The famous Stanford Prison Experiment in 1971 proved that everyone has the potential to become a monster. The scandal of Abu Ghraib finally prompted the man who devised the study to write of his experience. Karen Gold reports

Philip Zimbardo's freshly laundered lab coat gleamed as he walked towards the rat cages in New York University's experimental psychology lab. For the first time since he began a PhD his parents were to visit his hard-won academic milieu, a million metaphorical miles from the South Bronx tenement home where he was brought up. He hoped the laboratory, where he was learning to breed and test rats for enhanced curiosity, would impress them. Instead, his mother took one look and fled screaming. "I had forgotten how much she hated ratsbecause we always had them where we lived," he says. When he asked what was wrong she turned on him and exploded: "Have you gone crazy? You're not supposed to make rats smart, you're supposed to exterminate them. The last thing we need is smart rats."

It was not the first time Zimbardo the achiever had let showmanship outweigh humanity and common sense, and it was not to be the last. Throughout his early academic career, ascending from Yale to New York University to Stanford, this son of an unemployed Sicilian barber regularly conjured up social psychology experiments featuring hooded testers, electric shocks - some fake, some real - and a Lord of the Flies perspective on how far human beings could be pushed, or push themselves. It was only a matter of time before things caught up with him.

In the heady 1960s he was far from the only intellectual to be seduced by the sense of scope offered by studying the human mind. B. F. Skinner's rats were heading for perfectability; Stanley Milgram's notorious electric-shock obedience experiments and the Israeli state's Eichmann trial were plumbing human cruelty. The world was a psychological crucible, and US campuses its hotspots, with Zimbardo the teacher steering his students into non-violent protest and teach-ins against the Vietnam War. When he suggested investigating the antisocial effects of anonymity by creating a fake prison on campus in summer 1971, naturally the government's Office of Naval Research gave him the money.

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Looking back at the Stanford Prison Experiment, in which 18 middle-class young white men were randomly allocated to the role of guards or prisoners and for the next six days proceeded variously to humiliate, abuse, cringe from, bully and betray each other, what seems most striking is the lengths Zimbardo went to set it up. Not content with having the prisoners turn themselves in, he bribed the local police with TV coverage to have his "prisoners" officially arrested. He arranged uniforms, handcuffs, parole boards, letters home and chaplaincy visits. His junior researchers played officers in charge; he was the prison superintendent. "Of course the SPE was showmanship," he says. "We had costumes, we had a set. I prepared it as if it was a play or a movie. A lot of the research I do has dramatic flair, a dramatic quality. It's a dramatisation of psychological concepts."

But for guards and prisoners the dramatisation quickly became believable. One prisoner broke down after 36 hours, another after 48. (All had been ratified psychologically healthy beforehand). Due to last a fortnight, the SPE was halted after six days, when Zimbardo's Stanford colleague and future wife glimpsed the shuffling line of hooded, handcuffed "prisoners" in a hallway and, like his mother in the NYU lab, gave him a dose of reality.

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Horrified at his own complicity in prolonging the nastiness and deception, his repentance is nevertheless partial: "No one would let you do this kind of experiment now," he says. "Would I still do it if I could? Yes. It worked. With qualifications. I should have stopped it after 48 hours, when the second kid broke down. We'd demonstrated everything we wanted to by then."

What had he demonstrated? That, anonymised and subject to group conformity and non-restraining official structures, ordinary people were equally liable to both vicious aggression and self-confirming victimhood. It was a big result, and it caused a big stir. The SPE produced 12 hours of video and audio tapes; Zimbardo gave clips to TV companies as well as to researchers, prison campaigners and military academies. He wrote a couple of academic papers. And then?

Over the next three decades, Zimbardo the professor continued to pull in massive student audiences, lecturing to a thousand undergraduates at a time on introductory psychology, writing textbooks - 2.5 million sold - and becoming president of the American Psychological Association. His showmanship found respectable outlets: "My lectures have razzle-dazzle," he says. "Music, spinning cubes, steps descending into dungeons. I want to make my classes unique. Students are taking three or four classes a day; 30 years on, I want them to remember mine."

