportional representation systems (List PR, the Additional Member System or the Single Transferable Vote) all fit parties’ seats won closely to votes shares. Thus they are strongly supported by parties like the Liberal Democrats, UKIP, the Greens and so, that often pile up millions of votes in FPTP contests but may win few or no seats, because they are less commonly or rarely the largest party in a given local area.

Would you like to know more?

An accessible and British-focused account is provided by Patrick Dunleavy, Helen Margetts and Stuart Weir, The Politico’s Guide to Electoral Reform in Britain and this can be downloaded free of charge from the Democratic Audit of the UK.  Unfortunately, the Guide dates back to the first wave of electoral reform in the UK in 1999, but the Democratic Audit website also includes lots of useful material on recent voting system debates and controversies.

Note: This article gives the views of the author, and not the position of the British Politics and Policy blog, nor of the London School of Economics. Please read our comments policy before posting.

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