The Monster Study is the name of a stuttering experiment conducted by Wendell Johnson and Mary Tudor. Johnson wanted to show that the existing theories concerning language and stuttering were wrong because in the 1930 stuttering was thought to have an organic or genetic cause. He wanted to show the world that labeling children as stuttered can make their condition worse.
There were 22 subjects selected and 10 of them had speech problems. Five of the children were in group IA, who were told that their speech was fine and the other five were put into group IB, being told that their speech is “as bad as people say”. Another group of six children formed group IIA and they were told that their speech was not normal and needed improvement. The last group was IIB, the normal group.
The study lasted about 5 months and it seems that the ones who got praises managed to improve their stuttering while those who were constantly criticized stopped talking or became real stutters. People who worked at the orphanage were recruited and not informed about the real motives behind the study. They were used to make the labeling even more real among the orphans. After the experiment, the researchers tried to minimize the damage they done, but with little results.
The children were also never told that they were part of a study until 60 years later when there was an article about it in the newspaper.
For further reading about stuttering and this study you should check out this link.