Prof.Sheehan has ever answerd this question and revealed the essence of stutter, but disproven by psychological and physiological evidences so maybe it's just his personal reasoning but it's helpful to me: Blocks are induced from your contradictory activities in your mind:fears of stutter and urges to speak, called 'contradictory theory'. Also I hope some enthusiastic researching folks can offer the sources of this theory's evidence for its not just being reasoned by Sheehan.