Authorship and the 'Replaceability Principle'
These notes were written by Brian Scholl.

The current version of this document is: 7/10/99 (slightly cleaned up on 2/5/12).

Several people contributed helpful ideas and interesting discussion to these notes, including: Zenon Pylyshyn.  
Please forward suggested additions to
Writing and publishing papers, in most academic disciplines, is perhaps the single most important key to advancing the individual authors' careers, to generating interesting and useful discussion of one's ideas, and to advancing the field in general. It is thus also one of the primary sources of conflict. Authorship on a paper can signal that a person carefully designed a project; that they wrote part or all of the paper; that they had no role in designing or writing but that they nevertheless spent a significant amount of time and effort working on it; that they simply had some particularly good ideas relating to the project; that they belong to the lab in which the project was conducted, but did not participate directly; that they helped out with the data analysis; that they owned the equipment on which the experiments were run; that they simply hold a senior position in the lab in which the project was carried out but aren't even familiar with the specific project; etc. In those projects which involve more than a single person -- which is to say, most of them -- there are often disputes, misunderstandings, and lingering feelings of unfairness concerning which contributions do and do not merit authorship.  
Much has been written about how to manage authorship practices and disputes in laboratory settings, and much has also been written about the 'descriptive' state of the art in various fields -- essentially describing the sociological norms that seem to define authorship in different contexts (e.g. about the significance of the first and last authors on papers, etc.) and how to improve them (e.g. by requiring blurbs wherein each author explicitly states his or her roles in the project). My goal in these brief sketchy notes, in contrast, to think normatively: who should be an author on a scientific paper, beyond sociological norms?  
The 'replacability principle'
Authorship on papers can often be fairly judged, it seems to me, in terms of the replaceability principle. As a rough heuristic: anyone without whom the project could not have unfolded as it did should be an author; easily replaceable people, on the other hand, may not deserve authorship.

An obvious first caveat is that "unfolding as it did" needs to be assessed at the appropriate scale. Obviously, if you look too finely, everyone involved would be 'irreplaceable' -- perhaps because without them, a parameter would have been set slightly differently, or a sentence would have been worded in a subtly different way. But, using common sense, we can still differentiate counterfactual changes that would simply tweak an existing project, vs. those that would have made it a substantively different project.

Another obvious caveat is that there has to be some sort of 'statute of limitations' on this assessment. Otherwise, e.g., someone like Noam Chomsky should be an author on basically everything written in the field of linguistics in the past decade (since it would likely have been very different had it not been for his groundbreaking work many years/decades ago). (In some situations, the right cutoff might be a single paper: once published, all of its insight and wisdom becomes 'public domain' so that the authors needn't be included on any subsequent publications. In other situations, the right cutoff might be slightly longer.)

Applying the replaceability principle might mean, e.g., that the senior researcher and thesis student will be authors (since they collaboratively designed the project in the first place, and without them the project would never have seen the light of day); that another colleague might be an author (by virtue of having suggested an especially important and novel idea that radically altered the project); that the research assistants who programmed the experiment and ran the subjects would not be authors (because if they hadn't done that work, somebody else would have, without significantly altering the project); that a fellow lab member who suggested a minor improvement might not deserve authorship (because such minor improvements are part and parcel of any lab meeting, with any lab members); that the senior director of the research center where the research was conducted would not be an author (because his being in that position would probably not have at all affected the project, which could have been conducted in the same way elsewhere); that a colleague who consulted on the statistical analysis would not be an author (because somebody else could have provided the same consultation).

