This paper explores mechanisms explaining legislative voting unity among Canadian political parties from 1867 to 2011. Using all recorded votes from the House of Commons, we find partisan sorting—where elected MPs align with their party's ideological positions—and government agenda control significantly enhance party cohesion over time.

Key Findings: Partisan Sorting and Government Agenda Control are central mechanisms driving voting unity in Canada's Parliament.

Our analysis underscores the importance of integrating institutional rules and legislative agendas into models explaining parliamentary voting behavior. The results highlight how strict party discipline can emerge even within a diverse multi-party system, offering insights relevant to comparative political studies.