A psychologist/speech pathologist from the University of Iowa named Wendell Johnson and Mary Tudor, one of his students, conducted The Monster Study in 1939. (The study's name was given by Johnson's peers; read more to find out why!) Johnson was trying to find out if praise would reduce stuttering in children, and, conversely, if ridicule would induce stuttering.
Oh, it gets better.
The experiment was carried out over 6 months on 22 subjects, ages 5-15, who lived in an orphanage in western Iowa. (Gets around that annoying "parental consent" issue, decades before the phrase was even uttered.) 10 children in the experiment stuttered. They were separated into two groups: the experimental group, which was told that they did not stutter, and the control group, which was told that they did. Another 12 children who spoke normally were also studied: half of them were told that their speech was fine, and the other half were told that they stuttered (even though they didn't). I think the article itself sums the results up pretty well: "Letters between Mary Tudor and Wendell Johnson that were written shortly after the experiment ended showed that the children's speech had deteriorated significantly. Mary Tudor returned to the orphanage three times to try and reverse the negative effects caused by the experiment but lamented the fact that she was unable to provide enough positive therapy to reverse the deleterious effects."
So, I think that I've learned two important things from Wikipedia this time around:
1. Being a douchebag to children does not make them stutter.
2. Being a ward of the state sucks.