Since the 1970s it has been widely accepted that incumbent legislators in a number of countries representing single member districts have enjoyed an advantage in their bids for re-election. In the US, for example, the high incumbency advantage has meant it is often extremely difficult to remove a sitting legislator – so much so that Ronald Reagan was not entirely joking when he said that there was less turnover in the House of Representatives than in the Supreme Soviet of the USSR.
In a research note forthcoming in Electoral Studies, I investigated the extent to which incumbency has become a factor in the UK, examining the degree to which MPs are able to build up a local following and gain an advantage over challengers when they re-stand for election. The study looks at elections from 1983 to 2010 and uses two methods used by researchers in the US on the House of Representatives and US Senate where incumbency is such an important factor.
The key findings are that Liberal Democrat MPs enjoy very large incumbency advantages of (depending on the election and the method of analysis) between 5 and 15% of the vote. Where a Liberal Democrat MP gets in, they are often able to entrench themselves making it extremely difficult for Labour or more often the Conservatives to regain the seat. In several of the elections in the 1980s and 1990s it would indeed have been easier to have defeated a US member of Congress than an incumbent Lib Dem MP.
There appears to be a modest downtrend in the value of incumbency for the third party in more recent elections; as the party has grown larger, the novelty factor of having a Liberal Democrat MP is perhaps not as large. Yet the incumbency bonus enjoyed by the Lib Dems is still greater than for the other parties.
Labour and Conservative MPs are estimated to have incumbency advantages as well, although much smaller at 2% and 1% respectively, although the latter appears to be increasing. With the smallest incumbency bonus, the Conservative Party has been the main loser from the phenomenon. The note estimates that without incumbency effects, the party would have won 11 more seats in 2010, which would have been just enough to govern as a minority government with DUP support.