Do more distant collaborations have more citation impact?

Internationally co-authored papers are known to have more citation impact than nationally co-authored paper, on average. However, the question of whether there are systematic differences between pairs of collaborating countries in terms of the citation impact of their joint output, has remained unanswered. On the basis of all scientific papers published in 2000 and co-authored by two or more European countries, we show that citation impact increases with the geographical distance between the collaborating counties.