Why do we find large proportions of professional, policy-oriented groups in some interest domains, whereas other domains are dominated by amateur-run, members-oriented club-like groups? In this paper I measure the diversity of interest domains on the basis of the four-fold Streeck-Schmitter distinction of group-types (government, club, movement, firm). I assume that the diversity in a domain is directly related to its density. Based on the population-ecological Energy-Stability-Area (ESA) model, I predict the diversity of a domain to be positively related the 'energy' generated by policy making and its 'area' in terms of public support and economic activity. I do this in four ‘most-different’ systems in order to assess whether these systems mediate the hypothesized effects. In this way, I try to addresses three challenges in studies of interest groups systems: (1) aggregate assessments of groups-systems cannot be validly done based comparative studies of a couple of domains because interest mobilization varies strongly per domain, (2) the diversity of domains includes both the nature of the interest represented (e.g. business or citizens) and the organizational form but only the first is part of rare quantitative assessments, and (3) the explanations for domain-level diversity tend to be circumstantial and case-specific, and have only infrequently modelled using a large-n research design.
