There is a genetic component to it, but I'm the only person in my family who stutters. We believe it's due to stress in early childhood (which my brother suffered as well, but he speaks fine and I'm not). There was another stutterer in my class, on different occasion (small town) me and my family met his mom and paternal grandparents - all of them speak fine. If I would have children and they would stutter I would feel more prepared and ready to act than my family was.