New boundary proposals are unlikely to fix flaws that used to help Labour, but now give the Tories an edge

THE Conservative Party has long complained of the electoral system’s bias towards Labour. In 2005 Labour won a majority of 65 MPs out of 646 with a lead of just 2.8 percentage points; five years later the Tories failed to win a majority at all, despite leading Labour by seven points. The variation in constituency sizes is often cited as the reason for this: places where Labour does well, such as Wales, tend to have smaller constituencies, so it takes fewer votes to elect an MP.

Periodically, boundaries get redrawn to correct these anomalies. On September 10th the government announced plans to redraw Britain’s electoral map and create constituencies with roughly equal numbers of voters—cutting the current number of MPs, from 650 to 600.

But in the past two elections, the system actually benefited the Conservatives (see chart). This is because Labour’s earlier advantage was only partly down to constituency sizes. According to one study of the 2010 election, rejigged boundaries would have removed just 21% of pro-Labour bias. Other factors also contributed and their effects are now reversed.

One is the performance of smaller parties. From 1992 to 2010 the largest was the Liberal Democrats. When they won seats, it was typically at the expense of the Tories. This gave Labour an advantage. But in 2015 this factor had the opposite effect: the Lib Dems collapsed and the Scottish National Party emerged as the third-biggest political force at the expense of Labour, which was almost wiped out in Scotland. To the Conservatives this was worth 38 extra seats over Labour, according to Charles Pattie at the University of Sheffield and Ron Johnston at the University of Bristol.