Any issue of Nature today has nearly the same number of Articles and Letters as one from 1950, but about four times as many authors. The lone author has all but disappeared. In most fields outside mathematics, fewer and fewer people know enough to work and write alone. If they could, and could spare the time and effort to do so, their funding agencies and home institutions would not permit it.

Scientific papers have always contained two quantities — the increment of new science and the credit for its discovery. From the late 1600s until about 1920, the rule was one author per paper: an individual produced an increment of science and obtained a corresponding increment of credit. This symmetry was breached in the 1920s, diminished in the 1950s, and largely abandoned by the 1980s. Collaboration in multidisciplinary research is now universal as well as essential, and we determine from the list of authors who shares in the credit. Curiously, however, in most journals we are not told which of these did what part of the work, nor may we be certain (have we ceased to care?) who drafted the paper.

The ruling convention of multiple authorship is that all authors shared in the work more or less equally and, if the first author or two takes the role of 'first among equals', all listed authors take full credit for the contents of the paper. This is easy enough to swallow where three or four authors are concerned, harder when there are eight to ten authors, and almost impossible with twenty or fifty — let alone hundreds, as in some sequencing papers.

Credit: D. PARKINS

It would, of course, be possible to specify in detail, as in movie credits, who did what on a scientific paper: there is simply no widespread pressure to do so. Nature's editor Philip Campbell introduced a policy in 1999 of including a statement of author contributions in each paper (see Nature 399, 393; 1999). Although this is voluntary, authors in Nature are increasingly taking up the option.

Fortunately, where there are large numbers there are laws, and where there are laws there are results to be had. Lotka's law, obtained empirically by the mathematician Alfred Lotka in 1926 and many times confirmed, is a rough 'inverse-square law of scientific productivity'. For every 100 authors who each produce a scientific paper in a given period, there will be 25 authors who produce two, 11 who produce three, and one author who produces ten or more.

The appreciation of Lotka's law has allowed the continuation, in a world of clearly shared credit and hazily specified responsibility, of citation counting as the principal means of establishing scientific prominence and reputation. No matter how many co-authors you have, the more times your name appears on a scientific publication, the more productive you are assumed to be, and the more worthy of support. It can even be shown that Lotka's law predicts the ranking distribution of an author within an author list, and their climb up the scientific ladder.

The only natural force opposing the utility of Lotka's law has been Goodhart's law, from the economist Charles Goodhart: “Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes.” Once citation counting became established as a means to determine prominence, players began to 'game the system' based on their knowledge of that standard, and the metric ceased to have a close relation to the outcome it was designed to measure. Such attempts led to the somewhat occult business of impact factors, impact journals, author rank within a paper, and other such countermeasures to re-establish the utility of citation counting.

Until very recently, the combination of Lotka's law and impact factors at least held such 'author gaming' to a draw. Now cracks are appearing in the system. It seems that Lotka's law applies only when papers with 100 or more authors are rare. When these become common, the disjunction between the number of papers being counted and the number of authors entering the system per paper becomes so large that the power-law distribution of multiple authorships breaks down.

The ability of Lotka's law to equate frequency of authorship with scientific rank is buckling as more and more areas of science — genomics, proteomics, climate modelling and particle physics are the most prominent — regularly produce papers with more than 100 authors. Further evolution of the system is likely in the short term.

I predict that in those fields where multiple authorship endangers the author credit system we shall soon see institutionally initiated restriction on the number of authors. Paradoxically, this is likely to be endorsed by all parties as preferable to cinema-style specification of who actually did what. Most will prefer full credit for a few papers to little or no credit for many, considering where it matters most: university committees in charge of tenure, promotion and salary increments based on scholarly production. Given Nature's role in determining, as well as chronicling, how science is reported (see Nature 450, 1; doi:10.1038/450001a 2007), interested parties could watch these pages to see whether a trend towards more restricted authorship is emerging.