Existing research on political parties in the 19th century, mainly based on the experience of Anglo-Saxon countries, indicates that the development of unified legislative parties was an incremental and drawn-out process. Based on a comprehensive and novel dataset of roll call votes in the Reichstag of Imperial Germany, this study demonstrates that the German development was markedly different. From the outset, legislative party cohesion was high and comparable to party cohesion in current-day legislatures in similar types of separation-of-powers systems. Combining statistical and qualitative analyses of party group formation and legislative voting in the Reichstag, the study examines possible mechanisms that generated and sustained this high level of cohesion. The results indicate that strong ideational commitments of politicians played an important role for their creation of and self-selection into narrowly defined groups, which in turn resulted in a relatively fractionalised system but cohesively acting parties.
