Codecision and Institutional Change

This article examines the sources and processes of institutional change in one important aspect of EU politics – the legislative procedure of codecision – and shows how interstitial change of institutions that emerges between formal Treaty revisions and under specific conditions may be formalised in subsequent formal Treaty reforms. We develop two related models of Treaty change. First, in a ‘simple’ model, informal rules will be formalised in the Treaty text where all member states are in agreement, and will be rolled back when all member states oppose them; otherwise they will continue in existence at the informal level. Second, in a more complex framework, actors that have effective veto powers in a related arena may make credible threats that allow them to press member states into formalising informal rules, provided that member states are not unanimously opposed to this formalisation. We empirically assess our claims in the light of several instances of informal rules applied in the codecision procedure