They simply say that it is joint work with somebody.
Is it primarily because one is a better speaker than the other? Like, do some professors publish a lot but choose to stay behind the scenes and let their coauthors present the work?
An incomplete list:
(Thanks to the commenters who suggested further additions.)
The average immunology paper has 11 authors; so does the average molecular biology paper. Genetics papers average 21.6 authors. High energy and nuclear physics papers, 226.8.
In many cases talks will cover multiple papers, so there could easily be 50 co-authors involved. In many cases, these include authors from many different institutions, countries, and continents.
And talks are not a particularly big deal in many fields. In my fields, a moderately well-known scientist might give 5-10 talks a year. A post-doc looking for a faculty position might give far more than that.
Who is going to pay my fifty co-authors to travel to five talks a year? I have a hard enough time getting reimbursed for myself.
I don't even understand what the point would be. Presumably they'd sit in the front row? And ... smile and wave? Or would they run up to the podium when there's a slide they know about and take over for thirty seconds, and then run back to their seat?
The more I think about it, the more baffled I am. I guess the short answer is, There's no point, no one cares.
Some responses I don't think I've seen, yet, as well as some backup to former responses...
Oftenimes, other authors have different roles at the same conference, at the same time, such as presenting a different paper, serving a role at a division committee business or admin meeting, or they may be attending another panel session about work in their same area(s).
Sometimes it's because they are, for instance, on a different teaching schedule or invited profs at international universities, as opposed to just lacking monetary resources. Larger schedules conflict.
Monetarily and to my last point, many/most schools don't offer adequate travel expenses to co-authors. Before recently retiring, at the end of June, 2017, I had a budget of $2,500/yr., which, at least in my field, was EXCELLENT, and based on a long-term endowment from the E.W. Scripps Foundation, which most J-schools don't have, even though it could only be used @ $1,250/conference, for up to two conferences per year. While my colleagues would often be rather amazed at that base amount of funding--which could be supplemented by funds from other accounts, such as grants or other projects that rewarded one with monies toward university-based projects--at the same time, there were other restrictions on how that base money could be spent. If I were "SIMPLY" a second or later author on a single paper, I could only get up to 50% of the base funding -- $625. While, at many schools, that can be a good deal of subsidy, it doesn't go far, in covering expenses to major cities, for a few nights. It would barely cover the costs of the flight and maybe conference registration, leaving one with all the hotel and per diem costs.
As a tip to young scholars "new to the other side," to avoid such problems, it's always good to do some networking and/or to actually "co-present" a paper with a graduate student. Networking, one can usually pick up a side-gig as an officer in a division, let a research chair know--as soon as a paper is accepted--that you're willing to serve as a moderator, discussant, or panelist, in a discussion session (although the last often requires input well before even entering an article into competition). Co-presenting a paper with a graduate student who has worked on an article or is the main author has benefits of helping to "craft" your students and introduce them to others, afterwards, and it can also help them to get funding they might otherwise be unable to receive.
(Never underestimate the good that can come from crafting and polishing a couple/few rough diamonds or straight-up high potential grad students at a time. If well-trained, they can become great students and profs, at times going on to be lifelong colleagues you place in doc schools and/or as profs. With your main expertise as their starting point, they will often do very creative work that branches off slightly from or continues on, in your tradition, and they will often have some great insights, along the way.)
It may be that such grad students are simply unable to attend, for other reasons, such as moving schools, as they progress, or it may be that they didn't have a great deal of expertise/anything of substance to add to the specific discussion that I/another author didn't already know, in toto. In my field of media psychology, there are often 2-4 authors (or maybe 5-7, if a school has a system of "milking" authorships, by having several (usually grad) authors each spread out tiny duties (heheheh) from their projects, so that all, like, 7, could be on all 7 papers, as authors (Michigan State's Mass Comm used to be famous, for that in the '90s and early-mid aughts, until people caught on...)), but, say, two out of four of my students/co-authors simply coordinated my lab and ran our experimental participants, for the time, and then co-wrote the (always essentially banal/straightforward/specific) methodology section, because they had the most direct experience with the process. Such beginner or "helper" authors are rather "insignificant," in presentation, and any questions, should the section not have been specific enough for direct replication--or If I didn't specifically remember--can be referred to those authors, by email or whatever (even face-to-face, later at the conference) -- as can any others who may be more "important," if any of the other situations mentioned occurs, wherein a more "essential" author is unavailable at presentation. If seemingly worth it, you might set up to have coffee or a drink with your co-author and the questioner, if it seems worth the effort. It could yield a cite and future interest in your work.
All-in-all, most presentations--especially given time constraints--don't offer the opportunity for more than, say, two authors, at most -- and that's generally if they're both basically co-equals, bringing together different areas of expertise in a presentation. Otherwise, there's the mentoring grads model I expressed above.
!! Oh, yeah, and there's one last reason about which I just thought: the co-author is retired. My graduating doc student is presenting what is essentially his dissertation, which he defended this spring and which we'd since worked into a much more concise and expert piece--wherein I added a lot of my personal insights into theory and applications--but I'll be damned if I'm going to shell out $1,500+ on my own dime, to go help present that paper, in Chicago, next month (although I'm helping put together the presentation materials) -- especially on retirement money. He's presented several, at this point--including a starter, with me--and he knows my arguments in the article VERY well, having had to answer for them, in his defense (after submitting it to the grad school, finished, I tightened up and made the arguments more precise and forceful). Plus, it's his time to start carrying the torch, on his own, as my latest and perhaps best...
(My apologies for going a bit off topic, at times, but I seemed to read more into the question than was specifically articulated, although I do think fleshing it out, as such, should be very helpful. It sounds like a grad student question (as a matter of experience). This is also my first post on this site. I guess that's what you do, 12 days into retirement, at 4 in the morning. ;) Hope that all helped, in broader and specific senses. I'll get the hang of it...)
Thanks for the Opportunity, Carson B
In my field, the professor or leader of the research group is the mastermind behind the research going on in their labs. They will have a number of PhD students working on different parts of projects and then they will present the overarching story either in a conference talk or in a paper.
Sometimes, part of the work — significant for the results achieved but pointless by itself — has been performed by collaboration partners. In that case, the part of the talk usually sounds something like:
We sent this compound to the group of Mastermind to find out whether it glows in the dark.
Again, we have the presenting professor with the big picture who brings all loose ends together. It only makes sense that they do the talk.
The final slides are then usually the acknowledgements. Here, I am used to the professors saying exactly which PhD/Postdoc performed which work and which collaborator did what.
It’s not that those collaborators who are not on stage prefer sitting in the back or writing papers. Rather, they have different research interests, sometimes even sub-fields. They will go to those conferences that better fit their research, hold talks there and maybe mention the collaboration with the professor we just talked about if it fits into their story.
And it’s not that the PhD students and Postdocs prefer to sit back and write papers. They will either be in the lab working hard or they will be there listening, maybe presenting a poster.