They Don't Invent Them Like They Used To: An Examination of Energy Patent Citations Over Time

This paper uses patent citation data to study flows of knowledge across time and across institutions in the field of energy research. Popp (2002) finds the level of energy-saving R&D depends not only on energy prices, but also on the quality of the accumulated knowledge available to inventors. Patent citations are used to represent this quality. This paper explores the pattern of citations in these fields more carefully. I find evidence for diminishing returns to research inputs, both across time and within a given year. To check whether government R&D can help alleviate potential diminishing returns, I pay special attention to citations to government patents. Government patents filed in or after 1981 are more likely to be cited. More importantly, descendants of these government patents are 30 percent more likely to be cited by subsequent patents. Earlier government research was more applied in nature and is not cited more frequently.