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This article illustrates the contribution of mobile inventors and networks of inventors to the diffusion of knowledge across firms and within cities or states. It is based upon an original data set on US inventors' patent applications at the European Patent Office, in the fields of drugs, biotechnology and organic chemistry. The study combines the methodology originally proposed by Jaffe et al. (1993, Quarterly Journal of Economics, 108: 577-598) with tools from social network analysis, in order to evaluate extent of the localization of knowledge flows, as measured by patent citations. After controlling for inventors' mobility and for the resulting co-invention network, the residual effect of spatial proximity on knowledge diffusion is found to be greatly reduced. We argue that the most fundamental reason why geography matters in constraining the diffusion of knowledge is that mobile researchers are not likely to relocate in space, so that their co-invention network is also localized. In the light of these results, we revisit common interpretations of localized knowledge flows as externalities.

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