Determining Constituency Marginality in the UK Using the Expense Claims of MPs

A United Kingdom (UK) parliamentary seat is commonly referred to as ‘marginal’ if the majority is less than 10% of votes cast thus rendering the seat vulnerable on a swing of 5%. This paper investigates whether the spending behaviour of MPs on selected constituency service expenditure categories can offer insights on what constitutes a ‘marginal’ seat within the UK ‘first-past-the-post’ electoral system. The possible existence of a non-linear relationship between the expense claims of MPs and the size of the constituency majority provides the basis for such an insight. This paper thus investigates the empirical nature of this non-linear relationship using separate specifications based on quadratic and piece-wise linear splines in constituency majority size. The empirical analysis reported for the behavior of MPs appears broadly consistent with the conventional definition used to classify a ‘marginal’ constituency in the UK.