It is widely debated in studies of international negotiations why certain negotiators are more successful than others. Institutionalist and rationalist approaches claim that institutions and negotiators’ resources largely explain the outcome of negotiations, whereas constructivist approaches stress the importance of shared norms and values. The article asks to what extent the use of normative arguments explains negotiation success in EU treaty negotiations. We apply our approach to the negotiations leading to the Treaty of Amsterdam. We first define normative arguments as justifications for positions that refer to common norms and develop a concept of common values for EU constitutional negotiations. Second, we assess to what degree governments justify their positions by normative arguments using an automated analysis of position papers. Finally, we ask if such justifications increase success in negotiations. The results of our statistical models show that arguing affects negotiation success significantly and positively.

Types of flexibility – Tension: demands for diversity v. logic of uniformity – United Kingdom and differentiation under Maastricht and Amsterdam – Court of Justice (77/05 and 137/05) on Articles 4 and 5 Schengen Protocol: a clear signal disapproving UK's ‘pick and choose’ approach – Lisbon Protocols: unparalleled possibilities for differentiation coupled with coercive procedural constraints and threats of ‘expulsion’.