The Monster Study.
In 1939, Wendell Johnson of University of Iowa and his team hoped to discover the cause of stuttering by attempting to turn orphans into stutterers. Out of 22 young subjects, 12 of them were non-stutterers.
In the research, half of the group were exposed to positive teaching while the other group dealt with negative reinforcement. The teachers continually told the latter group that they had stutters. Notably, no one in either group became stutterers at the end of the experiment, but those who received negative treatment did develop many of the self-esteem problems that stutterers often show. The author’s interest in this experiment had to do with his own stutter as a child.