When I read very old, classic papers in biology, I am struck by how few sources they cite. Watson and Crick, 1953 cites 6 sources. Luria and Delbrück, 1943 cites 9. Sanger, 1977 cites 14.

This is in contrast to contemporary papers, which often have a whole page, in very small type, listing the sources.

What is going on? Am I correct in concluding that over the past century, the number of sources referenced by each paper has increased? What does this mean? Is biology (and perhaps other sciences) simply maturing and becoming more collaborative? Has the availability of computers and the internet made it easier to find more sources? Are standards higher nowadays about grounding your work in literature? Is it just that stuffing your bibliography with copy/pasted sources has become fashionable?

  • 'stuffing your bibliography with copy/pasted sources has become ------' dangerously easier! Tracking references in the library used to be a drag.
    – chris
    Dec 30, 2014 at 15:15

5 Answers 5


jakebeal has pointed at the more technical reasons for the increase of references. We should, however, not forget some of the more subtle, but perhaps also, more fundamental changes that has happened.

First, there is a gradual change in how science is communicated. Scientific papers have developed from letters that were read aloud in front of scientific societies and also published as personal letters. The scientific debate was more closely akin to debates between persons making observations. Obviously this was possible for the reason that there were very few involved in any one research question. So, in part the development seen is due to a development of publishing driven by changes in the form and volume of debate.

Second, research questions have become more complicated involving larger and larger groups of scientists with varying expertise. This increase in complexity also means references are no longer required to cover just a specific question but also information from adjacent or supporting fields.

Third, science is disseminated in smaller parts today that what was the case back in time. This is partly out of necessity related to the second point above. Additionally, and this is perhaps not the greatest aspect of developments, there is the pressure to publish due, mostly, to the fact that academic careers are measured in terms of number of publications and number of citations. The number of publications has thus increased for several reasons and hence also the number of somewhat relevant papers to cite. I will not get into bad behaviour such as self-citations here but it is clear that any system will have flaws and some people will make use of such flaws to benefit themselves.

So at least some of the change in number of publications is due to developments in the way we perform and communicate science and also changing pressure from the academic world on researchers to publish and be cited.

  • If science is disseminated in smaller parts, shouldn't this reduce the number of citations? Surely a tiny discovery would have less background work to reference than a large one?
    – Superbest
    Dec 30, 2014 at 16:31

Contemporary publications definitely cite more sources. There are likely quite a number of different forces in play, and I think that you have touched on a number of them in your question. In my experience and opinion, however, much of it can be derived from the basic purpose of citations: to acknowledge the context and foundations of a piece of work. Over time the "density" of context has increased in at least the following ways:

  • The number of active researchers has massively and progressively increased over the past century. This means that for any given area of interest, there are likely to be more people doing work that pertains to that area.

  • Frequency of publication has been increasing for a number of reasons, including the continually increasing ease of manuscript preparation and (more recently) the increasing use of publication metrics in evaluating researchers.

  • Improvements in information systems mean that it has become progressively easier to become aware of and obtain copies of the publications of others, such that there is less "excuse" for not citing a relevant publication.

Each of these increases the amount of information that can reasonably be considered relevant context and that a researcher is expected to be responsible for knowing about, and thus naturally the number of citations.


I tend to motivate this mainly with two facts:

  • As science advances and topics become more complex, research papers become more specific, so that one may need to put together many "little" pieces in order to deduce and/or motivate new insights.

  • Thanks to the internet and more advanced channels of communication, it is nowadays easier to discover and exploit the work of other researchers

Luckily, we have the second point! Otherwise we would be lost ;-)

I like to think of this as a world-sized brain which keeps on growing, where every researcher plays the role of a single neuron, and the network between them makes it possible to expand our knowledge.


Because as time goes on, more and more research is performed, and knowledge is accumulated and published. Early on, the literature is not abundant. Also, it is easier to identify and locate relevant literature now.


Although other answers have already mentioned this as one factor, I believe that the main driver for increased citations is simply the fact that it is much, much easier to rapidly and exhaustively search academic literature now than in the 1940s or 1950s. (In economic parlance, the "search costs" have decreased.) Academics who are operating now can easily use online search resources like JSTOR or Google Scholar to find papers on a topic of interest, and it is extremely simple to identify large numbers of related papers very rapidly. In my experience, I can begin research on a topic where I have no previous knowledge of the literature, and within a few hours I can comfortably identify twenty or thirty relevant papers.

If one compares this to the difficulties of research through libraries in the 1940s and 1950s, the difference is quite staggering. In those days, even finding one paper would have required travelling to the library (no sitting on your computer in the office for you!), searching printed index-cards, and finding physical copies of the paper in the stacks. It could also require journals to be physically transported between libraries before being available to you. Obscure works might be present in only a small number of libraries, and it would be difficult to be alerted to their existence at all. While it is certainly true that there are other factors at play (e.g., the accumulation of more work over time), my suspicion is that most of this is down to diminished search costs.

  • 1
    Indeed, it is easy to forget what searching the literature was like. I spent hours and days in the library when I was in graduate school, seeking all possible papers relevant to my work. Twenty-five years later, I revisited one of the topics in my dissertation. It took about five minutes on the internet to find several papers I had missed in my pre-internet search. May 7, 2020 at 17:05

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