It really doesn't matter what you major in. My impression is MSTP programs are much more like graduate programs than medical school programs--that is, they individualize applications and want to see achievement rather than the more "personal" stuff. For instance, if you published articles on Nature on classification of subatomic particles and got a recommendation letter from Stephan Hawking I can bet you can get into Princeton's physics program even if you majored in English Lit. For MD/PhD it's the same thing. The problem, of course, is this: are you gonna be able to do serious scientific research without taking a few required background classes and fulfill requirement for a different major at the same time? For a lot of us, the answer is no. In physics it's ALMOST impossible to read specialized contemoprary articles without at least a full undergraduate degree. It would've been really nice for me to major in African Civ, but I simply ran out of time.
Now as far as the "personality" issue, I'm sure they do look at you and say, oops, ain't gonna be a doctor I tell ya; but I bet they'll think twice before rejecting a 19 y o got three first author articles on Cell, no matter how bad his personality skills might be. (not me...ha i wish.)
Good luck.
I've discovered that ever since I've started the whole application thing I make less and less sense.
