Party Group Cohesion in the European Parliament: Tracing the Bias in Roll-Call Votes

Abstract

To understand legislative behavior and party politics in the European Parliament (EP), scholars to date have heavily relied on roll-call vote (RCV) data. However, only a subset of all EP votes is taken by roll call. Furthermore, it has been argued that EP party groups may be requesting RCVs for strategic reasons. This can lead to a selection bias in the data and, consequently, wrong findings such as overestimated party group cohesion. Researchers have put forward alternative hypotheses about how voting cohesion might differ between recorded and non-recorded votes as a result of party signalling or disciplining (Carrubba 2006, Carrubba 2008, Thiem 2006, Hug 2009). In this paper, we test these partially conflicting hypotheses and find evidence for an alternative hypothesis, namely that roll-calling simply takes place on more important votes. We rely on new data and draw on the fact that since the EP rule revision in June 2009 all final legislative votes are automatically taken by roll-call. Thus, the selection bias that might have affected former analyses disappeared for final legislative votes. We compare pre and post 2009 voting records, which enables us to trace the revealed level of party group cohesion. The analysis shows that the relative cohesion of party groups on final legislative votes has on average increased after the EP rule revision. This indicates that relying on RCVs can lead to underestimating rather than overestimating group cohesion - a finding that contradicts previous hypotheses.