Utilizing data from a survey of 60 national public interest group founders, this paper addresses two related questions: (a) What factors explain the (relatively) recent proliferation of public interest groups? (b) What factors determine in which policy areas public interest group activity is most likely? The results of the data analysis suggest that several factors have contributed to group proliferation. Among the most important are law group start‐up costs, the spread of affluence and education, an increase in patron activity, and rapid societal change. The results also suggest that public interest group activity is most likely in issue areas of interest to well‐educated, affluent whites. Ultimately, however, the data suggest there is bound to be a great deal of uncertainty in the public interest group universe. Because group start‐up costs are so low and the entrepreneurial pod is so large and varied, virtually any type of public interest group dealing with any issue could form at any