Localized Knowledge Spillovers : Evidence from the Spatial Clustering of R&D Labs and Patent Citations

Patent citations are a commonly used indicator of knowledge spillovers among inventors, while clusters of research and development labs are locations in which knowledge spillovers are particularly likely to occur. In this paper, we assign patents and citations to newly defined clusters of American R&D labs to capture the geographic extent of knowledge spillovers. Our tests show that the localization of knowledge spillovers, as measured via patent citations, is strongest at small spatial scales and diminishes rapidly with distance. On average, patents within a cluster are about three to six times more likely to cite an inventor in the same cluster than one in a control group. At the same time, the strength of knowledge spillovers varies widely between clusters. The results are robust to the specification of patent technological categories, the method of citation matching and alternate cluster definitions