I was one of the worst cases. It was nearly impossible for others to understand me.
The school kids made my school years a living hell. I went through speech therapy twice a week for years with little improvement.
One day I suddenly stopped stuttering. I realized it was an epigenetic issue from my mother.
During WWII she escaped by a hair being caught hiding Jews from the Nazis.
When she told me the story, I never stuttered again.