If you're among the very high percentage (90% according to [this](http://www.mnsu.edu/comdis/isad10/papers/rasskazov10.html) particular study) of those who speak mostly fluently when alone, you can infer there is a significant social trigger that comes to play when stuttering. It may not be general social anxiety - one may feel very relaxed around others despite stuttering and still enjoy being around others. As you've pointed out, social anxiety may be a symptom of stuttering rather than a cause.   That said, how can you possibly discredit stuttering from being a psychological problem? Assuming you're part of this very large percentage that speaks fluently when alone, PWS (Person Who Stutters) are consciously aware when someone enters a room (or a phone call), which in turns triggers stuttering. Remove a person or any social interaction from the room, the stutter disappears. Since we've already established that most PWS have not suffered brain trauma and have no neurological abnormalities (otherwise they would not be able to speak thoughts and stutter as severely even when alone), what remains is a psychological awareness/trigger of being in a social situation and a corresponding negative psychological response.