"Although the number of citations does not increase by raising the number of authors, but co-author citations increase by raising the number of co-authors. For example, in clinical medicine the percentage of co-author citation is more than self-citation because of the high number of co-authors and team working in this field. Of course, co-author behavior has no relation with author himself/herself, but continue to have relations with his/her co-authors. So, we propose that co-author citations should not be considered for calculating h-index of highly-cited researchers"
From: Leila Dehghani, Reza Basirian Jahromi, Mazyar Ganjoo. 2011 Citations to highly-cited researchers by their co-authors and their self-citations: How these factors affect highly-cited researchers' h-index in Scopus. Webology, Volume 8, Number 2