The level and causes of party unity are under-researched topics in parliamentary democracies, particularly in comparative perspective. This article presents a non-formal model explaining party unity in legislative voting as the result of individual legislators’ decisions reacting to the incentives and constraints created by their respective institutional environments. Hypotheses derived from the model are tested against empirical data on party unity in 11 western parliamentary democracies since 1945. On the system level, central party control over nominations and intra-parliamentary resources as well as the strength of parliamentary committees with regard to policy decisions are shown to affect party unity as expected by the model. On the level of individual parties, governing parties are less unified than opposition parties and larger parties show higher unity than smaller ones. Both results shed doubt on frequent claims in the literature.
