Discontinuity of Output Convergence within the United States: Why Has the Course Changed?

Has the progress of output convergence changed within the United States? This article examines the output convergence among U.S. states for the last five decades by making several improvements over the extant literature. By applying a battery of convergence tests designed to capture nonlinear transitional dynamics to real output per worker data (i.e., nominal values deflated by state-level price), we find that output convergence has not been a feature of the continental United States since the 1970s. Instead, output convergence has proceeded among four subgroups within which constituent states have certain characteristics in common. Our regression analysis suggests that state-level characteristics related to technology and human capital play a crucial role in accounting for the formation and composition of convergence clubs, in agreement with the recent theoretical models of growth and development (e.g., Aghion et al. 2009; Gennaioli et al. 2013b). The level of technology, proxied by patents, turns out to be a consistently significant determinant even after controlling for endogeneity, suggesting that frictions in the diffusion of technology and human capital may have led to clustering of states with different levels of productivity. Our results therefore cast doubt on the common view that diffusion of knowledge and technology across state borders is frictionless. (JEL O47, O51)