(9 months, no blog entry…)

This week, I read two articles on very different aspects of citation patterns. The first one was an analysis, by Stuart Cantrill over at the Nature Chemistry blog, of the journal's impact factor citation distribution. The second was a very enlightening Science paper from 2012 on Coercive Citation in Academic Publishing, and it got me wondering: I have never, so far, experienced coercive citation from an editor (i.e. instructions from an editor asking to add more citations to their own journal). However, I have been in the past — several times — advised by wise and knowledgeable (read: older) colleagues to make sure I was including same-journal references before submitting a manuscript. Usually it comes with some logic behind it, like “it makes the editor less likely to judge the paper out of scope if they see a lot of citations from their own journal”. But although that logic could apply to specialized journals, it makes no sense for more generic journals…

So, I wanted to look at the citation patterns among the “top three” general audience chemistry journals, namely Nature Chemistry, JACS and Angewandte Chemie. I took a sample of 2000 papers each from the 2010–2014 period (only 1200 for Nature Chem, which publishes fewer papers), and looked at the distribution of references from those papers. Let's first look at the most-frequently cited journals for each source:

Most-frequently cited journals

First, we see that in all three journals, JACS and Angewandte are the most cited journals (in this order). This makes sense: they really are the top general journals in chemistry and publish a lot of papers every year (much more so than Nature Chem, this difference in volume of publications explaining the much lower spot of the later). After that, you can begin to spot differences in the journals cited: Nature Chem features more heavily interdisciplinary journals Science, Nature, and PNAS. On the other hand, Angewandte (and JACS to a smaller extent) clearly features more citations to subfield-specific journals, and in particular organic chemistry journals. This would reflect a heavier focus of Angewandte on organic chemistry and synthesis, something that is regularly mentioned in chemistry circles (mostly by people outside molecular chemistry)! On the other hand, the only subfield-specific journal to make it into Nature Chem’s top 10 is a physical chemistry journal, the venerable Journal of Chemical Physics (which is one of my personal favorites).

Next, let's focus on citations between the three journals themselves:

Same-journal citations

For column X (source) and row Y (citation target), the table tells you what's the percentage of citations to Y in X. For example, 12.1% of the references found in Angewandte are to articles in Angewandte, while 14.8% are to papers in JACS. What is interesting (to me) is to look at same-journal citation, i.e. whether papers in journal X are more likely to be cited in this same journal than in other journals. And… it is the case, by about 2% to 3% in each case. So there is a small but significant excess of same-journal citation. I can see three explanations possible:

  1. One explanation may be that these journals have different audiences, and therefore it is natural that they feature more self-citation than other journals. I think this is definitely not true in terms of the subfields of chemistry, since all three are general chemistry journals with broad readership.
  2. There might be a geographical effect, with more US authors in JACS and more German authors in Angewandte… but are you really more likely to cite your (geographical) neighbor rather than the work of chemists from another continent? I do not think this can account for the differences observed.
  3. The final reason is that there may still exist, consciously or unconsciously, a tendency to include (or favor) same-journal references when writing a manuscript for a specific journal.

Let me know in the comments or on Twitter what you think! There surely are other possible reasons I have failed to see…

PS: on the topic of geographical diversity of these journals, you can go back and see my earlier post on the globalization of chemistry as seen through publications in the field…