Today we understand that stuttering has many possible causes. It may run in some families, an inherited genetic quirk of the language center of the brain. It may also occur because of a brain injury, including stroke or other trauma. Some young children stutter when they're learning to talk, but outgrow the problem. In some rare instances, it may be a side effect of emotional trauma. But you know what it's not caused by? Criticism.
In 1939 Mary Tudor, a graduate student at the University of Iowa, and her faculty advisor, speech expert Wendell Johnson, set out to prove stuttering could be taught through negative reinforcement -- that it's learned behavior. Over four months, 22 orphaned children were told they would be receiving speech therapy, but in reality they became subjects in a stuttering experiment; only about half were actually stutterers, and none received speech therapy.
During the experiment the children were split into four groups:
- Half of the stutterers were given negative feedback.
- The other half of stutterers were given positive feedback.
- Half of the non-stuttering group were all told they were beginning to stutterer and were criticized.
- The other half of non-stutterers were praised.
The only significant impact the experiment had was on that third group; these kids, despite never actually developing a stutter, began to change their behavior, exhibiting low self-esteem and adopting the self-conscious behaviors associated with stutterers. And those who did stutter didn't cease doing so regardless of the feedback they received.