This contribution conceptualizes the European Union (EU) as a system of differentiated integration characterized by both variation in levels of centralization (vertical differentiation) and variation in territorial extension (horizontal differentiation) across policy areas. Differentiation has been a concomitant of deepening and widening and has increased and consolidated as the EU's powers, policy scope and membership have grown. Turning to explanation, the contribution attributes the pattern of differentiated integration in the EU to the interaction of interdependence and politicization. Differentiation among the member states (internal differentiation) results from supranational integration under conditions of high interdependence and politicization. By contrast, external differentiation (the selective policy integration of non-member states) occurs in highly interdependent but weakly politicized policy areas. These constellations are illustrated in case studies of differentiation in the internal market, monetary union and defence.

We present a new dataset on decision-making in the European Union (DEUII) that revises and expands a previous dataset. Researchers are using this new dataset to address a range of research questions regarding the inputs, processes and outputs of the EU's legislative system. The dataset contains information on 331 controversial issues raised by 125 legislative proposals that were introduced between 1996 and 2008. For each of these controversial issues, the dataset identifies the policy alternative favoured most by each of the main political actors: the European Commission; the European Parliament; and each of the member states' representatives in the Council of Ministers. This information was assembled during 349 semi-structured interviews with key informants. This article describes the dataset and identifies its relevance to several research agendas in EU studies

The collection of articles in this special issue provides a comprehensive analysis of European Union decision-making during the Eurozone crisis. We investigate national preference formation and interstate bargaining related to major reforms of the Economic and Monetary Union. The analyses rely on the new 'EMU Positions' dataset. This dataset includes information about the preferences and saliences of all 28 EU member states and key EU institutions, regarding 47 contested issues negotiated between 2010 and 2015. In this introductory article, we first articulate the motivation behind this special issue and outline its collective contribution. We then briefly

Research on differentiated integration (DI) in the European Union has burgeoned in recent years.However, we still know little about citizens' attitudes towards the phenomenon. In this article, we argue that at the level of individual citizens, liberal economic values increase support for DI.Stronger preferences for equality, in contrast, make opposition to the concept more likely. Similarly, concerns about discriminatory differentiation at the member state level lead citizens to oppose DI.We test the theoretical claims by analysing survey data on citizens' attitudes towards a 'multi-speed Europe'. Supporters of DI, indeed, are marked by liberal economic attitudes. In contrast to general EU support, we do not find robust correlations with socio-demographic variables. Moreover, the data reveal striking differences amongst macro-regions: support for DI has become much lower in Southern European states. We attribute this opposition to negative repercussions of the Eurozone crisis.

The article analyses how enlargements affect the speed of European Union (EU) decision-making. In line with rationalist theories of group choice, we argue that enlargements increase the costs of organizing decisions, i.e. transaction costs. Increasing transaction costs, in turn, slow down EU law-making. We test this theory by estimating Cox regression models that incorporate time-varying covariates on all directives, regulations and decisions submitted by the European Commission between 1976 and 2006. In contrast to previous analyses, we show that an increase in group size indeed slows down EU law-making.