Think about walking a straight line on the ground. It's easy and you don't think twice about it. But if you’re put on a balance beam, walking the same straight line between two 100 meter tall buildings, it suddenly feels scary and you’d find it much harder to balance. The fear messes with your head and impairs your ability to perform an otherwise simple task. Even though you know how to walk, being up so high makes you nervous and shaky. Stuttering is a similar thing (for some of us, anyway). When you're alone or feeling safe, words can come out easily. But when you're in front of others and feel scared or worried about messing up, talking can feel as hard as walking on that high beam. In my experience, it’s an anxiety induced disorder and is closely tied to our self beliefs, however subconscious they may be.