Stuttering is definitely linked to self-confidence if I go by experience. The only times where I stutter now (cause I've got it mostly under control) is when a situation intimidate me in some way. So yeah I think seeing a psychologist for things like social anxiety could help some people much more once they have some techniques to help with the stuttering itself. I don't know if it's linked to a particular disease (I don't think so) but it's a neurological problem too so it could be.