Also noticeable in the cumulative probabilities is that the U.S. tends to both make and receive more citations than other countries. Note, for example that the entry in the U.S. column contains the largest figure other than the diagonal in every row, and the U.S. row contains the largest figure other than the diagonal in every column except Germany. This result could be driven by differences between the U.S. and other countries in the propensity to patent. If the U.S. has a low propensity to patent, then each patent granted represents (on average) a larger chunk of knowledge, which could result in more citations made and received per patent (Cabellero and Jaffe, 1993).

It is more likely, however, that the propensity of U.S. inventors to patent in the U.S. is greater than that of foreigners (Eaton and Kortum, 1996). That is, U.S. inventors are more likely to take out a U.S. patent on a trivial invention than are foreigners. All else equal, this should make the average citation rate to and from the U.S.-invented patents lower than the corresponding rates for foreign-invented patents. Since we find the opposite, this may be evidence confirming a view of the U.S. as the most open and interconnected economic and technological system.

There are also some interesting pairwise effects. The U.S. and Great Britain are “closer” to each other (in terms of overall probability of citation) than any other country pair, suggesting a possible effect of common language. Japan is closer to the U.S. than it is to any of the European countries. Britain and France are closer to each other than to Germany, but closer to Germany than to Japan. Note, however, that not all of these differences are statistically significant.

Figures 2 through 6 show the effects of these parameter differences graphically, and also present a useful pictorial comparison to the raw data presented in Figure 1. Each Figure presents the estimated citation functions for citations to one of the countries, with the different lines within each Figure corresponding to the different citing countries.