it's impossible to prove a negative (e.g., you don't get stuttering from x y or z). However, from what we do know about stuttering, it emerges between the ages of 3-5. 75% of preschoolers who stutter recover without intervention within 2 years of onset. Those who go on to persist to stutter into adulthood typically stutter throughout their lives. So while we can't say that bullying does not cause stuttering, it's difficult to make the case that it does. Most 3-5 year olds are not subjected to bullying. What I think you're getting at is the reaction to stuttering. A person's reaction to stuttering can certainly be influenced by bullying (at least in school age children). But in this case it's not that the bullying is causing the stuttering, but it's increasing the negative reaction to stuttering.