Mobility of inventors and the geography of knowledge spillovers. New evidence on US data

In this paper we exploit new data on US inventors in Organic Chemistry, Pharmaceuticals, and Biotechnology to revisit the JTH test of the localization of knowledge spillovers (Jaffe, Trajtenberg, and Henderson; 1993). We find that inventors who patent across different companies contribute extensively to the observed citation patterns, both directly (through personal self-citations) and indirectly, by linking the various companies via a social network conducive to more citations. To the extent that the geographical mobility of these “cross-firm” inventors is quite limited, the resulting social networks and citations patterns are found to be bounded in space. We conclude that spatial distance, as measured in the JTH experiment, is just a proxy for a much more important variable, such as social distance between inventors. In a similar vein, we show that technological distance, introduced by Thompson and Fox-Kean (2005) to question the soundness of the JTH experiment, is also a proxy of social distance.