Dopamine has clearly been a key factor. I am starting to think you are just trolling because this is easily view able and searchable information. I don't have the time to provide you with every study as I am busy but you can easily search through [here](https://www.ncbi.nlm.nih.gov/pmc/?term=dopamine%20and%20stuttering) with the answers. Anti-psychotics have had such an impact in some people's stutter that it caused large changes in their lifestyles.