The first-past-the-post electoral system is well known for its tendency to produce big mismatches between a party’s national vote share and share of seats. As the election effectively consists of 650 local contests, the overall outcome depends not just on the total number of votes cast for each party, but also crucially on where these votes are cast.

Parties vary greatly in terms of how efficiently they turn votes into seats

In practice the system has tended to favour the top two parties, Conservatives and Labour, at the expense of other Great Britain-wide parties. For example, the combined Con-Lab vote share in both the 2010 and 2015 General Elections was around two-thirds, but their combined share of seats in both elections was around twenty percentage points higher, at 87%.

The 2015 General Election provided a particularly extreme example of how the geographical distribution of a party’s support affects the number of seats it wins. UKIP and the Green Party only won one seat each despite securing by far their highest ever vote tallies – 3.88 million for UKIP (12.6% vote share) and 1.16 million for the Greens (3.8%). Because their support was spread relatively diffusely across Great Britain rather than heavily concentrated in specific regions, these large vote tallies were not effective in winning seats. For its part, the Scottish National Party’s 1.45 million votes in Scotland (4.7% UK vote share) translated into 56 seats.

The 2017 Election did however produce a substantial narrowing of the discrepancies between vote share and seat share, attributable largely to the upsurge in support for the two largest parties and the related decline of UKIP and the Greens. The Conservatives and Labour again both won a higher share of seats than votes, but this time the gap between combined vote share (82.3%) and seat share (89.1%) closed to just 6.7 percentage points. For Labour the share of votes (40.0%) and seats (40.3%) were almost identical.

GE 2017 reduced the gap between vote share and seat share

The disproportionality between votes and seats can also be calculated in terms of votes-per-seat-won: in 2017 the Conservatives got one seat for every 43,018 votes, while for Labour the figure was one seat for every 49,152 votes. It took many more votes to elect a Lib Dem (197,665) and Green (525,665), but far fewer to elect an SNP MP (27,931).

UK General Elections 2010 - 2017, votes, seats and candidates by party

The votes that made the difference in 2017

Yet another way of examining the relationship between votes and seats is to quantify the proportion of votes cast for a party that was decisive in winning seats for that party. In some seats, a party may amass large majorities well in excess of the number of votes needed in order to be ‘first past the post’. In seats where the party’s candidate is defeated, however, the votes for that candidate can be considered in retrospect to have been cast in vain.

The chart below breaks down the votes cast nationwide for each of the eight parties that won at least one seat in the 2017 General Election, based on whether the votes were:

  • decisive’ (directly contributing to a candidate coming first in a constituency – equal to the runner-up’s vote tally plus one),
  • excess’ (votes for winning candidates over and above the amount needed to come first) and
  • redundant’ (votes cast for defeated candidates).

On this basis, the party with the most efficient distribution of votes was the SNP, in that 53% of their votes were decisive in securing victory for SNP candidates, whereas only 9% of their votes were ‘excess’ votes for victorious SNP candidates above the total needed for victory. This latter figure is a reflection of how closely contested the constituency races were in Scotland – as we discussed in another blogpost, a remarkably high proportion of seats won by margins of less than 1% were in Scotland.

In GE 2017, the SNP's votes were distributed most efficiently - 53% directly contributed to the election of an SNP MP

The Green vote was the least efficiently distributed among those parties that one at least one seat, closely followed by the Liberal Democrats. 94% of votes for the Green Party across the UK were for cast for unsuccessful candidates. The remaining 6% consists of the 30,149 votes cast for Caroline Lucas in Brighton Pavilion, where her majority over the runner-up was 14,699. 89% of votes cast for the Lib Dems went to losing candidates.

To sum up – while the 2017 General Election did not produce the same extreme disproportionality between votes and seats as was the case in 2015, it remains the case that the electoral system rewards the parties very differently for the votes they attract.

Picture credit: Elections 2010-48 by AdamKR; Creative Commons Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)