Should a subset of member states of a federation be allowed to form a sub-union on some policy issue? When centralization is not politically feasible, allowing an enhanced cooperation agreement among a subset of countries permits the latter to gain benefits which would otherwise be lost....

This contribution develops a theory on the impact of differentiation on integration and unity among EU member states and discusses empirical evidence from four policy areas. According to the theory, the centripetal effects of closer cooperation among willing EU members on initially unwilling...

Around the world many countries are about to break apart, stay together, join in federations or build supranational organisations. The political debate of the European constitution is embedded in this context. The question arises which policies have to be shifted to the central European level...