Are marginals different? Evidence from British elections 1950–2015

We analyse the results of British general elections from 1950 to 2015. In our model, voting is both instrumental and expressive, and is driven both by ideology and the perceived valence of different parties. On most assumptions the model predicts that the safer the seat the lower the swing. The exception is where ideological factors are relatively dominant in instrumental voting, and valence factors are relatively dominant in expressive voting. In this case the highest swings might be in the safest seats. Alternatively swing might peak at intermediate majorities, and this is what we find when we look at swings between Conservative and Labour in seats held by one or other of these parties. We also find that marginals behave more distinctively when the national result is expected to be close or when there has been another general election recently; and that at least some voters have a sense of what is a ‘bellwether’ seat i.e. one that would be marginal in a close election. However in those seats where the main contest has been between the Conservatives and Liberal Democrats, the swing is positively related to the closeness of the contest.