The students who write me for advice about going to grad school often have a unique challenge: they've already been infected with the "interdisciplinary" bug. My own work tends to range across disciplinary boundaries: from philosophy to theology to literature to social science. Those students who have a wide-ranging curiosity, and aren't easily content to simply burrow deep into one discipline or subdiscipline, often ask about "interdisciplinary" graduate programs. Indeed, many of my favorite students have struggles deciding whether to apply to doctoral programs in philosophy, theology, or literature.
But here's another "I-hate-to-break-it-to-you" reality: disciplines remain the gatekeepers of the academic world. While there's all sorts of interesting inter-, multi-, and cross-disciplinary work now happening in the academy, you earn the right to do that work by first proving your mettle within the rigors of a discipline. Training in a discipline is a discipline--an apprenticeship and a formation, an inculcation into a community of scholarship (this is actually described quite well in Kuhn's Structures of Scientific Revolutions). It is within a discipline that you learn to dot your i's and cross your t's. Even if ultimately your interests spill over the boundaries of a discipline, you need to take a sort of vow of chastity and restrain yourself from such experimentation in order to earn your stripes within a discipline. While there are a few "interdisciplinary" programs, it's too easy to emerge from them without this sort of discplined rigor--which is why it also tends to be difficult for graduates of such programs to find jobs. They fall between the cracks.
This is because of the other reason that disciplines remain academic gatekeepers: teaching. At the end of the day, you're going to be hired into a department. (There are a very few exceptions to this rule, but they're so few and far between that you should ignore them.) You're going to make your home in a disciplinary corner of the college or university, even if they might grant you license to roam from there a bit. While you might be permitted, even encouraged, to cross disciplinary boundaries in your research and writing, your "day job," so to speak, is going to be teaching. And the courses you teach are usually going to have a prefix: PHIL 200, THEO 101, ENGL 338, REL 100, etc.
So as you're contemplating which disciplinary portal you should pass through, ask yourself this question: Which 101 course would I be most happy teaching? While you're probably going to be dreaming about all the cool advanced seminars you'll be able to teach with majors, you need to keep in mind that your bread-and-butter is going to be teaching the introductory course in your discipline, the so-called "101" class. While you might find it hard to determine whether you're a philosopher, a theologian, or a literary critic, you do need to decide which introductory class you'd like to spend a lifetime teaching. Is it THEO 101 or PHIL 101 or ENGL 101? Those are very different sorts of courses. You've probably taken all of them. So ask yourself: which of those do I really want to teach? The answer will tell you which sort of graduate programs you should apply to.
Next time: Choosing schools (1): philosophical theology.