One of my graduate students has decided she no longer wants to write a medieval dissertation; another field calls to her more strongly.
You'll live with your dissertation topic for a long time, both while you're writing it and afterwards, when you're turning it into articles or a book. It's how you learn to do real research; it forms the basis for everything that comes afterward. So it's important that the topic move you, excite you, trouble you in a good way. If my student feels more excitement about her new field, then of course she should go and plow it, sow it, tend it, harvest it. And I said so, while also saying that should the new field not work out for any reason, she'd be welcome to come back to the old one.
There are two "buts," one for her, one for me. For her: I hope this isn't just last-minute panic. She was about to defend her dissertation proposal. It seems to me this change came about very suddenly, though she may have been thinking about it much longer than I've been aware of. And sometimes of course it is only when push comes to shove that you realize what really matters to you.
For me: I don't have that many students who want to write medieval dissertations. People don't come to Large Regional U to study the Middle Ages, usually. Our best students are here because they are place-bound; many students don't come in with a clear idea of what field they'd like to work in. Even if they enjoy their classes in medieval literature, few of them have the languages to write a dissertation on it. And my other medievalist colleague and I do hold the line on languages. So it was exciting to have this student come in a few years ago with a declared interest in the Middle Ages and with adequate language training. She has taken several classes with me and been my research assistant; I was her teaching mentor when she taught a literature course. We have always got along well. She is smart, conscientious, organized, and in many ways a kindred spirit. I was looking forward to supervising her dissertation.
At a different sort of school, this would not be a big deal. At a different sort of school, an advisor might even be glad to have one fewer dissertation to read. But I am not likely to get two or three students in every in-coming class who want to be medievalists. I have another student further along in the process, and I am on a committee for one who has a medieval component to her work. After the student who has just decided to change areas, there's no one else likely just now. In fact, it took 10 years to collect these three students. Having them made me feel I was in a golden age, one I knew would pass . . . but hoped it would not pass so quickly.
I'm as sure as one can be that it's not a personality clash or problem between us. I've encouraged her through exams and papers for other classes, I've found her opportunities to present at conferences, I've read work in a reasonably timely manner (not so fast as would be ideal, it's true, but not so slow as to cause problems). She has consulted me on various academic problems and on the work-life balance. At the same time, we're in no way enmeshed; we are both married and have satisfactory personal lives.
So this is professional. I'm a little disappointed on the personal level, but she'll still be around; we can continue to chat about our cats. What I want is partly (I admit) the department status that comes from successfully supervising a dissertation to completion, partly the experience of watching someone's ideas develop, finding the ways that her work can spark insights for me as well as the ways that I can support her growth as a scholar. At its best, supervising a dissertation creates a sort of study group, with useful feedback for both sides.
Primarily I want my student to be happy in her work as I am in mine, able to form the kinds of ties with her dissertation committee that I made and still enjoy with mine. It's pure selfishness to want her back, and to hope that maybe this is the sort of wobble I went through in my third year of grad school, when I started taking classes in another area entirely, only to decide after a few weeks of them that I could not bring myself to do that work, even if it might have been more sensible. I don't think she's trying to do the sensible thing. I think she just realized where her heart really lies; and better now than after struggling through a chapter or two.