Here is an article from Professor William Webster at BSA. Essentailly, this write up shows how genetic potential influences the functioning of our brain (interhemispheric interference), which leads to dis-fluency, which generates negative emotions- and how this becomes a learned behaviour over the years. Importantly, he also discusses how we can undo it: how our "thinking" can influence our brain. Very good write up if you want to understand why we stammer. Read