At the same time he set up a centre for studying shyness and programmes to help shy adults. He chose shyness to study next, he says, because it is a self-imposed prison, reminiscent of the one created inside the heads of the SPE "prisoners". If, in 1971, a young, highly ambitious academic carried out one of psychology's most famous and far-reaching experiments, why had he never written a book about it?

He had come close. Soon after the SPE ended, he sat down to transcribe the tapes. He had a title - Prisons of the Mind - chapter headings and a publisher's advance. Four years later he cancelled the book contract and sent the money back. "I became so depressed at how I had allowed the suffering to go on for so long," he says. "So I put the whole thing away." And there it might have stayed, if in 2004 a young army reservist had not blown the whistle on his fellow all-American soldiers serving as military police holding Iraqi civilians in Abu Ghraib prison.

Zimbardo was in Washington when the story broke, watching on TV, immediately thrown back 30 years. Within hours a former Stanford student, now a radio journalist made the same connection and rang him. Weren't the Abu Ghraib pictures very like footage from his professor's long-ago Stanford Prison Experiment, he asked.

One media appearance followed another; in the end Zimbardo did 40 in a fortnight. He coined a soundbite and stuck to it: the military claimed the Abu Ghraib soldiers who had set dogs on naked prisoners, pretended to electrocute them and forced them to simulate sex, were rotten apples. Might they not, asked the professor, like his nice Stanford boys instead be normal apples decaying in Iraq's rotten barrel?

He had the chance to answer that question when the legal team for one of the Abu Ghraib accused asked him to act as an expert witness. His first instinct was to say no. He wanted to be on the good guys' side this time. He tried to set up a fund for the whistleblower, now in protective custody and fearful for his life. No bank would take the money. In the end he said yes. It did no good. He read 11 government reports, viewed 2,000 images of pornographic violence instigated and recorded by these all-American girls and boys, learnt of the raw sewage, 40-day shifts and collective licence to which they were exposed, flew to a military base in Naples to give evidence by video link to the US court martial in Iraq... and the judge took an hour deciding to give "Chip" Frederick, his client, eight years. Frederick also lost his 22-year army pension, and was stripped of his nine service medals on the courthouse steps.

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This public humiliation, the vindictive official showmanship, seems to have resounded deep in Zimbardo. He had been there before. Back in the South Bronx ghetto, as a small child, he learnt first-hand about survival. "I was a skinny kid," he says. "I was never going to be a physical leader. But I could see where the power was and align myself with it. I would make up games and stories. The other kids would say: 'What are we going to play today Phil?'" Desperate to belong, he accepted initiation rituals - stealing fruit, climbing trees, looking up women's skirts - and hierarchies: "There were seven kids in the gang. The Disney film had just come out, so we called ourselves the Seven Dwarves. I was six years old. I was Dopey."

Since he was the youngest, the police would pick on him, banging his head against their squad car and throwing him inside it to make him tell them where the gang's toys were hidden. But above all he was shamed. Charity clothing social workers mocked him for pausing to choose between two pairs of second-hand trousers. The dentist - he tells this story, like everything else, in the present tense - allowed a trainee to break a tooth he was pulling out of his mouth, because "these people (poor people) couldn't be trusted not to fake pain". Now 74, Zimbardo still feels it: "I said to myself: what do I have to do to individuate myself? To prove I am not just one of these ignorant masses?"

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Shame, power, good guys, bad guys - the SPE had them all, and so did Abu Ghraib. Finally, 30 years on, he decided it was time to write his book. It was hard work. "I went back to the video tapes," he says. "As I'm watching them, I'm getting emotionally involved. I'm saying stop the study, it's done, you've got what you wanted, what's wrong with you? I'm getting angry with this Phil Zimbardo guy."

