I will get down voted as this community does not like to hear it, but for the very most stutterers it's a form of anxiety. The normal form of spontaneous fluent speech is disrupted because the exposure to pure free emotion (as your voice never lies, just like the eyes) is so scary, that people repress it immensely. The moment I started to treat it like an anxiety disorder is the moment I finally learned to talk fluently, as I finally learned to just express my emotions instead of focusing on not making mistakes. The problem I notice with many if not most stutterers, is that try to convince themselves they are not afraid of mistakes, but actually really are. Vulnerability is what will cure most of it, and most of you have been telling yourself you're already vulnerable, but there's so many deeper layers. Until you really understand this, the stutter will persist, just as in people with OCD; the harder they try not to do something, the more they will have that specific compulsion. ​