Party policy and group affiliation in the European Parliament

Systematic empirical research has yet to explain how national parties join political groups in the European Parliament. This article first demonstrates, using original empirical measures from expert surveys of party positions, that EP party groups consist of national parties sharing similar policy positions. Secondly, using Bayesian/MCMC methods, the paper estimates the policy determinants of group affiliation using a (conditional) multinomial logit model to explain that ‘party group’ choice is largely driven by policy congruence. Finally, predictions from the model identify national parties not in their ‘ideally congruent’ EP groups. The findings suggest that the organization of and switching between EP groups is driven mainly by a concern to minimize policy incongruence between national and transnational levels.