Innovation Spillovers across U.S. Tech Clusters

The vast majority of U.S. inventors work for firms that also have inventors and plants in other tech clusters. Using merged USPTO–U.S. Census Bureau plant-level data, we show that larger tech clusters not only make local inventors more productive but also raise the productivity of inventors and plants in other clusters, which are connected to the focal cluster through their parent firms' networks of innovating plants. Cross-cluster innovation spillovers do not depend on the physical distance between clusters, and plants cite disproportionately more patents from other firms in connected clusters, across large physical distances. To rationalize these findings, and to inform policy, we develop a tractable model of spatial innovation that features both within- and cross-cluster innovation spillovers. Based on our model, we derive a sufficient statistic for the wedge between the social and private returns to innovation in a given location. Taking the model to the data, we rank all U.S. tech clusters according to this wedge. While larger tech clusters exhibit a greater social-private innovation wedge, this is not because of local knowledge spillovers, but because they are well-connected to other clusters through firms' networks of innovating plants. In counterfactual exercises, we show that an increase in the interconnectedness of U.S. tech clusters raises the social-private innovation wedge in (almost) all locations, but especially in tech clusters that are large and well-connected to other clusters.