The Stanford Prison Experiment

That was the question that an experiment in 1971 set out to answer when Stanford University professor and psychologist Philip Zimbardo used funding provided by the U.S. Office of Naval Research to create a make-believe prison in the basement of a university building. Male students were offered the chance to take part in the two-week experiment (and to make $15 per day). All applicants were carefully screened to ensure that they were healthy, emotionally and psychologically stable. 24 men were selected out of 75 that had applied. These men were then randomly assigned to play the role of either prisoners or guards in the "prison."
Initially, nobody was particularly concerned: after all, these were stable, intelligent young men who understood that they would be playing a role for just two weeks. Some people even considered that the experiment was pointless because everyone involved knew that they were simply acting roles for a limited time, so they couldn't be expected to behave in the ways that real prisoners and guards did.
Nonetheless, the experiment began on August 15th, 1971, and just six days later, on August 20th, it was abruptly and unexpectedly terminated after the "guards" had become brutal and sadistic and the "prisoners" had become withdrawn, fearful, and apathetic. There were real concerns that someone might end up being seriously hurt or suffer long-term psychological damage.
How could a group of nice, healthy, intelligent young men suddenly transform into sadistic thugs? How could another equally intelligent group of young men suddenly become apathetic victims of this brutality? What did this say about the malleability of human behavior, even when everyone involved knew that this wasn't real?
The Stanford Prison Experiment has become a classic in psychology, but its results were so startling that they received much wider interest from people trying to understand the nature of good and evil. If you put a good person in a bad place, perhaps the outcome will also inevitably be bad? That was the conclusion that Phillip Zimbardo drew from the experiment, but not everyone accepted this, and some even believed that Zimbardo had rigged the experiment to make the outcome as dramatic as possible. These doubts have ensured that although this has become one of the best-known psychological experiments ever, it isn't mentioned in many psychological textbooks and some people refuse to teach it because of concerns about its honesty.