Research seems to suggest that a combination of factors is involved. Stammering is at root a neurological condition, based in the wiring of the brain. Studies have shown differences in the anatomy and functioning of the brain of those who stammer compared with most other people.
Genetics are relevant, at least in many cases. Someone with stammering in the Family seems more likely to develop a Stammer Themselves.
As an issue that affects communication, Stammering can have a Deep and Lasting Psychological Impact - which in turn can Affect and Aggravate Stammering.