Reaping the winner’s bonus

Our ‘first past the post’ electoral system has two key characteristics when translating votes into seats.

First, the most popular party receives a ‘winner’s bonus’ of seats. Some regard this as a strength, increasing the chance of majority-run council administrations, but others believe it to be unfair.

Second, is the fate of smaller parties. There is a high threshold for success that punishes small parties. Parties winning a quarter of local authority vote share will not win any seats if votes are uniformly distributed. Concentrating votes in a small number of wards or divisions is the most effective strategy.

Previously, we used the ‘effective number of parties’ to demonstrate that most councils are two-party dominated, but the identity of those parties changes over time and place (see ‘Measuring the strength of local parties’, first 655). 

Overall, between 1973 and 2020, Ns (where ‘N’ is the number of parties and ‘s’ references seats) averaged 1.9 to 2.3 and is currently 2.2.

Another approach for evaluating party systems is to substitute seats with vote share. Nv is calculated in the same way. An election fought between two parties that sees a 50/50 split in votes means Nv=2 – a clear two-party system. 

Had the votes instead divided 75 to 25 per cent, then Nv falls to 1.6 since one party is much larger than the other.

Nv has ranged between 2.3 and 3.6 since the 1973 reorganisation. The gap between Nv and Ns (averaging 0.8 over the last five decades) shows how votes are translated into seats in council elections.

Following the 2017 county council elections, for example, Ns is 2 but Nv was 3.1. 

Devon county illustrates important consequences of the voting system. The Conservatives reaped the winner’s bonus with a 44 per cent vote share delivering 42 seats – 70 per cent of the total. 

Liberal Democrats finished second in votes (22 per cent) but won only seven seats. Labour has the same seat total with a much smaller vote share. Green and UKIP candidates combined received one in 10 votes but won only one seat between them. 

Labour benefited as a small party because its vote was focused on Exeter. The Liberal Democrat vote was more evenly spread – in three districts, it received a quarter or more of votes but won only a single division each time.

Cumbria provides a contrary example with a smaller gap between votes and seats. Again, the Conservatives headed the poll but the share of votes and seats this time was almost identical, as were those for Labour and Liberal Democrats. 

The explanation lies in the geography of vote distribution. Labour’s support is concentrated in the towns while the Liberal Democrat vote is strongest in Eden and South Lakeland.

Similar outcomes are evident in the Welsh unitary councils in the same electoral cycle. 

In Swansea, Labour received four in 10 votes but won two in every three seats. Despite receiving one in four votes, only eight Conservatives – 11 per cent of the total – were elected. 

By contrast, the Liberal Democrat vote was more efficiently distributed. The party chose not to contest some areas, ran modest campaigns in others and targeted winnable seats in just four divisions.

More rural areas in Wales show a smaller disparity between vote and seat shares.  Pembrokeshire’s distribution of seats is very close to the spread of votes, a fact largely determined by the strength of Independents. Many of these seats are ‘no go’ areas for the main parties, whose focus appears to be identifying the more vulnerable Independents.

Variations in the vote/seat relationship are evident too on much smaller geographical scales, for example, the densely populated London boroughs. 

After the 2018 elections, the average effective number of parties is 2.8 for votes but 1.5 when seats are considered. 

Barking and Dagenham returned only Labour councillors (Ns is 1, therefore) but three in every four votes were cast in its favour with Nv measuring just 1.6. 

Tower Hamlets, too, has a small value for Ns (1.1) but a much larger one for Nv (3.4). This is because Labour candidates were opposed by candidates standing for a wide range of parties with little or no prospect of victory.

Merton provides a different example of how votes and seats are more closely related because party strengths are geographically based. Labour dominates in the east of the borough, the Conservatives in the west, with Liberal Democrats occupying a corridor of seats in the centre adjacent to the Merton Park ward won by the Residents’ Association.

You can find out the vote shares in your local authority by visiting


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