Hello, this is an interdisciplinary issue: I started my thesis in the faculty of media studies, in the humanities' tradition the "author" is an important concept and, unless you actually sit down with one or more other persons to write the article, you don't have co-authors added after your name.
Now I moved to another faculty to finish the phd, psychology, and my supervisors are choosing (pretty arbitrarily) a long list of co-authors to add to my name in the title. As all my studies where in the Humanities, I can't help feeling that my work is diminished and brutalized by this method, can you please explain me where this tradition come from and how much of interference with my work should I accept or when should I hold my ground? Should I have a say in the choosing of the co-authors?
Thank you very much

Co-authorship is actually really important in some fields. Firstly, it means that everyone is fairly credited. Secondly, it can actually be a bit of a selling point if you have co-authored with prof big head.

On the other hand, gift authorship is wrong.
To be a co-author generally you should have contributed to the paper writing (this can be a 'its fine) and approved the final version. In addition, you should have had some involvement in the design / concept of the study, data collection, or data analysis.

Of course, author order is another issue of controversy. First, second, third and last are generally most important.

well, I guess the problem arises from the fact that my new supervisors know nothing (like: zero, zilch) of my topic, and they put a lot of pressure in hiring me really fast, so I didn't have the time to learn about co-authoring, otherwise I would have thought twice about accepting. Can you please tell me how much of a say I have in choosing these co-authors? And how should I reply if these people who have NO idea of what I am writing about start saying "this is stupid, write this instead?" Do I get any kind of leverage?

There are discipline-specific standards for co-authorship to which you can refer (alias, keep in the back of your mind) when discussing with your sups.

For psychology, check the APA manual. As Thesisfun rightly said, the general rule is that anyone who is co-author has contributed significantly to the paper. This usually means contributing to the design of the study, data analysis, or data interpretation, plus contributing to the writing in some form and approving the final version - which also means taking responsibility for it.
Giving technical assistance, helping in the data collection/preparation, or giving a quick feedback on early versions of the paper, alone, is not enough for co-authorship (a note in the acknowledgements at the end of the paper may help in these cases).

Furthermore, as general rule, the supervisor(s) of a PhD student is/are usually listed as co-authors (right or not it's another issue), with the main supervisor as last author. I know this is foreign to humanities, and it may be really frustrating at first, but be assured that if you publish in a psychology journal readers will know how to interpret the list of authors.

You say your sups are choosing co-authors 'arbitrarily'. What do you mean? Usually sups have reasons for deciding who should be listed - even if the reason is that it may be good for you to have a certain name there, or that in the future you may want to profit from a certain person's advice (or that Dr. X should be there because he got the money for the project - already experienced).

Have you tried to speak clearly with your sups about the co-authorship issue? This is something that should be clarified at best before starting writing. As first author, you should have a say on who else is on the list. However since you're new to the discipline you may want to listen to your sups, since they know best what standards should be used in this situation...

the thing is, the thesis is already written, most of it, that is why they offered me only 6 months of scholarship to finish it,
so it's really "from my brain", so to speak, and there is very little time to discuss the papers
And the topic of the papers is the topic I have been working on for 3+1 years, of which they know nothing about. For funding reasons I understand they want more of my topic in their department, because it is really popular, but they know nothing about it. When they hired me I asked them several times if they understood that my thesis came from another disciplinary area with other methods and to read it well to see if it could fit in their department, I guess they didn't read it

I am very confused why you moved department if your new department cannot support you.
Perhaps you need to speak to your supervisors about your concerns about authorship!

I didn't understand that they could not support me before, they were very pushy on how they needed my work for their research group, because on the paper their research group is about my research topic, but in practice there are only 2 out of 50 researchers who do what I do. I had left the previous department one year before, for personal incompatibility with the supervisor (made me spent the first two years rewriting the introduction), without plans of finishing in this new particular department. Well, I guess the only way is too see how the first co-authors are going to behave, it is good to know that I do have a little say in who these co-authors should be.

Unless the co-authors on the list contributed to the paper, don't put them on it. This is your work, and you would be fully justified in digging your heels in over this matter.

On any paper, you might have listed a first author, a corresponding author and potentially any number of co-authors. There's also the thorny issue of "courtesy" or "guest" authors.

The first author is obviously the main author, essentially the person who pulls the material together and compiles the paper.

The corresponding author is the person who liaises with the journal editors, either via postal or direct contact or via the website electronic contribution services. The first author is normally but not necessarily the corresponding author.

Co-authors are people who have contributed to the paper to varying degrees. They may have helped the first author directly in writing the paper or simply contributed material the first author has incorporated into the paper. Practice has been with papers I've been involved in to include authors in order of contribution, first author first, probably corresponding author if not the first author second, then co-authors in order of decreasing contribution.

"Courtesy" or "Guest" authors have gone last, normally supervisors or others when they have not contributed directly to the paper. The rights and wrongs of this we can debate all day long and I don't think a person should be listed unless they directly contribute.

However, I remember my second supervisor got very upset when he wasn't listed on one of my earlier papers. He was included just to keep the peace, even though he played no part in the paper whatsoever. With later papers, he simply wasn't made aware a paper was being written and that was common practice with other members of my group.

Ian