Are you sitting comfortably? Estimating incumbency advantage in the UK: 1983–2010 – A research note

This note adapts two models commonly used to estimate the incumbency advantage that US members of Congress enjoy – the ‘slurge’ and the Gelman-King Index – to provide comparable estimates for UK MPs. The results show that Liberal Democrats enjoy extremely large such advantages on a par with those of US Congressmen of between 5% and 15% of the vote. Labour and the Conservatives have incumbency advantages at around 2% and 1% respectively. The note estimates that effects could have changed the outcome in as many as 25 seats in some elections, and that they cost the Conservatives the chance to govern alone after the 2010 election.

► I assess the degree to which sitting UK MPs acquire an electoral advantage.
► The study uses two methods popular in US literature, ‘slurge’ and ‘Gelman-King’.
► I find that MPs of all parties gain an incumbency advantage.
► Lib Dem MPs have the larger such advantages than Labour or Conservatives.
► Incumbency is significant enough to change the result in as many as twenty seats.