Meghan has a fun old post asking what is, or will be, your “old school science cred“. The scientific thing you’ve done, or will do, that will one day cause future grad students to look at you and think “Jeez, you’re old.”
Here’s a way to get “old school science cred” that didn’t make it into that post: cite a “personal communication” from someone. Citing “personal communication” was a thing back in the day–but not so much any more.
Here’s a table of the number of papers in Ecology that include at least one instance of the phrase “personal communication”, in 5-year chunks (sorry, too lazy to make a graph):
1981-1985: 403 papers in Ecology cite at least one “personal communication”
1986-1990: 387
1991-1995: 345
1996-2000: 333
2001-2005: 295
2006-2010: 231
2011-2015: 146
2016-2020: 145
Notice that the number of Ecology papers containing the phrase “personal communication” has dropped by well over 50% since the early ’80s, even though the number of papers published by Ecology has increased a lot since the early ’80s. So the percentage of Ecology papers citing “personal communication” has cratered, just within the professional careers of active senior ecological researchers.
I highly doubt this trend is specific to Ecology, I’m sure you’d get similar results if you looked at Am Nat or JAE or Oikos or whatever.
I can think of a few not-mutually-exclusive hypotheses to explain this. Ecology journals publish less natural historical work than they used to (though there’s been a bit of a rebound in recent years). I feel like natural historical papers are particularly likely to cite personal communications for natural historical information that is stored in the heads of natural historians rather than written down in a citable source. Maybe there’s also just a growing expectation on the part of reviewers, editors, and graduate advisors that authors will cite written sources whenever possible. Perhaps because written sources are thought to be easier for others to check for themselves. And maybe there’s a feedback loop–people stop citing “personal communication” because they don’t see anyone else doing it.
I don’t know that this is all that important in itself. I don’t think the decline in “personal communication citations” is either a problem to be solved or a victory to be celebrated. Times change, and one little symptom of changing times is that ecologists mostly don’t cite “personal communication” anymore.
Dynamic Ecology 
Interesting! I treat four kinds of “weird citations” in “The Scientist’s Guide to Writing”: pers. comm., pers. obs., unpubl. MS, and data not shown. I suspect that unpubl. MS is in decline due to preprints, and “data not shown” due to online supplements. Pers. obs. has always struck me as kind of silly (why would you take my word for it if I say “Pers. obs.” but not if I don’t?). And now pers. comm. is in decline? Maybe by the 4th edition or so I can drop that whole section!
Yeah, I bet “data not shown” has cratered. Though I’m going to try to get “not shown” into my next paper for a statistical analysis. I’m already doing the analysis two different ways and showing that they lead to the same conclusion. I have in fact checked three other ways of doing it, and they all lead to the same conclusion as well, but showing them to readers feels like overkill. So my plan is to not show them and ask readers to trust me. Or, maybe I’ll say “not shown” but also include the R code to run those other three analyses in the Dryad file.
That alternative-analysis thing is the case where I’ve used “data not shown” and I’m not even convinced that it’s worth shoving the alternatives in an online supplement. Perhaps posting the R code in Dryad is a nice intermediate compromise.
Yeah, basically nobody reads the online supplements. Even reviewers often don’t read them carefully.
I should do a post on this, because this is something on which practices vary across scholarly fields. Economists for instance are all about including a bunch of different analyses of the same data, that all lead to the same conclusion. They call them “alternative specifications” or “robustness checks”. They’re included in the main text of the paper. But I’ve also seen economists complain about the practice. Arguing that it makes papers difficult/boring to read, and that it gives a false sense of security when the data itself has some serious flaw or limitation.
Maybe we should do a poll on this: do you, as a reader or reviewer, want authors to try to anticipate and address your questions by doing alternative analyses? If so, where do you want to see those alternative analyses: in the main text, in an online supplement, or in the R code?
Now I’m curious about the age distribution of authors citing “personal communication”. Is it only senior people who cite “personal communication” these days?
Just one data point, but I’ve cited “personal communication” maybe a handful of times. I published my first paper in 2013, so definitely not senior.
At least so far as I recall at the moment, I’ve used it mainly in regard to management-related things that aren’t well documented in the literature, but where I have a sense that some appeal to authority helps show I’m not just making things up.
I sometimes cite “pers. comm.” or “pers. obs.” when describing study sites – for example, regarding communication by the staff or managers of a protected area or my own personal observation of a site where I have worked more extensively. I published my first paper em 2011. 🙂
Could be interesting to compare these data to the frequency of “personal communication” citations in journals that publish more natural history than Ecology does. American Midland Naturalist, entomology journals, ornithology journals, etc. That would speak to whether the decline in “personal communication” citations is due to a decline in natural history content in Ecology. As opposed to due to changing citation practices that span both more and less natural historical areas of biology.
Compelling post, Jeremy! As an ole codger, I’ve drifted away from the “personal communication” thingy because it’s become so easy to pinpoint citation(s) where the same thing has been said. Usually, I’ll just copy & paste the “personal communication” into a search engine, and then I’ll find several papers saying essentially the same thing.
One of my recent papers–I think the one in PLOS Computational Biology–attempted to say “data not shown” and this was shot down by the editorial staff. I wonder if some of the decline in “personal communication” is because the journal refuses to take it.
(I am embarrassed to say, the journal was right not to take it, because when I had to show it I discovered a mistake. Oh well–better in review than after publication.)