I used to think it was really clear. I've been talking with my man Gabe Brostow, and it's not so much. Different fields conceive of it differently, and even within a field the borderline questions can get to you.
"Authorship" is who gets their name on a publication. "Authorship order" is who gets their name first, second, etc.
First I'll talk about the differences between fields.
In most of the humanities, single-authored papers are the norm. Even if the primary author is a graduate student and has had extensive advice on the paper from the advisor, the student is the only author. This is the norm in philosophy, English, film studies, etc. In scientific field it's common to have the advisor's name on papers. The problem with the single-author culture is that it gives the advisors little external motivation to have and mentor graduate students. You can't put your grad students' papers on your CV.
In computer science, the field I'm most familiar with, it's a matter of course that 1) your advisor mentors you a lot, and 2) that her* name goes on every paper. The principle investigator on a grant (the "PI") usually gets last authorship on every paper coming out of the lab. This means that although first author is the most prestigious, last author is the second most prestigious. She's usually the one who masterminds the whole lab's mission.
Gabe tells me that in engineering, you don't actually talk about your research with your advisor. You talk about where to publish, who should be on your committee, things like that. He tells me that if an idea comes up while talking to your advisor, you can't use it in your dissertation. (!!) Nonetheless, the advisor gets her name on your publications. That seems very weird to me; since my working heuristic is that people get their names on the publications when they have made a significant intellectual contribution.
I taught this heuristic to my undergrad cognitive science methodology class recently. One of the students ended up coming to my lab to work. We brainstormed the next step in the experiment, and she helped. She aske me later if she'd made an intellectual contribution and should she therefore get authorship. I liked her being up front about it. I told her that I was glad she told me that, because if she wants authorship we will make her work for it. She agreed to.
However, that project already has four authors on it. Is there any harm in adding another? Possibly. In some sense it dilutes the apparent contribution of everyone else. I also don't want resentment in the other authors who have done much more work. Finally, though it's not an issue with this many authors, if the next person is a third author then it changes the way it gets cited in many publications in the text. For example, if there's a single author it will look like (Davies, 2009). If there are two authors it will look like (Davies & Haigh, 2009.) If there are three it sometimes** gets shortened to (Davies, et al., 2009). So adding a third author reduces the name recognition being built for the second author. This is not a problem im many computer science publications, though, because citations occurr by number, as in . You have to look at the references section to know who is being cited. In the humanities it's not a problem either, since the full citation is often in the footnotes.
I don't want everyone in my lab to have their name on every paper, especially since I have many students in my lab. I am thinking of implementing a general rule (which can have exceptions) that the lab meetings are for freely given advice. That is, if you are talking about your research in the lab meeting, and someone gives you advice, that does not necesarily mean that that person gets their name on a paper. You give them the same courtesy. If you want your name on the paper, you have to help write it or do something outside the general lab meeting to earn it-- and in that sense I'm pretty generous with authorship. I want to encourage a culture in my lab of collaboration and mutual help. I want people to feel free about coming to the lab meeting and asking questions and not worrying about having to add authors to their every time they do. Of course, there can be exceptions, such if someone in the lab meeting has a really, really great idea and should get credit for it. But even in this case I would hope the response would be to have the person join the project and help in other ways.
I'm very interested in other thoughts on the topic. (If you leave a comment and don't sign your name it will appear as from "anonymous.")
Pictured: Somebody making a paper.
* The convention I use for the he/she thing is that if I'm referring to someone of higher status, like an advisor in an advisor/student relationship, it's a woman; else it's a man.
** In psychology (APA) style the first time you cite it all authors are listed. Subsequent citations get the et al. SIGGRAPH (The prestigious computer science conference of special interest group: graphics) does the et al. thing from start.