How often do Ph.D. holders hold faculty positions in different, but related, fields from their Ph.D.?

Question

This is something that I thought about a while back when reading (I forget where) about someone who held a Ph.D. in chemistry, but focused on physical chemistry throughout his Ph.D. years and actually became a physics professor.

I was wondering how often this happens. For instance, do electrical engineering Ph.D. holders with very physics-oriented research become physics professors ever? What about vice versa? Or what about, for instance, applied mathematicians specializing in cosmology. Do they ever become physics professors?

More important question here: does this open up the possibility in which one can apply for faculty positions in multiple departments, thereby increasing one's chances of a faculty position? Or does the interdisciplinary nature of it actually limit the candidate's faculty choices?

Note, when I say someone is a professor in another field, I mean, for example, they received a Ph.D. from a department of chemistry and were subsequently employed by a department of physics.

Answer 1

For interdisciplinary researchers, this sort of thing can happen fairly often. For example, if you are a chemist working on ecological applications, you might well have started out in a chemistry department and end up in an ecology department, or vice versa. This happens even more so when dealing with disciplines that may be categorized differently by different institutions: for example, is work on medical devices part of biology and the natural sciences faculty or part of electrical or mechanical engineering? Generally, however, if it's going to happen, there is at least some logical trail that you can see how the faculty member got from point A to point B.

Answer 2

This has happened in the past, especially in some emerging fields. For example, I know a professor of electronic engineering, whose PhD was actually carried out within a mathematics department, studying cryptography. From there, the practical applications of cryptography in physical systems led towards electronic engineering. At the time of his PhD, computer science was quite different to what we have today, and indeed it wasn't hugely unusual for people to complete their PhD in mathematics, yet end up working in a different (yet related) department.

While not precisely what you asked, in my experience it isn't at all unusual for people to make fairly significant changes in field, when working in postdoc positions. I've seen people from civil engineering working brilliantly in electronics groups, and vice versa. They can then end up forging a successful career in those groups. The impression from what I've seen is that if someone is doing work that's sufficiently inter-disciplinary that it's related to a group or department, provided their skills and expertise are in line with the department at the time, they are not disadvantaged.

Obviously this might depend on location and discipline, although I know many people who moved into computer science in the early days, from different (often completely unrelated) fields.

Answer 3

Adding to the existing great answers (about interdisciplinary research and emerging fields), I'd add perspective by asking, "why are there obstacles at all?". After all, traditionally the degree is awarded as a deliberately non-specific "doctor of philosophy", that happens to be in recognition of advancing the state of knowledge in some specific area, not as a "doctor of chemistry" or "doctor of linguistics" or whatever.

At issue is that in our increasingly specialized academe, hiring is usually done at the departmental level. Therefore, those hiring committees ask, "will this candidate contribute to the reputation, research output, and breadth and depth of expertise in our department?" And also, "will this candidate help us train graduates in our discipline, and/or instruct service courses in our discipline to the broader student body?" And, frankly, "is this candidate like me, so that I can confidently assess their capabilities and fit?"

A senior, respected scholar with sustained research output, that will have necessarily broadened out of the narrow area they started their work in years ago, will present little concern based on what exactly was the topic and awarding department of their Ph.D. years ago. And this will be easily outweighed by the breadth and reputational benefits they presumably bring.

For more junior hiring, it depends on the nature of the position. Where an open position is targeted on an interdisciplinary area, or the search is being run by a specific research team or "Institute" or "Centre", which may well be a part of a conventional department but has some autonomy, there will likely be a lot of openness to candidates from adjacent fields. If it's a hard-fought generic departmental position -- "we've finally got approval to hire another assistant prof in our physics department" -- a newly minted Ph.D.-Chemistry (that did their research in the intersection of physics and chemistry) will face a lot more skepticism: "Will they be able to build and fund a physics lab? Can they teach Physics 101? Which upper year Physics courses will they be qualified to teach?"

The implications is that if you're junior and doing something interdisciplinary, that's great. But I would also consciously build and demonstrate your broader credentials in the standard field you think is most likely to employ you. And you'll have to work harder if you want to prove that in more than one field, or if it's a field that's different from the name of department that will sign off on your "doctor of philosophy" (sic).

Note: Written with a North American perspective. Hiring dynamics can be quite different in e.g. the continental European model.