Party Policy and Group Affiliation in the European Parliament

Abstract

While much attention has been paid to party politics in the European Parliament (EP), systematic empirical research has yet explained the basis on which national parties affiliate with the EP political groups. Our paper addresses this lacuna in three ways. First, we use empirical measures on party positions to demonstrate that EP party groups consist of national parties that share similar policy positions. Second, we explain party group as driven by policy congruence, using Bayesian/MCMC methods to estimate the policy determinants of group affiliation using a (conditional) multinomial logit model. Finally, we use the predictions from the model to identify which national parties are not in their ?ideally congruent? EP groups. Data consist of an originally constructed dataset of national party affiliation in EP groups at the time of the 2004 EP election and national political party policy positions from expert surveys of party positions. Our findings suggest that national party organization and switching into EP groups is driven mainly by a concern to minimize policy incongruence between the national and transnational levels.