This study examines cooperation networks in legislative decision-making in the European Union, and asks why certain pairs of member states cooperate more intensely than others. The analysis focuses on decision-making before and after the recent historic enlargements of the EU, thereby assessing how the EU is adapting to its new membership. Signaling theory and network theory provide the theoretical framework for this study. These theories make key propositions regarding the formation of political ties, and have been applied to study legislative politics in other political systems, notably the US Congress. Signaling theory posits that political actors are more likely to form a relationship if they have similar policy preferences. Signaling theory therefore explains variation in network ties with individual-level variables. Network theory predicts that the likelihood of a tie between two actors depends on the presence of certain relationships with other actors. For instance, two actors are more likely to cooperate if they share many transitive linkages with other actors. Network theory therefore explains variation in network ties among actors with characteristics of the network in which they are embedded. Our study integrates and analyzes two new datasets on legislative decision-making in the EU. The first dataset contains information on cooperation networks in the Council of Ministers. The second dataset contains information on the policy positions of each of the member states on 331 controversial issues raised by 125 of the most important legislative proposals from the period 1999-2009.
