I've been doing some research into the causes of stuttering and came across this great article: https://academic.oup.com/brain/article/132/10/2747/330765 TL;DR - Stuttering comes from an underdeveloped region of grey matter in the brain. It's a very interesting read, but very dense and I'm at work so I can't give a full synopsis. But I am working on a book that will hopefully do a much better job of explaining this study and what it means.