Two decades of the existence of post-communist party systems provide already a good opportunity to juxtapose these systems with party systems in established Western European democracies. Introducing two newly compiled datasets on Eastern Europe, the presented paper aims to compare the historical development of Western and post-communist party systems with a particular focus on the changes of party regulations in the post-war and post-1989 era, respectively. The first original dataset covers composition of cabinets in post-communist EU Member States, the second one concurrent changes of party regulations in this region. A large-N quantitative analysis of all EU Member States is carried out which shows correlations between cabinet formation, changes in ruling coalitions and reforms of party laws. To explain these correlations and their causality, a rational choice model is constructed that incorporates essential features of the coalition/policy bargaining link.
