His co-authorship was a concern because it was mostly with more senior people (including his advisors), and I was worried that this potential hire might not be making a significant contribution and that his productivity might decline if he lost his relationship with those co-authors.
When considering hiring, tenure, and promotion, the considerations should be centered on whether the candidate has/is likely to make a significant (original/insightful) contribution to the body of knowledge in his or her chosen area of study.
Co-authorship comes into the equation when considering how much of a contribution an individual makes. If all of the publications are with senior scholars (especially the same one or two), then one could be suspicious that the candidate might not be adding that much. On the other hand, if all or most of the pubs are with junior scholars, then one can be a bit more certain that the candidate is bringing something to the table.
What is the best way to figure this out? Besides considering how the candidate has actually indicated how much of each pub is their own work (percentages are often included in the candidate's application--this book is x% mine, y% my co-authors), one could actually read the person's body of work to detect the ideas and methods that are consistent even as the co-authors vary (that is, those making the decision might actually practice some social science of their own and engage in comparative analysis). Or one could depend on outside reviewers who read the body of work and evaluate. One could also consider where the person's pubs are--in top ranked and highly selective presses and journals?
Of course, this issue is most important for hiring, a bit less for tenure and not so much at all for beyond that, as the question really revolves around whether the candidate is going to be a productive scholar. This is a big concern early when the record is pretty thin and at tenure since you are making a decision about someone's trajectory. Beyond that, the record is pretty clear about what someone is going to do because they have already established a post-tenure pattern that is likely to endure. Indeed, at that point, there are other measures of whether a person has made a contribution and has an international reputation--citations, membership on editorial boards, reviews of the existing work, etc.
With the possible danger that folks with limited vision might misunderestimate one's co-publications, why co-author?
- To bring different skill sets together. For instance, I have co-authored with other people so that I can add cases to my study without becoming an expert myself in Eritrean politics.
- To bring together different research resources. For instance, I am working with a pal from grad school who works at the National Defense University, and thus has connections to the US military. He is working with me, in part, because of my connections to the Canadian Forces.
- To tackle very difficult projects, more minds at work might be of assistance. I am currently applying for a grant to do a very difficult project on diasporas, collecting much data and trying to figure out some pretty complicated questions.
- To bring contrasting perspectives together so that the final product is the result of some intellectual wrestling.
- To ease one's workload. I could have written my second book by myself, but it got done faster and the work itself was better through the efforts of Bill Ayres the non-terrorist. Bill's work allowed me to work on other projects as well as lead the department in phd supervision.