The Wolf in Sheep's Clothing? Policy Preferences and Legislative Party Switching

It is incontestable that political parties belong to the most important institutions within parliaments. The members of parliament (MPs) represent and act for the party under which they were elected, but sometimes they decide to change their legislative affiliation; a phenomenon known as “party switching”. Defecting MPs alter the party configuration that was set by elections and compromise democratic representation and the party’s accountability. Previous studies show that MPs are, among other things, motivated to switch due to ideology and policy concerns. In doing so, studies argue that MPs seek to affiliate with parties that have similar ideological and programmatic goals. However, existing studies have mainly compared the ideological distance between MPs in order to control the influence of ideological congruence. Moreover, MPs’ ideological preferences are derived from their voting behavior, that nevertheless is deeply affected by party discipline and therefore does not really reveal where MPs stand ideologically. Based on the literature on policy linkage, we argue that MPs are more likely to defect if there is a mismatch between their own policy preferences and those of their party. Taking advantage of the dataset of the Comparative Agendas Project (CAP) and a unique database of party switchers in Europe, we are able to measure MPs’ policy preferences on several policy domains and to determine the effective distance between switchers and their parties. This way we can test in a more effective manner whether policy concerns drive defectors. Empirically, we analyze the impact of policy preferences on party switching in Belgium, Italy, Switzerland, and the United States from 1995-2012. Our quantitative analysis provides strong support for our hypothesis, albeit it shows variation across parties and policy domains. These findings are not only relevant in order to understand legislative party switching, but also to grasp how policies influence legislative behavior.