I'm back in California after two wonderful weeks at Summer School in Trento, Italy. I learned many things, both great and small. One of them was resolving the mystery as to the organization of PhD dissertations in economics and management. In engineering, computer science, and sociology, dissertations are generally organized as a report on a one project -- a cohesive whole. In contrast, many or most dissertations in economics and management are collections of "essays", usually numbering three. For a long time, I couldn't figure out why. Now I know.
The driving force is the hiring criteria in the academic job market in economics and management. I was told by a professor that, to even get an interview at a top university, an applicant had to have one or more publications in a high ranking journal, and that having many conference publications or a top-quality dissertation were not enough. Therefore, from the start of the dissertation process, candidates are guided toward journal publication, even at the expense of fully researching the thesis as is done in engineering, computer science, and sociology.
Another reason, I suspect, is a shortage of post-doc positions in economics and management, particularly the latter. I don't have any data to back this up, so maybe I'm wrong, but it seems that PhDs in management are pushed into the job market for tenure-track positions immediately after completing their dissertations. If this is so, then it would explain why they would be structuring their dissertation as a collection of three "publishable units".