Abstract
Abstract Between 1883 and 1919, Andrew Carnegie funded the construction of more than 1,500 public libraries across the United States, reducing the costs of accessing knowledge for millions. We study the effect of these libraries on innovation. Patenting in recipient places increased on average by 10%–12% in the 20 years following library construction relative to a novel control group of cities that applied for but did not build libraries. We show that access to scientific knowledge and increased collaboration opportunities are possible mechanisms.