It's definitely not fear or anxiety that causes stuttering. Those are natural human emotions and that would mean everyone would stutter. I stutter sometimes alone talking to my cat, even though it's way smoother and my hearts not beating out of my chest but it still happens with no fear or anxiety in the situation. Only time I truly don't is when im singing, it's freaking gone! its awesome... But man I wish we could get this shit figured out. There's barely any knowledge in this world about stuttering.