When Do Applicants Search for Prior Art?

There is concern that patent examiners lack the resources, capabilities, and incentives to properly identify the prior art against which patent applications are evaluated and that, as a result, they issue a large number of low-quality patents. In this context, the extent to which applicants have incentives to contribute prior art is an important question. This paper uses data on examiner and applicant citations in U.S. patents to examine this question. The data show that applicants contribute a surprisingly low share of citations to previous patents and routinely fail to identify even their own previous patents. However, there are also stark differences across fields. Within fields, and even within firms, there is self-sorting: applicants contribute more prior art for their more important inventions. The results suggest that incentives to search for prior art vary across industries and inventions, which reflects underlying differences in the strategic reasons for obtaining patent protection.