Maybe I'm splitting hairs, but ... For adults who stutter the vast majority have atypical brain activity during speech production and language processing tasks compared to adults who do not stutter. In addition there are differences in brain structure in adults who stutter and school-age children who stutter. When stuttering is in it's "advanced" form there is something fundamentally different about the way the brain generates speech. That said, we do not know whether this atypical activity and structural changes are the cause of stuttering or the result of compensatory changes attempting to manage the disorder. For the adult it doesn't matter--adults who stutter have atypical brain activity. Yes, stuttering is more likely to occur in social environments. But this increase in severity and frequency is not "stuttering" it's "stuttering more often." Compare it to the basketball player shooting free-throws. Some players can sink one after another in practice, but in a game they "choke." Our mental state can influence the frequency of stuttering just as it can influence the frequency of missed free throws. It does not mean that our mental state causes stuttering or missed free throws