  It is well known in bibliometrics that the average number of citations per
paper differs greatly between the various disciplines. The differing citation
culture (in particular the different average number of references per paper and
thereby the different probability of being cited) is widely seen as the cause
of this variation. Based on all Web of Science (WoS) records published in 1990,
1995, 2000, 2005, and 2010 we demonstrate that almost all disciplines show
similar numbers of references in the appendices of their papers. Our results
suggest that the average citation rate is far more influenced by the extent to
which the papers (cited as references) are included in WoS as linked database
records. For example, the comparatively low citation rates in the humanities
are not at all the result of a lower average number of references per paper but
are caused by the low fraction of linked references which refer to papers
published in the core journals covered by WoS.
