As an Associate Editor of over a dozen journals, I have seen an increase in the number of submitted articles with multiple authors.
In a recent issue of The Chronicle of Higher Education, an article by Paul Voosen had the following interesting table from Thomson Reuters Web of Science:
In the past 30 years, as the above table reveals, there has been a precipitous drop in the number of single-authored papers. In Computer Science, for example, only 9% of the 2012 scientific articles were single-authored, whereas, in Mathematics, the percentage of single-authored papers in 2012 was 31% with the Social Sciences having the highest percentage of 38%.
A paper by Ding, Levin, Stephan, and Winkler, published recently in Management Science, had some fascinating facts: A number of studies have identified a significant increase in the number of coauthored papers by individuals at different academic institutions and in different countries, as well as in the number of coauthors per paper. An analysis of approximately 13 million published papers in science and engineering from 1955 to 2000, for example, found an increase in team size in all but one of the 172 subfields studied, and average team size was found to have nearly doubled, going from 1.9 to 3.5 authors per paper (Wuchty et al. 2007). Adams et al. (2005) found similar results for the top-110 research universities in the United States, reporting that the average number of authors per paper in the sciences grew by 53.4%, rising from 2.77 to 4.24 over the period 1981–1999.
Growth in the number of authors on a paper is due not only to a rise in collaboration within a
university—and an increase in lab size—but more importantly to an increase in the number of institutions collaborating on a research project. A study of 662 U.S. institutions that had received National Science Foundation (NSF) funding one or more times found that collaboration across these institutions in science and engineering, which was rare in 1975, grew in each and every year between 1975 and 2005, reaching approximately 40% by 2005 (Jones et al. 2008). Collaboration has increased internationally as well. The Levin et al. (2009) study of authorship patterns across a wide array of four-year colleges and universities in the United States found that the percentage of papers with one or more international authors went from 6.6% in 1991 to 19.2% in 2007.
I recall my dissertation advisor at Brown University, Professor Stella Dafermos. Her many papers published mostly in the 1970s and 1980s were mainly single-authored except for earlier ones with her dissertation advisor and several with her doctoral students. I recall also being told that it was important to have single-authored papers before promotion to Associate Professor and tenure. Luckily, I achieved both at UMass Amherst just 4 years after getting my PhD at Brown.
These days, interestingly, several of my former doctoral students were urged (and are being urged) to have one or more single-authored papers before they come up for promotion and tenure whereas I know of folks who are up for tenure that have never published a single-authored paper (all these cases I am mentioning have PhDs in the Management Science/Operations Research/Industrial Engineering/Operations Management areas and are in business schools). Some are also told that papers with their advisor do not count "as much." Of course, there is also the pressure to publish in "premier" and "A" list journals.
I continue to write single-authored papers but very much enjoy collaborations and working also with doctoral students (present and former) as well as with other colleagues across the globe.
And these days, with a very tight deadline -- of tomorrow -- we are trying to finish a paper with 7 co-authors (based on our NSF project).
Thanks to Marc Abrahams in his, Scientists Find Safety in Numbers, for noting that: "If there were a prize for the largest number of co-authors, it would
have gone to the 2,512 people credited with writing "Precision
Electroweak Measurements on the Z Resonance," which appeared in the
journal Physics Reports in 2006." He also reminds us that: "at least one prize has been awarded for the highest number
of co-authors. In 2003, the Ig Nobel prize for literature went to the
approximately 976 co-authors of a medical study published in the New
England Journal of Medicine. That paper also had the distinction of
having 100 times as many authors as pages."
The "optimal" number of co-authors will depend, hence, on the project/problem and very much on the field and subfield. Coauthors may provide expertise, proper instruments, act as sounding boards and assist in the data analysis, etc.
Also, the "optimal" number of co-authors may depend on what stage one is in one's career and the expectations of your environment.
Clearly, the data show that single authorship of scientific articles has decreased dramatically. Let's, nevetheless, hope that we are still educating students who "can stand on their own feet," if need be.