Note: Columns are the countries from which citation donors are based; rows are the countries which receive citations. The 1000 articles considered received 3393 citations.
We found that North Americans cite North Americans more often (54%, n = 1,294) and Europeans cite more European papers (59%, n = 596) (x2 = 314, d.f. = 1, p < 0.001). Both Americans and Europeans receive citations from the rest of the world less often than from their own continent (x2 = 314, d.f. = 2, p < 0.001).
We also explored numbers of self-citations. From the total of 3,393 citations that our 1,000 records received, 691 (20%) were self-citations. We found no difference between the self-citation practices of the Americans (20% of their citations were self-citations) and the Europeans (21% were self-citations) (x2 = 0.01, d.f. = 1, p = 0.919).
In addition, we investigated how many records of the 3,393 publications were international research collaborations and found that 22% (n = 746) were collaborations between two or more countries.
The above results are similar to those between US and Europe (Table 8). It seems that considering North America (US and Canada) rather than US alone does not materially alter our conclusions.