Big question. Really can´t answer with authority. Just want to share an idea that can get us closer. Stuttering is the sympton. Is the repetition of words. Like this; s-s-s-s-tuttering. The word does not give us useful information on expading our understanding. You see, I can stutter because I have Parkison Desease and I can stutter because I have a blocking behaviour since child (like most of us) and the word does not discriminates. To the eye of the public we have the same condition. But is clear that we don´t. There are two different causes in the example. A better word instead of stuttering could be blocking behaviour. Blocking tell us that there is something we don´t want to experience. So we block. Behaviour says that it is learned. So it can be unlearn. When you think of block you think of freezing (at least I do). You are block so you can´t move. You can linked that with some kind of fear response of your body and mind. Well... At least you can debate and think of some type of explanation, cause or connection. With the stuttering you got nothing. It´s like having influenza virus and calling it the sneeze.