The UK electoral system now decisively favours the Conservatives

Before the election, it was commonly assumed that the bias in the electoral system would favour Labour, as it has done since the 1990s. But Tim Smith illustrates how the skew on May 7 actually benefitted the Conservatives and contributed to their unexpected victory.

In the wake of the largely unexpected Conservative election victory, it was said that pollsters and political scientists had a lot of explaining to do after so many incorrect forecasts.  However, this author correctly predicted that the Liberal Democrats would do worse than was assumed , and also that the electoral system might well favour the Conservatives for the first time since 1987 which also turned out to be the case.  In this blog piece I will explain what has happened and the consequences for the next election.

At this election the two-party bias (or skew) in the electoral system moved from a pro-Labour bias of 54 seats, to a pro-Conservative bias of 48 seats, meaning that if the two parties had won the same number of votes, the Conservatives would have won 48 more seats than Labour.  The table below shows the decomposition of factors that result in this bias.  These can be obtained algebraically using Brookes’ decomposition method, as adapted by Johnston, Rossiter and Pattie. For a full explanation of the factors please read this piece on the Ballots & Bullets blog.

Conservative bias

As the table shows, the pro Conservative bias of 48 seats is the largest in the last eight elections, and it is the first time since 1987 that it has been in the Conservative Party’s favour.  The largest contributor to this shift was third party victories, which swung from a Labour lead of 21 seats to a Conservative lead of 39 seats.  The pro-Labour element of this had been mainly due to the fact that there had been far more Liberal Democrat MPs in seats where the Conservatives would otherwise have won than in those where Labour would otherwise have won.  The collapse of the Liberal Democrats to just 8 seats eliminated most of this.  Meanwhile, the SNP landslide in Scotland then pushed the bias in the other direction making Labour the primary victim of third party wins.  Some of this has been partially offset by a decline in the pro-Tory effects of wasted third party votes.  Wasted UKIP votes appear to have fallen more equally than Liberal Democrat ones had done.

The other key factor behind this swing in electoral system bias has been efficiency, known in US literature as ‘the gerrymander’, which moved from a neutral position to a pro-Conservative factor of 55 seats.  In the UK system the boundaries are not deliberately gerrymandered by partisan redistributions, but nevertheless, they now very much favour the Conservatives whose votes are much more efficiently distributed.  When the parties’ vote shares are equalized, Conservative wins waste far fewer surplus votes than Labour, with the latter now tending to pile up larger but ultimately unnecessary majorities in safe seats.  The reason for this big increase in Conservative efficiency was caused by their very strong performance in the right places, i.e. marginal seats, and this was helped by the large number of first term incumbents standing for re-election for the first time.  Labour did best in its safest English seats.

Some authors had warned that the differential size in constituencies could cost the Conservatives their majority at the election after their failure to ensure major boundary changes. The total size related bias favoured the Labour party by 24 seats in 2015.   The pro-Labour bias due to differences between the nations shrunk due to Labour’s wipe-out in Scotland, which is slightly over-represented, and due to the two Conservative gains from Labour in Wales which is very much more over-represented.  The size bias within the nations, due to the fact that Labour seats tend to see their electorates fall relative to Conservative ones over time, increased from 9 to 17 seats, as the current boundaries which are based on December 2000 electoral data have become another five years older.

Finally, there was a small decline in the pro-Labour bias from abstentions (differential turnout).  The turnout rose more in seats where the Labour party are ahead of the Conservatives on a projected equal vote, and this reduced the abstention bias from a Labour lead of 31 seats to 24 seats.  The turnout in Labour areas remains substantially lower than in Conservative areas and there appears to be no sign that this is likely to change much.


Equalisation of constituency electorates with a major boundary review is going to be a high priority for the new Government. Should the legislation end the over-representation of Wales and slight over-representation of Scotland relative to electorates in England, and succeed in equalising electorates within nations, then the Conservatives could theoretically find themselves defending a majority of 36 seats instead of 12.  Some might argue that as the system is now favouring the Conservatives, the party should proceed more cautiously in order to avoid accusations that they are trying to load the system further to their advantage.

It has been noted here that Labour’s path back to a majority is going to be extremely difficult and on a uniform swing (even before boundary changes).  The party would need to be 12.5 percentage-points ahead of the Conservatives to win a majority.  Boundary changes would likely make this even worse and might mean Labour would need the kind of lead they achieved in 1997 in order to win a majority of one.  Worse still for Labour, there are now a further 38 Conservative first term incumbents in seats gained from other parties, who, if they stand for re-election in 2020 are likely to gain their sophomore surge, making it harder for Labour, or indeed the Liberal Democrats, to win the seats back and deprive the Conservative government of its majority.

This article originally appeared 28 May on the Notts Politics Ballots & Bullets blog. It is reposted with the author’s permission.

About the Author

Tim Smith is a part time Politics PhD student at Nottingham University

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