Various types of contributions that should and should not merit authorship
Here are some thoughts, scattered among discussions of the various roles one can play in a research project, on what sorts of contributions should and should not merit authorship by this heuristic criterion.  
  • A solitary researcher 
    In increasingly rare cases, there may be an individual (at any level) who, by herself, designs and implements a project, runs subjects, analyzes the data, and writes a paper completely independently, or with only very minor input from others. There would be little opportunity for authorship disputes here. But note that without further refinement, it is possible that even this single author might not 'deserve' authorship according to the replaceability principle! This might be the case, e.g., for an an extremely straightforward and obvious followup experiment to previous work (perhaps even explicitly suggested by the authors of an earlier prominent publication): here, if one researcher hadn't conducted the project, others certainly would have, and likely without many substantive differences -- so that even the lone researcher wouldn't be irreplaceable!  
  • Senior researcher/Lab director 
    Many senior researchers and lab directors in cognitive psychology are still very actively involved in programming experiments, running subjects, analyzing data, and the other day-to-day business of experimental psychology. But many others are not. They will be intimately involved in the selection of research directions and in the design of experiments, but the actual implementation is often left to students and other more junior researchers. Individual differences here are often central, of course, to the ill-defined project of choosing of a research advisor in the first place. Some advisors are happy to view the mentor/student relationship as one of didactically-oriented collaboration; others use their students as little more than cheap labor. Some advisors are happy to work on whatever areas or projects interest their students; others may explicitly require, e.g., that post-docs spend 56.25% of their time working on their advisor's projects. Conventions regarding authorship for senior researchers varies widely, and can be especially sensitive given that it is they who often decide how and when to write papers, and who will participate. Many will claim authorship for themselves using the same criteria as they do for anybody, gauging whether they have had a significant -- and perhaps irreplaceable -- impact on the project. In most cases, of course, authorship is definitely deserved, since it is often they who conceived and designed the project initially. Other times, however, senior researchers will claim authorship on papers simply by virtue of the work having been done in their labs; this seems unfair: if the senior researcher is really just an advisor on a project which the student (or other junior researcher) devised and implemented independently, then they are performing an eminently replaceable function. (On the other hand, if the donation of rare equipment or financial support was sufficiently rare or unlikely, then perhaps such a role might be deserving of authorship after all?)  
  • Thesis students (both graduate and undergraduate) 
    Many research projects are conducted as a collaboration between a senior researcher and a student working on the project as a PhD thesis, a Masters thesis, or an undergraduate honors thesis. In most such cases, there is no opportunity for authorship disputes because both will have contributed major parts of the project's design. In some cases, however, there will arise situations wherein a thesis student essentially does all the work for a project which was largely or even completely designed by their advisor. This situation is ripe for misunderstanding. The replaceability principle might here suggest that the student should not be an author on the resulting paper (of "their own project"!) -- though of course this would violate all sorts of norms in our field.  
  • Research assistants 
    Many labs often employ research assistants to help out with (or even take charge of) scheduling and running subjects, constructing stimuli, and other similar tasks. In the majority of such cases, these RAs should probably not be authors on the resulting papers: if they hadn't done these tasks, somebody else would have done the same work, in largely the same way. RAs, in other words, often do not contribute anything irreplaceable to a project. (In addition, of course, RAs are often employed literally, and are compensated via work-study credit or even a salary.) However, even this can be a gray area. Many future grad students get their start as RAs who regularly have particularly good ideas, and in our lab, for instance, RAs have sometimes been authors on projects to which they contributed an inordinate number of eventually-crucial, irreplaceable ideas. In some extreme cases, I suppose that an RA might merit authorship simply by dint of having put an extraordinary amount of work into a project, but then again, there are many labs wherein RAs put in significant amounts of time and effort for nothing more than 'research experience'. More often, RAs will start out by helping out on projects for which they will not be authors, but will gradually assume more responsibility for the actual design of the experiments, and may then take on a project on which they will play a primary role and on which they will be an author. Similar comments here apply to situations of statistical consultation, etc. I know of students who have been made authors on papers simply for doing the straightforward statistics for a probability-impaired senior researcher. This seems inappropriate to me, since if a particular statistician hadn't done the straightforward work, another would/could have.  
  • Other students, colleagues, lab members, and random passers-by who offer ideas 
    Another common situation that results in authorship-related conflict concerns anybody who offers a clever idea that promises to improve a project. Often, this will arise from lab meetings, where a lab member who is not directly involved in the project offers a helpful suggestion. This can also happen as a result of a colleague commenting on a manuscript, or perhaps making a suggestion at a scientific meeting. Most such suggestions, in my opinion, do not merit authorship, simply for practical reasons. As a matter of course, any lab meeting or scientific talk will result in many such ideas, and it is not feasible to grant authorship to so many people. In the context of lab meetings, especially, such suggestions are often 'repayed', as it were, in similar suggestions when the next project is discussed. There can then be a standing rule of sorts that everybody is expected to contribute ideas to all projects during lab meetings, but that such input does not typically confer authorship, since it evens out in the long run. But of course the replaceability principle suggests that some 'random' suggestions -- the really good ones! -- are deserving of authorship, since even an offhand comment, if precious enough, could completely alter the trajectory of a project. (But the decision of what counts as a really good or essential or novel idea is obviously subjective, which is why all of this requires some explicit discussion. Often a colleague will feel slighted because they were not offered authorship for their extraordinary groundbreaking suggestion, whereas the current authors view this particular suggestion as obvious, mediocre, or as one which they have heard independently from countless sources.)