Reference per year increase with time question.

[Steven A. Morris]

I have noticed that in a lot of specialties, the average number of
references listed per paper increases over time.  In a collection of papers
I have on anthtrax research, for example, the mean number of references per
paper is 12 during the 1940's through the 1970's and increases to about 30
references per paper for the 1980's to the present.

Has anyone studied this effect? Does it mean that authors cite a wider body
of knowledge as the specialty evolves?  Or is it just that social
conventions of citing are changing? Or maybe it's an artifact of trends in
the Science Citation Index journal coverage over time?

Can anyone point me to any papers that discuss this effect?

Thanks,

Steven Morris
Oklahoma State University