I believe stuttering is mainly a bad habit. I've read somewhere, that Colin Firth "learnt" how to stutter, before he was in the movie "King's Speech" and it was very hard for him to unlearn that after the movie was finished. And don't forget that most people stutter in some way (like repeating words) and have speech-related fears, but they don't make a big deal out of it and still don't have the identity of a stutterer.