FPTP

• Polls ahead of the UK election not only point towards defeat for the government, but a veritable trouncing.
• Some multiple regression and post-stratification (MRP) polls, which enable seat-level analysis, suggest record margins of defeat.
• A huge Labour majority is likely to come off the back of a disproportionately small number of votes.

Follow the losers

• It’s common for the number of seats in parliament that a party wins to be highly disproportionate to its share of the overall vote.
• Looking at the vote shares of election losers between 1929 and 2019, there is an almost perfect correlation of -0.9 between the losing major party’s vote share and the proportionality of the election outcome.
• I’ve plotted the vote shares of the losers against the Gallagher index, which is the most widely used measure of proportionality.

Basically, as the losing party’s vote share approaches 50%, which is when we are witnessing a highly competitive two-horse race, FPTP can produce very proportional outcomes, with Gallagher index scores of sometimes below 5 (scores more typical in proportional representation systems). But as the losing major party’s vote share drops below 40%, Gallagher index scores get into double figures.

Why so disproportionate?

• This is a very linear effect – the lower the vote share the second party gets, the more disproportionate the overall election result.
• The first scenario is when the party system fragments and more votes go to third parties.
• The other scenario is a very asymmetric two-party race, where the second-largest party heavily trails the winner.
• Whenever one party moves clearly ahead of the other, disproportionality reaches levels higher than ever witnessed in the UK.

2024: a perfect storm

• It is fragmented and there is significant asymmetry in the polling for the two main parties.
• But the expected disproportionality in the upcoming election will be down to the Labour lead more than the degree of fragmentation.
• In the UK’s FPTP system, both major parties’ seat shares depend heavily on how much of a lead the winner has over the loser.
• The difference of an increase in the Labour lead by just a couple of points can change a lot under FPTP.

For proponents of electoral reform, the upcoming election is set to provide a mixed bag. On one hand, a massive Labour win renders it unlikely that the party itself would contemplate reform. On the other, anything near a two-thirds majority for Labour on the back of a vote share barely above 40%, together with severe under-representation of the Conservatives, could expose the distorting nature of FPTP more than any election in living memory.
Heinz Brandenburg does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.