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We assess the role of ethnic ties in the diffusion of technical knowledge using a database of patents filed by US-resident inventors of foreign origin, identified by name analysis. We consider 10 leading source countries, both Asian and European, of highly skilled migration to the USA and test whether foreign inventors’ patents are disproportionately cited by (i) co-ethnic migrants (‘diaspora’ effect), and (ii) inventors residing in their country of origin (‘brain gain’ effect). We find evidence of the diaspora effect for the Asian but not the European countries, with the exception of Russia. A diaspora effect does not necessarily translate into a brain gain effect, most notably for India where no such effect is detected. Neither does a brain gain effect occur solely in conjunction with a diaspora effect. Overall, diaspora and brain gain effects carry less weight than other channels of knowledge transmission, most notably co …