While it is known that different types of interest groups emerged at different points in time in the history of the United States (at least in large numbers), it is less clear why this was so. Why, for instance, did trade and professional associations (representing businesses or individuals based on their professions) emerge decades earlier than most public interest or citizen groups (where memberships are organized around political passions)? Why did most labor unions emerge even earlier than the associations? More precisely, what social and economic conditions in the United States stimulated the emergence of these different kinds of organizations? Were the different forms of interest articulation and aggregation (professional memberships versus passion-driven memberships) reactions to different kinds of stresses in society? In this paper I use the birth years of interest groups active in Washington, DC and the American states in 2011, drawn from the well-known directory Washington Representatives, to chart rates of organizational formation over the entire history of the United States. Using data on changes in national and state populations, state and national GDP, and other economic and social variables capturing eras of history, I analyze trends in interest group rates over nearly two hundred years.
