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. Parliamentary voting is the fundamental mechanism for making collectively binding decisions in modern democracies. Formally, these decisions are made by individual deputies with a free mandate only accountable to their voters, though the reality in parliamentary democracies looks different: usually, the members of parliamentary party groups (PPGs) vote together PPGs are the relevant actors in parliamentary business. 1 Parts of the literature have reacted to this by treating PPGs as unitary actors, with regard to both policy decisions and coalition formation. 2 Party specialists, on the other hand, emphasise the heterogeneity of political parties. 3 Given the diversity of interests within parties, party unity in legislative voting cannot simply be assumed, but needs to be explained. What makes individual deputies vote in line with their parliamentary parties? Even though this question has recently received increased attention, 4 there are still only very few comparative studies on this building block of the theory of parliaments. Ulrich Sieberer is a Lecturer in the Department of Social Sciences at the University of Mannheim. This article developed out of a Master's Thesis at the University of Mannheim. The author is grateful to Peter Graf Kielmansegg and Franz Urban Pappi for their advice and supervision of the thesis. Thanks go to André Kaiser, Bernhard Miller, Wolfgang C. Müller and three anonymous referees for valuable comments and suggestions. Thanks are also due to John Carey, Philip Cowley and Asbjorn Skjaeveland, who shared unpublished data or offered advice on calculating Rice scores.

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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. Parliamentary voting is the fundamental mechanism for making collectively binding decisions in modern democracies. Formally, these decisions are made by individual deputies with a free mandate only accountable to their voters, though the reality in parliamentary democracies looks different: usually, the members of parliamentary party groups (PPGs) vote together PPGs are the relevant actors in parliamentary business. 1 Parts of the literature have reacted to this by treating PPGs as unitary actors, with regard to both policy decisions and coalition formation. 2 Party specialists, on the other hand, emphasise the heterogeneity of political parties. 3 Given the diversity of interests within parties, party unity in legislative voting cannot simply be assumed, but needs to be explained. What makes individual deputies vote in line with their parliamentary parties? Even though this question has recently received increased attention, 4 there are still only very few comparative studies on this building block of the theory of parliaments. Ulrich Sieberer is a Lecturer in the Department of Social Sciences at the University of Mannheim. This article developed out of a Master's Thesis at the University of Mannheim. The author is grateful to Peter Graf Kielmansegg and Franz Urban Pappi for their advice and supervision of the thesis. Thanks go to André Kaiser, Bernhard Miller, Wolfgang C. Müller and three anonymous referees for valuable comments and suggestions. Thanks are also due to John Carey, Philip Cowley and Asbjorn Skjaeveland, who shared unpublished data or offered advice on calculating Rice scores.

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