Special interest groups exert a great deal of influence over political outcomes in the United States. Understanding the determining factors for the formation of special interest groups is important. However, the literature has excluded the role of spatial neighbors. This article employs
spatial econometric techniques to discriminately analyze the factors determining the number of special interest groups in a state. While geographic location is not a factor, gross state product, state general expenditures, union membership, and the percentage of manufacturing employment relationships
between states are crucial in the formation of special interest groups across states.