When Does Party Unity Hinder Representation? A Model of MP Behavior

• In Event: Polarization at Both the Elite and Voter Level

Abstract

While party unity in a legislature can advance the policy agenda of a party, pressure for a unified vote can also decrease the accountability of individual MPs to their constituents and can contribute to polarization. But what explains variation in party unity within a country? We model a situation in which MPs in single member districts are accountable to a constituency whose median varies stochastically across a set of issues. The floor median MP’s constituency therefore aligns with the dominant party in parliament on a majority of issues, but probabilistically favors the minority party’s position for a subset of issues. The proportion of issues on which the median MPs constituency deviates from the party unity position is therefore a function of the heterogeneity of their district’s median preference across issues. We model the costs to MPs of deviating from the party unity position as an inverse function of party majority size, so that the highest costs of deviating occur when the party holds a bare majority and the lowest costs are incurred when the party holds a larger majority. The model therefore parameterizes party unity as a function of MPs’ district heterogeneity over issues and the majority margin in parliament. We test the model’s predictions about party unity in the context of British divisions in the House of Commons, using survey data to estimate the distribution of median preferences across issues by district.

Authors

• Marika Danielle Csapo, Institut Barcelona d'Estudis Internacionals

• Zsuzsanna Blanka Magyar, Institut Barcelona d'Estudis Internacionals