Patent Citations and the Geography of Knowledge Spillovers: A Reassessment

Jaffe, Trajtenberg and Henderson (Quarterly Journal of Economics, 108(3):577-98, 1993) developed a matching method to study the geography of knowledge spillovers using patent citations, and found that knowledge spillovers are strongly localized. Their method matches each citing patent to a non-citing patent intended to control for the pre-existing geographic concentration of production. We show how the method of selecting the control group may induce spurious evidence of localized spillovers. This paper reassesses their findings using control patents selected under different criteria. Doing so eliminates evidence of strong intranational localization effects at the state and metropolitan levels, but leaves largely unaffected evidence of international localization effects.