Pay attention to other people when they speak. Most stammer occasionaly but they dont even tend to notice(it drives me nuts). They stumble on words but when they recall the moment they dont have the stutter memory. Meanwhile we only have the stutter memory so we go down a path of self pity/loathing we get embarrased and the next time we open our mouths we stutter even more. more shame =more stuttering and more stuttering = more shame its a hellloop. I never noticed non stutterers expiriencing blocks. As to your question. The science behind it is a mistery. Lee Lovett(google him) believes its only psychological and coaches people for free. Tons of success stories from him. Meanwhile studies show one day its a damaged brain the other day its only psychological and it changes always . Meaning the root is unknown.