We have discussed elsewhere at great length the advantages and disadvantages of using patents and patent citations to indicate inventions and knowledge links among inventions (Jaffe, Henderson and Trajtenberg, 1993; Trajtenberg, Henderson and Jaffe, 1997; see also Griliches, 1990). Patent citations perform the legal function of delimiting the patent right by identifying previous patents whose technological scope is explicitly placed outside the bounds of the citing patent. Hence the appearance of a citation indicates that the cited patent is, in some sense, a technological antecedent of the citing patent. Patent applicants bear a legal obligation to disclose any knowledge that they might have of relevant prior inventions, and the patent examiner may also add citations not identified by the applicant. In related work, Jaffe, Fogarty and Banks (forthcoming) examined in detail the patents that did and did not contain citations to a specific set of important NASA patents. The conclusion was that, while citations contain much “noise” in the form of apparently spurious implied connections, on the whole they do provide useful information about the generation of future technological impact of a given invention.

The analysis in this paper is based on citations to patents granted between 1963 and 1993. We examine a set of “potentially cited” patents whose primary inventor resided in the U.S., Great Britain, France, Germany or Japan, and which were assigned to corporations. There are a total of about 1.5 million such patents. About 65% of these are from the U.S. (i.e.., their inventors reside in the U.S.), 17% are from Japan, 10% from Germany, 5% from Great Britain, and 4% from France. We then examine all citations made to these corporate patents (whether or not the citing patent is itself corporate) from patents granted in any of these five countries between 1977 and 1994. There are about 1.2 million “citing” patents, and they made a total of about 5.0 million citations to the set of “potentially cited” patents that we are considering.