I don't think stutterers can completely stop stuttering, but it can be brought down. BTW even non-stutterers have imperfect speech at times and often trip on their words and make funny noises. How would a psychologist prove their theories? They can only prove it by examples of people who've overcome their stutters. A famous example is Rowan Atkinson. He still stutters, but he is able to speak fluently when he's acting on stage.