There's nothing wrong with stutterers' tongues or vocal folds or breathing. Stutterers are not more nervous, don't have worse self-esteem, and are not "psycho," as some movies have portrayed stutterers.

Stuttering is a developmental disorder. Some experts believe that stuttering develops from the normal mistakes all children make when learning to talk ("normal disfluencies"). While most children can pick themselves up after a stumble, some children get into a vicious cycle of trying harder to talk, tensing their speech-production muscles too much, and getting more stuck.

Other experts have found that severe stuttering can develop almost overnight in young children. They believe that stuttering may not develop gradually from normal disfluencies. Genes have been found associated with stuttering, so these experts believe that a genetic defect causes something in the child's brain to trigger stuttering.

Although the origin of stuttering is not clear, everyone agrees that childhood stuttering can develop into a severe physical and psychological disability. Adults who stutter can have physical symptoms, including:

• Breathing abnormalities during stuttering, especially upper chest tension.
• Laryngeal blocks, which cut off airflow during stuttering.
• Articulation problems, including tension in the lips, jaw and tongue, and prolonged or repeated sounds.
• Secondary or "escape" behaviors, such as head jerks, eye blinking, or facial grimaces.
• Neurological abnormalities visible via brain scans.

Adult stuttering psychological symptoms can include:

• Avoidance of feared sounds, words, and speaking situations. For example, the stutterer may avoid making telephone calls.
• Substitution of another word.
• "Anti-expectancy" speech behaviors to prevent stuttering such as speaking in a monotone, or affecting an accent.

Some stutterers are so good at avoidance that their co-workers and even their spouse or family doesn't know that the individual stutters. Even though their speech sounds fine, these "covert" stutterers can be crippled by severe psychological fear and anxiety.