What are the negative effects to the first author when adding coauthors?

Question

There are many questions on this site which involve authorship disputes and ethicality.

In fact, there are often people who are greatly distressed when their contributions to a paper are "lesser" than they expected, for whatever reason. This could range from having a co-first author added, or having other secondary authors added to the authorship list. Another famous example woudl be the Alpher-Bethe-Gamow paper regarding Big Bang nucleogenesis, where the first author Alpher greatly resented the addition of Bethe to the authorship list.

However, I do not understand why adding co-authors would be a bad idea. Surely, the fact that you were helped by another person means that they should be co-authors on the paper, as long as they made significant intellectual contributions.

None of the fields mentioned in this question regarding authorship norms had any explanation of the potential detrimental effects additional authors have to the first author.

What, then, are the possible negative effects that the first author can suffer if other authors are added to the paper?

Note: I do not intend to ask regarding issues where the original author is removed from consideration, only those where the first author is kept, but other coauthors are added to the authorship list.

Answer 1

I think for some people, the principle is important: they didn't feel that the person whose name has been added contributed enough, and humans hate the feeling of giving up something unfairly.

That said, there is a very real cost in terms of credit. This will not be consistent across different people looking at your record, but every new co-author decreases the chance that the important part of the work will be attributed to you. There was actually a recent study about this which found that women pay an especially large price on this score: when they do joint work with men, the evidence suggests the men tend to get the credit (the actual observation of the study is that their chance of getting tenure do not improve). You'll note though, even for men a solo paper improves chances of tenure by more than a jointly authored paper with anyone.

Answer 2

Following the link to the Alpher-Bethe-Gamow paper I found that there is an answer for this particular case that seems important. From the Wikipedia page:

He [Alpher] felt that the inclusion of another eminent physicist would overshadow his personal contribution to this work and prevent him from receiving proper recognition for such an important discovery.

For this paper is seems save to assume that many people will consider that the order of the authors has not been decided by contribution. I am not sure about the rules for author order in physics at that time but for sure people would immediately get the intended pun. Hence, I can understand Alpher's concern.

So in fields where order of the authors is not by contribution (or for special paper like this one), having one more big shot in the list of authors may really lead to the wrong perception. In fact, for PhD students in math is in not at all unusual to have papers written directly under the supervision of the advisor appearing with only the PhD student's name on it, even if the supervisor may have contributed some ideas. In this way it is ensured that the work will be perceived as the PhD student's work.