Lowery et al (2008, 1245-46) are ‘somewhat pessimistic’ about comparative theory formation on interest representation and see only room for ‘careful, narrow comparison across relatively similar systems’. In this paper I provide an example of how such research may be designed and therewith indicate the conceptual limits and possibilities of comparative theory formation on interest groups. I focus on organizational populations of interest organizations in three ‘most different’ cases (UK, F, D (and NL)). I borrow from various ‘theories of the middle range’ on agenda-setting, political organization and institutions, in order to make the population ecological model of interest organisation comparable across countries. I relate this research strategy to recent progress in, among others, the study of group strategies and influence, and conclude that there is reason for cautious optimism in the comparative study of interest groups.
