Stuttering is caused by a neurological defect where the brain improperly processes its own speech. When we do not hear our own speech we stutter less. That is why we stutter less when we whisper, sing, talk in unison or use airpods with noise cancellation. This is why the McGuire Program is successful because we do not hear our own voice but hear a changed voice. This is also why the Edinburgh Masker and Delayed Auditory Feedback (DAF) are successful because they Mask or Delay hearing our own voice.