Party System Structure and Institutionalisation in the European Parliament

What factors shape the structure and institutionalisation of the European Parliamentary party system? This paper is particularly interested in explaining the choices of national parties to join the ‘non-core’ party groupings of the European Parliament—those outside the EPP, Socialists and Democrats and ALDE. Drawing on party manifesto data, this paper assesses whether the structure and stability of the EP party system is best predicted by national party system structure and stability, EP institutional incentives and constraints or by processes of Europeanization.  It argues that institutional incentives play a role in the initial party group membership choices of the new member states of the EU.