I'm no expert but I'm guessing there are many causes. I do know that my brother-in-law (52 yrs.) had stuttered most of his life until he took special therapy to control it about 15 years ago. They traced it back to the fact that he was born left-handed but was trained/forced by his parents to use his right hand as a baby. I don't know if the stress of this event caused his stuttering or if his brain function was modified by the force of becoming right handed as a baby caused the stuttering.