The literature on party group switching in the European Parliament contends that members re-affiliate primarily for strategic reasons. This article advances the discussion by also considering the occurrence of non-strategic switches which follow the collapse of weakly institutionalized groups. Using an original dataset which includes DW-Nominate scores (1979–2009), I operationalize policy-seeking behavior among strategic switchers by deriving member- and delegation-to-group policy distance variables. The pooled logistic regression models using a penalized maximum likelihood estimator make it possible to address quasicomplete separation, and the results show that members from large groups and delegations have significantly lower odds of switching. Further, as members or delegations become incongruent with their group, the odds of switching increase. The study has important implications for research investigating the relationship between weak party institutionalization and parliamentary behavior.

The literature on party group switching in the European Parliament claims that members who are ideological outliers have the highest odds of changing their group label, but is it true that the most incongruent legislators are also the most successful at switching groups? This article seeks to determine whether or not voicing dissatisfaction by casting votes against the party line is associated with an increased probability of party group switching. Using logistic regression with a penalized maximum likelihood estimator, the analysis of ambitious switching (1979–2009) uses loyalty and policy distance variables to show that moderate, not extreme, outliers have the highest odds of exiting their group. These findings revise what we know about the relationship between legislative voting behavior and party switching, and they have important implications for examining the effect of policy-seeking on party switching in national parliaments.