In my opinion stuttering is almost 100% psychological there is no true biological answer as to why we stammer. All humans have an inbuilt mechanism that keeps us alive, our subconscious These are the thought processes that continually whirll away in our brains without our control. In my eyes, stammering is a fault in our subconscious for some reason we attribute speaking with a fear response and this makes us tense up and block on the word. The only way to truly stop stammering is to some how control this fear response. I am still working on it.... but I know for sure with time and practice it is possible