From causes to consequences: Investigating the effects of differentiated integration on citizens’ EU support

Research on differentiated integration has paid considerable attention to its causes. However, we know very little about its consequences. Using the synthetic control method and interactive factor models, this article investigates the effects of differentiated integration on citizens’ support for the EU. We find that in cases where member states are granted an opt-out or are allowed to integrate into a policy area they were previously excluded from, support increases. In contrast, support decreases when member states are not granted a requested opt-out or are excluded from a policy area they would like to join. These findings carry important implications for the EU's legitimacy. While differentiated integration has the potential to enhance citizens’ legitimacy perceptions, it can also undermine them simultaneously.