The theme of the book is that we must understand, judge and condemn not only individuals but also the situations and systems that created them. He can see that writing it was a kind of rehab: "In the SPE I was the system," he says, "I let it happen." But he also hopes to feed into the Zeitgeist. Law faculties now want to talk about how a judicial system could be more widely focused, he argues.

The book's website offers visitors the chance to vote on whether George W. Bush and Donald Rumsfeld should be held responsible for Abu Ghraib, and 85 per cent say yes, although wider public discussion of the book's political message in the US has so far been zero. The Lucifer Effect is immensely long, a blow-by-blow account of every SPE minute, and lots from Iraq too. The publishers tried to make him cut it. "I said no: the detail has to be in because I am never going to tell it again," Zimbardo says.

While he chose to frame the issues in terms of good and evil, because he thought that would make people read it, he has personally had enough of evil. Instead, he wants to concentrate on what makes a hero; what enables a very small number of people, like the Abu Ghraib whistleblower, to step away from the crowd.

Back in 1971, he couldn't step away. He still had hopes of the system. "The military told me they used the SPE tapes to train people not to behave like our guards," he says. "I used to quote that as a good outcome of the research. Now I know they also used it to train interrogators to break people. I had no idea they were doing that."

The Lucifer Effect by Philip Zimbardo is published by Random House, ?18.99.

Extracts from The Lucifer Effect

"The morning-shift guard trio decides that the prisoners must consider the guards too lax... They decide it is time to stiffen up. First, they institute a morning work period, which today means scrubbing down the walls and floors. Then, in the first stroke of their collective creative revenge, they take the blankets off the prisoners' beds in cells 1 and 2, carry them outside the building and drag them through the underbrush until the blankets are covered with stickers or burrs. Unless prisoners don't mind being stuck by these sharp pins, they must spend an hour or more picking outeach of them if they want to use their blankets.

"Prisoner 5704 goes ballistic, screaming at the senseless stupidity of this chore. But that is exactly the point. Senseless, mindless arbitrary tasks are the necessary components of guard power. The guards want to punish the rebels and also to induce unquestioning conformity.

After initially refusing, 5704 reconsiders when he thinks it will get him on the good side of Guard Ceros and gain him a cigarette, so he starts picking out the hundreds of stickers in his blanket. The chore was all about order, control, and power - who had it and who wanted it."

"The Stanford Prison experiment reveals a message we do not want to accept: that most of us can undergo significant character transformations when we are caught up in the crucible of social forces. What we imagine we would do when we are outside that crucible may bear little resemblance to who we become and what we are capable of doing once we are inside its network. The SPE is a clarion call to abandon simplistic notions of the Good Self dominating Bad Situations. We are best able to avoid, prevent, challenge and change such negative situational forces only by recognising their potential power to 'infect us', as it has others who were similarly situated."

The thoughts of Zimbardo's peeers

Ilona Boniwell , who has co-authored papers with Zimbardo relating to positive psychology: "Through his experiment we found out more about human nature than most other experiments ever conducted in the field of psychology, but it's impossible nowadays to run his experiment. You would never be able to get past an ethics committee."

Steve Reicher , head of the school of psychology at St Andrews University, and one of authors of The Experiment , televised by the BBC, which challenged Zimbardo's work: "Many critical details of the study are still not in the public domain, the research was never published in peer-reviewed journals and the ethical problems with Zimbardo's specific study have led him to argue that similar research never could be done again. The upshot is that we cannot address many key questions about the processes that produced them: why did only some guards act brutally and not others? Why did the brutal guards appear to exert authority over the others? Was it due to Zimbardo's own leadership in the study? Can his extreme situationism account for what happened in his study, let alone the wider phenomena he purports to explain? These questions are all the more urgent due to some of the political uses to which Zimbardo's work has been put, such as excusing abusers and torturers as helpless dupes of circumstance."

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Mark McDermott , reader in psychol-ogy at the Univer-sity of East London, who has worked both with Zimbardo and on the BBC experiment: "It's too easy to say that because the study took place x number of years ago it's no longer relevant and we have moved on. We would not have had Abu Ghraib if that were the case."