When concentrating your vote flips over into being a disadvantage

There was a fascinating discussion on my Twitter timeline with Rob Ford, Will Jennings, Iron Economist and many other distinguished people, triggered by concerns about the Liberal Democrat revoke A50 policy.  In short: the concerns expressed by some are that the Liberal Democrats might get the total majority they would need to enact this Revoke with a mere 30-35% of the vote, and that would be way short of the 50% endorsement sought by those wanting a referendum.  And the fact that they could get this majority with just 35% was bolstered by the modelling I did ages ago in this popular blog post.

Which blog post I still stand by in outline, but developments since have shown up even more glitches in the system, including this feature: the LibDem seat total climbs very slowly at first, but then at some point it rockets as all sorts of seats fall.  What this highlights is how having a very evenly spread vote across all constituencies is a massive disadvantage below a certain threshold, and only flips over to being an advantage when you hit the 30s in terms of vote share.

There is a corollary: learning how to concentrate your vote share is essential if you want to go above a small number of seats, as a small party: contrast the fortunes of the SNP and UKIP in the 2015 election.

Since that post, I have written a number of others exploring methods a model user might concentrate the LibDem vote share and get a different result; generally speaking, the outcome was about 20-30 more seats for the LibDems when their vote share is in the high teens/low 20s. Again, just what you would expect.

What I thought I would also share before heading off for my nighttime cocoa: that same variable becomes a disadvantage for the LibDems if they are looking for a majority. In other words, they begin to pile up pointlessly large majorities rather than gain more seats – just as hit the Tories in 1997, say, or Labour in 2017, when their votes did not go as far as they might in seats.

Here is a graphical representation: first the behaviour of party seats when there is no use of “historical LibDemmyness” in the machine (Solid line) and second the same relationship with a high degree of LibDemmyness and a little tactical voting

The dotted line suggests a much higher threshold for the LibDems is needed to get a majority – but still in the 30s. Maybe 5 percentage points higher.  And none of this loopy, “450 seats plus” style outcome.  Another reason to doubt whether a purely smooth swing is what we might expect.


Published by freethinkingeconomist

I'm a mid 40s, former special adviser (Downing Street 2017-19, BIS from 2010-14), former FT leader writer and Lex Columnist, former financial dealer (?) at IG, student of economic history, PPE like the rest of them, etc. This blog has large gaps for obvious reasons. The name is dumb - the CentreForum think tank blog was called Freethink, I adapted that, we are stuck now.

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