This paper analyzes the historical development of party voting unity in Canada, Australia, the Union of South Africa and New Zealand. Our dataset includes the complete legislative voting records of all legislators who sat in these legislatures before 1930. In order to analyze the development of party voting unity, we propose to simultaneously estimate the influence of individual member characteristics and the legislative agenda on the voting behaviour of members. Preliminary findings suggest that cohort and replacement effects, as well as some aspects of the legislative agenda, explain why party unity has increased over time in these legislatures. Ultimately, the goal of this research is to evaluate different theories of party development in Westminster-style parliamentary systems.
