  The importance of collaboration in research is widely accepted, as is the
fact that articles with more authors tend to be more cited. Nevertheless,
although previous studies have investigated whether the apparent advantage of
collaboration varies by country, discipline, and number of co-authors, this
study introduces a more fine-grained method to identify differences: the
geometric Mean Normalized Citation Score (gMNCS). Based on comparisons between
disciplines, years and countries for two million journal articles, the average
citation impact of articles increases with the number of authors, even when
international collaboration is excluded. This apparent advantage of
collaboration varies substantially by discipline and country and changes a
little over time. Against the trend, however, in Russia solo articles have more
impact. Across the four broad disciplines examined, collaboration had by far
the strongest association with impact in the arts and humanities. Although
international comparisons are limited by the availability of systematic data
for author country affiliations, the new indicator is the most precise yet and
can give statistical evidence rather than estimates.
