Industrial agglomeration policies may limit competition. This study develops, validates, and applies a novel approach for measuring competition based on the comovement of markups and market shares among firms in the same location and industry. Then this study develops a model of how this reduction in competition affects aggregate income. In this paper, the authors apply their approach to the well-known special economic zones (SEZs) of China. This paper estimates that firms in SEZs exhibit cooperative pricing almost three times as intensively as firms outside SEZs.