In 1971 researchers at Stanford University created a simulated prison in the basement of the campus psychology building. They randomly assigned 24 students to be either prison guards or prisoners for two weeks.
Within days the "guards" had become swaggering and sadistic, to the point of placing bags over the prisoners' heads, forcing them to strip naked and encouraging them to perform sexual acts.
The landmark Stanford experiment and studies like it give insight into how ordinary people can, under the right circumstances, do horrible things — including the mistreatment of prisoners at the Abu Ghraib prison in Iraq.
What is the distance between "normal" and "monster"? Can anyone become a torturer?
Such questions, explored over the decades by philosophers and social scientists, come up anew whenever shocking cases of abuse burst upon the national consciousness, whether in the interrogation room, the police station or the high school locker room.
Hannah Arendt coined the phrase "banality of evil" to describe the very averageness of the Nazi leader Adolf Eichmann. Social psychologists pursued the question more systematically, conducting experiments that demonstrated the power of situations to determine human behavior.
Dr. Philip G. Zimbardo, a leader of the Stanford prison study, said that while the rest of the world was shocked by the images from Iraq, "I was not surprised that it happened."
"I have exact, parallel pictures of prisoners with bags over their heads," from the 1971 study, he said.
At one point, he said, the guards in the fake prison ordered their prisoners to strip and used a rudimentary sex joke to humiliate them.
Professor Zimbardo ended the experiment the next day, more than a week earlier than planned.
Prisons, where the balance of power is so unequal, tend to be brutal and abusive places unless great effort is made to control the guards' base impulses, he said. At Stanford and in Iraq, he added: "It's not that we put bad apples in a good barrel. We put good apples in a bad barrel. The barrel corrupts anything that it touches."
To the extent that the Abu Ghraib guards acted, as some have said, at the request of intelligence officers, other studies, performed 40 years ago by Dr. Stanley Milgram, then a psychology professor at Yale, can also offer some explanation, researchers said. In a series of experiments, he told test subjects that they were taking part in a study about teaching through punishment.
The subjects were instructed by a researcher in a white lab coat to deliver electric shocks to another participant, the "student."
Every time the student gave an incorrect answer to a question, the subject was ordered to deliver a shock. The shocks started small but became progressively stronger at the researcher's insistence, with labels on the machine indicating jolts of increasing intensity — up to a whopping 450 volts.
The shock machine was a cleverly designed fake, though, and the victims were actors who moaned and wailed. But to the test subjects the experience was all too real.
Most showed anguish as they carried out the instructions. A stunning 65 percent of those taking part obeyed the commands to administer the electric shocks all the way up to the last, potentially lethal switch, marked "XXX."
Dr. Charles B. Strozier, director of the Center on Terrorism and Public Safety at John Jay College of Criminal Justice in New York, said the prison guards in Iraq might feel that the emotions of war and the threat of terrorism gave them permission to dehumanize the prisoners.
"There has been a serious, siesmic change in attitude after 9/11 in the country in its attitude about torture," Dr. Strozier said, a shift that is evident in polling and in public debate. In the minds of many Americans, he said, "it's O.K. to torture now, to get information that will save us from terrorism."
Craig W. Haney, a professor of psychology at the University of California, Santa Cruz, who was one of the lead researchers in the Stanford experiment, says prison abuses can be prevented by regular training and discipline, along with outside monitoring.
Without outsiders watching, Professor Haney said, "what's regarded as appropriate treatment can shift over time," so "they don't realize how badly they're behaving."
"If anything," he said, "the smiling faces in those pictures suggest a total loss of perspective, a drift in the standard of humane treatment."
Experiments like those at Stanford and Yale are no longer done, in part because researchers have decided that they involved so much deception and such high levels of stress — four of the Stanford prisoners suffered emotional breakdowns — that the experiments are unethical.