To use the whip or not? Whether and when party group leaders use disciplinary measures to achieve voting unity

Party group leaders seemingly use a variety of disciplinary measures to achieve unified voting behavior in party groups. However, there is little systematic knowledge about the nature of such disciplinary measures and the interaction between leaders and group members remains a black box. Most studies of party voting concentrate on institutional variables such as electoral systems and take disciplinary measures as a given. This study presents a new way to measure party discipline, based on data from semi-structured interviews with 76 party group leaders and experts from five European parliaments. The discipline index provided here is the first to consider in a systematic way the means available to reprimand or reward party group members. In a second step, the study shows that disciplinary measures can compensate for a leader’s lack of control over candidate selection processes, and relate to group composition or size.