>Are you saying that something in the brain is pre-wired to be a better guitar player?
Yes, whether due to nature or nurture, I would say certain people will be faster at learning how to play the guitar, and thus, be a better guitar player given an equal amount of exposure. Of course, you could always put more time into it, but there is only a finite amount of time available to a given individual.
No citations, but I believe this agrees with the general consensus. In fact, I would be greatly interested if anyone could provide studies showing the contrary.