Abstract
This paper explores the patterns of citations among patents taken out by inventors in the U.S., the U.K., France, Germany and Japan. The authors find (1) patents assigned to the same firm are more likely to cite each other, and come sooner than other citations; (2) patents in the same patent class are approximately 100 times as likely to cite each other as patents from different patent classes there is not a strong time pattern to this effect; (3) patents whose inventors reside in the same country are typically 30 to 80% more likely to cite each other than inventors from other countries, and these citations come sooner; and (4) there are clear country-specific citation tendencies; e.g., Japanese citations typically come sooner than those of other countries.