So far from the journal articles and studies I’ve read, the root cause (or one of the root causes) of stuttering is an imbalance of the neurotransmitters (including dopamine) in the basal ganglia area of the brain that’s responsible for speech-motor control. Because of this, our internal “timing cue” for verbal speech is thrown off and difficult for us to control. This is also the reason why stutterers tend to not stutter at all, or stutter very little, when speaking simultaneously in chorus with one or more other people (where their voices act as an external timing cue to assist us).