If someone who supervises graduate students ± postdocs is considering a major career move, such as leaving for another institution or leaving academia entirely, what should s/he tell the advisees and when should s/he tell it?
If an adviser is definitely going to leave, one way or another, in the near future (i.e., in a time frame that will definitely affect the advisees), s/he should alert the advisees to the imminent situation. Then it is the responsibility of the adviser and the department to have a plan in place that minimizes the negative consequences for the advisees.
If, however, there is great uncertainty about whether a move will take place and the adviser is merely checking out some options or responding to some feelers from other universities, the adviser should not necessarily alert the advisees to all these possibilities. The advisees will likely hear rumors, but unless there is something definite coming down the road, there's no point in being on constant alert for something that may never happen. Of course, if an adviser wants to be open about all this with the advisees, that is fine too, as long as the situation is clearly explained and the information doesn't add unnecessary stress.
When I was applying for jobs to leave University 1 (owing to there not being a job there for my husband), I didn't tell my advisees anything specific because one strong possibility was that I would stay at University 1. They knew I was looking around, but I did not give them constant updates about my applications or interviews.
Once my husband and I both had tenure-track offers from University 2, I delayed telling my advisees because University 1 came up with a counter offer of a tenure-track position for my husband. We agonized in indecision for weeks. There were days when we were definitely staying at University 1, and then there were days when we were definitely going to University 2. Once we made the final decision to go to University 2, I told my students, and I made sure to tell them before they heard it from others.
Even so, I did not pack up my office and leave that instant. In fact, I stayed another year at University 1. The offer/decision came so late in the academic year that I didn't want to cause problems for University 1. I had teaching commitments and I had one grad student who was going to finish his thesis that year. My only other student had failed his oral prelims long before my decision to leave. He decided to switch advisers, and later dropped out for reasons unrelated to my departure. A new grad student who had not yet started at University 1 was quite content to do his grad studies at University 2.
At University 2, I have not discussed with my advisees my possible opportunities to leave because there have been no opportunities that advanced to the stage where this was necessary. There were rumors about my leaving, some of which made me laugh, they were so bizarre, but I really had nothing specific to tell my group. I wasn't actively trying to leave, and I didn't see the point of going through the complex details of the many different ways that some other universities were exploring the possibility of luring me away.
As I mentioned on Friday, my university was proactive about giving me some reasons to stay; I never got to the point of bringing an offer to the table and asking for a retention package. If I had, then it would have been appropriate to inform those in my group who would be affected by a possible move.
So, in my opinion, a general guideline for what/when to tell is to consider the likelihood of your departure. If it's very likely (even if not definite), tell your group in time for plans to be discussed. If you are in the midst of an indefinite process involving varying degrees of seriousness that may or may not result in something at some point, then the stress to your group of keeping them informed of all the twists and turns of the complex process might not be worth it.
I know of cases in which an adviser did decamp rapidly and without warning, leaving others to deal (or not) with abandoned students and postdocs, but more typical is for adviser and advisees to work out a plan. The more senior advisees will either stay where they are and be advised remotely (possibly involving some visits back and forth) or will physically move to the new university but get their degree from the original one. Other advisees may be at a stage at which they can switch institutions and get their degree from the new place. Others may choose to stay where they are and switch advisers.
I'm sure it's annoying to hear rumors and to not know what's going on, but if it really bothers you so much, you could ask your adviser a direct question about it. Either you will get a non-answer, in which case you should respect the fact that you don't have a right to know everything about your adviser's professional decisions despite the fact that they affect you, or you will get some information that will either be comforting or disconcerting.
Academics are mobile, just like people in other professions. It's stressful for advisees to live with the uncertainty of a possibly departing adviser, but I recommend trying to keep the level of paranoia about this particular issue as low as possible and keep the focus on research until you it's clear that a move is actually going to happen.
11 months ago