This is part of a three-part series on the UK’s usage of Alternative Vote. For Part 2, see here. For Part 3, see here.

On May 5th, 2011, the UK held a referendum on adopting the Alternative Vote system, or otherwise known as Instant-runoff voting (IRV). Supported by the Liberal Democratic party as part of a coalition agreement with the Conservatives, the referendum ended in a decisive loss for those advocating for a change.

Our model attempts to calculate the results of the 2015 election if the election had been conducted under the Alternative Vote system.

UK-AV

With these results, running off assumptions described below, we found that a change to Alternative Vote would not have led to a major shift in the composition of Parliament if voting habits remained the same. Whilst it would be expected that the Liberal Democrats would benefit from an AV system as the party on the center, there are several reasons why this may not have happened:

  1. Although large amounts of Labor and Conservative Voters would have switched their support to the Lib Dems for second choice, this would only happen if the Liberal Democrats had more votes than Labor / Conservative. In the 2015 election, this was not the case.
  2. Athough they may be considered on the “center” for traditional left-right wing politics, with the rise of the protectionism vs populism debate, the Liberal Democrats could no longer be considered to the center. With votes from UKIP being redistributed to the Conservatives, it is clear why the Conservatives could have gained the most in this setup.
  3. The district magnitude in AV is still one – which means that a nation still tends towards a two-party system. If the Liberal Democrats are unable to muster enough support, they will still be unable to overturn this system, regardless of their position on the center.

By this, it seems that switching to an Alternate Vote system would not have had too much of an impact in the 2015 election. However, conclusive results cannot be drawn – which is why we need to analyse elections beforehand – see Part 2 for the case of 2010.

Appendix

We made the following assumptions of redistribution on the UK’s domestic politics:

  • 40% of fringe-party voters would vote for Labor, 40% would vote for UKIP, and 20% would vote for the Conservatives.
  • Out of the Conservative voters, about half would choose the Liberal Democrats first, and 30% would choose UKIP next. This uses assumptions of pre-Brexit Britain – we assume that a large portion of conservative voters were mostly centrist, and euro-skeptic voters would largely stay home.
  • Out of Labor voters, 50% would choose the Liberal Democrats second, and 40% would choose UKIP (given it’s working-class support). Once again, this is before Labor’s strict leftward lurch.
  • Liberal Democratic Voters were relatively evenly split between Labor, Conservatives and the Greens (40%, 35%, 25%, respectively).
  • UKIP voters would prefer mostly Conservatives and Labor to some degree (60% and 30%).
  • Green voters would mostly prefer the Liberal Democrats and Labor.
  • In Scotland, the SNP would mostly prefer the Liberal Democrats. Only the 25% of the Liberal Democrats would support the Scots, the other major voters of these parties are not likely to have the SNP their second choice. A similar situation prevails in Wales.
  • In Northern Ireland, the Ulster Unionists and Democratic Unionists would prefer each other, whilst Sinn Fein and SDLP would prefer each other. Each party has a minority that supports the Alliance.