I think psychology plays an enormous enough roll but I can't logically conclude it's all psychological. It shouldn't be an issue to begin with. Many people face psychological hurdles of every which magnitude all their lives yet they do not block. They, at most, stumble on something, shake from nerves, but are able to talk. We, on the other hand, do not stop at one word or 2. Once it begins, even if we are passed the word, we likely still continue stuttering from the onset until it is over. "It" being that which started from the onset. Whether it came during a certain conversation point like asking to repeat something or asking your name, or came from someone walking by, once the onset begins, unlike others, we tend to be entrapped with the faulty mechanism so to speak. Psychological issues, nerves, etc, doesn't bring that kind of long lasting impact for the masses.