We investigate the importance of co-ethnic networks and diversity in determining immigrant inventors’ settlements in the United States by following the location choices of thousands of them across counties during the Age of Mass Migration. To do so, we combine a unique United States Patent and Trademark Office historical patent dataset on immigrants who arrived as adults with Census data and exploit exogenous variation in both immigration flows and diversity induced by former settlements, WWI, and the 1920s Immigration Acts. We find that co-ethnic networks play an important role in attracting immigrant inventors. Yet, we also find that immigrant diversity acts as an
additional significant pull factor. This is mainly due to externalities that foster immigrant inventors’ productivity.