Make up your mind! Opinion formation and polarization in EU referendums

In its early decades, EU integration relied on a permissive consensus among the population, which broke down with the creation of the European Union. Since then, national governments have increasingly felt the need to hold referendums as part of the integration process. As such referendums on EU integration were intended as instruments to strengthen the legitimacy of the EU, we investigate in this paper whether they have changed citizens' outlook on the EU. Because referendums force voters to take a position on an issue they may have previously given little thought to, more people may hold an opinion on the EU after a referendum, thereby further eroding the permissive consensus. Moreover, as more citizens form an opinion for or against EU integration, EU referendums may have led to increased polarization in attitudes towards the EU. Based on time-series cross-sectional analyses of aggregated Eurobarometer survey data from 15 countries covering the period 1972--2012, we show that the share of citizens holding an opinion on the EU increases around an EU referendum. However, this does not necessarily lead to increased polarization. Complementing our aggregate analysis with panel surveys on EU referendums in Norway and Sweden, we show how more and more people move from having no opinion to being for or against the EU over the course of a referendum campaign. Our findings add to the understanding of issue-based polarization and help to explain the increasing politicization of the EU in recent decades. Beyond EU integration, delegating decision-making to citizens may increase the legitimacy of decisions but may also, in certain cases, widen societal divisions over the issues put to a vote.