Bias in the composition of interest communities is often explained by reference to variations in the collective action constraint facing voluntary and nonvoluntary organizations. But with the exception of literature on PAC formation, studies of direct institutional mobilization are rare. More often than not, their mobilization advantages vis-à-vis problems of collective action are simply assumed. This paper fills this gap by testing the collective action hypothesis on direct institutional mobilization. We argue that the PAC studies are flawed as tests of this hypothesis; they study the wrong mode of political activity and use selective samples and limited research designs. We develop a new test using state data on seven types of institutions to solve these problems. We also compare the collective action problem facing institutions to the related problems facing voluntary organizations. We find strong evidence of collective action problems in institutional mobilization, problems that make interest populations of nonvoluntary and voluntary organizations appear far more similar than commonly thought
