Looking through the literature, you see some papers with twenty-plus authors and others with only two or three. Maybe it is just a difference between papers coming out of industry versus those produced by academia. Sometimes I have to wonder if all twenty authors really contributed much at all to a six page paper, or maybe it is just a system of "you pat my back and I'll pat yours" (or "I'll include you on this publication if you include my name on your next one"). On the other hand, how many key people were left out of the typical two author paper? Does the undergrad who helped synthesize a key compound (through a method you carefully worked out for weeks beforehand) get left out of the author list? How about a visiting scientist that contributed ideas and demonstrated new techniques to a group? Is she kept on a publication because of the contribution of original ideas and thoughts, or do you leave her off because she didn't physically complete any of the actual experiments?
Is it enough to contribute time/manual labor? Is it enough to contribute only ideas? Or is it necessary for one to contribute both to be worthy of authorship? I've heard people say that it's only really necessary to look at the first and last author of an article, and if that's the case the debate over authorship doesn't really matter.
I'm for citing ALL of those involved in a project, no matter how small a part they played (meaning inclusion as authors, in the acknowledgments or in the references section under "personal communication"). What are your opinions?