Briefing: Why we elect London’s mayor with the Supplementary Vote

While many people still don't understand it, SV is a simple voting system that empowers voters


On 5 May, Londoners go to the polls to elect the Mayor of London. Each voter gets two preferences, in an electoral system known as Supplementary Vote (SV). SV is a decent system with which to elect a mayor— it empower voters.

What is SV?

Many London voters aren’t aware they can express a second preference in the London Mayoral election, according to a friend of mine who’s been working phone banks at London Labour HQ.

Apparently one voter, when greeted with the news that they could vote for Sadiq Khan even if another candidate was their first choice, thought that they were being invited to engage in fraud:

“Two votes? No bloody way. Not me.”

It would not be fraudulent to vote twice on 5 May. It would be to make use of the SV system. Londoners on the electoral register get a first preference, and then, if they so wish, a second preference as well.

When votes are counted up, the electoral authorities begin by adding up first preferences. If anyone receives 50 per cent or more of the vote, they win. Well done them. No need for another round.

If no one gets 50 per cent, there’s a second and final round. Now second preferences come in. All but the top two candidates are eliminated, and the second preferences of those voters whose first preference candidates have been eliminated are tallied up and added to the first round totals.

After the re-allocation of second preferences, whichever of the two candidates has more votes is the winner.

Isn’t this just the same as Alternative Vote?

No. You could say that SV is a sort of hybrid of the Alternative Vote (AV) and First Past the Post (FPTP); and it lacks some of the disadvantages of both.

Like AV, SV is a preferential voting system. But in AV candidates need more than 50 per cent of the vote to win. If the leading candidate does not get 50 per cent of 1st preferences first time round, the candidate with the fewest first preferences drops out and their second preferences are then tallied up. And so on and so on, till a candidate gets over the 50 per cent mark.

In SV voters are given only two preferences and only two candidates can have their tallies added to. So there is only one round of tallying; the winning candidate doesn’t need to get the votes of 50 per cent of the electorate.

As in FPTP, they simply need the most votes.

Why not just use AV?

SV has advantages over AV: because there are only two rounds of tallying, it does not get nearly as messy, and it is easier to understand.

During the Labour leadership election appeared to baffle many commentators; and back in 2011, during the AV referendum campaign, AV certainly flummoxed one John Humphreys.

AV can certainly lead to some strange outcomes. A candidate who finishes third on first preferences and third on second preferences can in theory win an AV contest. This can seem a little odd. To a large extent, SV precludes such possibilities.

And SV also has some of the advantages of AV. It is a system that tends to enable winning candidates to receive significant support among voters.

What is more, you the voter can vote for your favourite candidate—perhaps Sian Berry (Green) or Caroline Pidgeon (Lib Dem)—without worrying that your vote is almost certainly wasted given that they’ve got little chance of winning.

Under SV, your vote won’t be wasted if and when your second preference vote comes to be tallied.

Then the Supplementary Vote is the best?

SV is not without disadvantages. It is all very well using your first preference for your favourite candidate and your second preference pragmatically. But in order for you to do so, there have to be two candidates out in front and you have to know who they are.

This is easy enough in the 2016 London Mayoral contest which is a two-horse race.  However, in a different contest, it could be very unclear what pragmatics dictates. If it’s not clear who is in first, second and third place, then voting outcomes are likely to be distorted by tactical voting, so that certain candidates could be unfairly advantaged / disadvantaged.

SV, like all preferential systems, punishes fringes parties – for this reason, it is best suited to the election of stand-alone elections (such as for mayor). As a replacement to FPTP for elections to the British Parliament, it has all the same problems as AV.

SV is not perfect. Nor is any electoral system.

I am voting in London on 5 May – given it’s SV, how do I vote?

For your first preference, vote with your heart.

For your second preference, vote with your head. Khan and Goldsmith are way out ahead in the polls. So if you haven’t put one of them as your first preference and you want your vote to be counted, you should choose between them for your second preference.

Khan vs Goldsmith – who does SV favour?

Preferential voting systems tend to favour candidates close to the political centre, wherever that may be. London, it is sometimes said, is a left liberal city. So all in all, SV would seem to favour Khan.

Certainly it has worked in favour of Ken Livingstone who has run in all 4 of the past London Mayoral contests, and had significant second preferences wins three times out of four.

In 2012, Livingstone, although he was eventually defeated, mopped up many more second preferences than Johnson: it was not widely reported at the time but, thanks to SV, the scale of Livingstone’s defeat was significantly narrowed.

Will SV make a difference to the outcome in London 2016?

Is it likely that whoever wins under SV, a different candidate would have won if the election had used FPTP?

It is certainly possible. But if the data from past mayoral contests in England, all of which use SV, is anything to go by, it is not very likely. In 51 races for mayors, there have been just three instances where the eventual was not also the winner on first preferences.

So if it makes so little difference, why favour SV?

It is fair to say that many Londoners don’t understand the mechanics of the SV system but, because of its relative simplicity, it still enables voters to cast their vote effectively. What’s more, it has still done a good job of conferring legitimacy on the office of London Mayor – to date, winning candidates have always got a decent chunk of the vote.

SV also enables voters to express themselves, even where they don’t like either of the top two candidates. And, by the same token, it encourages smaller parties to field candidates, and to raise new issues.

Peter Wiggins is a freelance journalist. Follow him on Twitter

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