I've recently read on it being a physical issue as well. However, I can't help but wonder if there aren't psychological 'after effects' in some people who stutter (eg. me). It's probably all a conditioned response to the physical tendency to lock my vocal cords, but I find that my flow of though is hindered and not smooth. My reading isn't smooth (probably because when I read, I talk to myself, and my brain has been conditioned that I stutter when I talk). Things like that. So yes, stuttering might be caused solely by physical inclinations, but it could have psychological consequences.