I don't think it's stress and/or anxiety it's more like situational awareness, at least in my case, if that makes any sense. I used to think if I could be as anxious when talking with my boss as I am with my friend I'd stutter equally as much or not at all. Got me some Xanax, never felt more relaxed in my entire life and still managed to stutter the way I usually do. But I guess it's better to stutter and don't give a damn about it than stutter and feel some anxiety.