We all know the story of the Stanford Prison Experiment. It has been a staple of introductory psychology textbooks and lectures for nearly fifty years (see Griggs, 2014).

Ordinary young men were randomly divided into Prisoners and Guards; within a short time, the Guards become so brutal and the Prisoners so victimised that the study – originally scheduled for two weeks – had to be cut short after only five days.

These findings seem to demonstrate the terrifying power of the situation over individuals. Philip Zimbardo has always insisted that he had to do nothing to produce such toxicity. The participants simply slipped ‘naturally’ – and perhaps unconsciously – into their roles as vicious guards or broken prisoners.

But now, a half century later, dramatic new evidence has emerged that challenges Zimbardo’s account. Our textbooks and our lectures will have to be rewritten. The story of what happened in the SPE and why such brutality occurred will have to be retold.

Over the years, several scholars had expressed doubt over Zimbardo’s version. But the problem was that much of what actually happened in the SPE remained opaque and so the story in the textbooks went unchanged. We knew that the study had been recorded but the recordings were not in the public domain. And since evidence from the SPE was never actually published in a peer-reviewed psychology journal, methodological details of the study were also not in the scientific domain.

This all changed recently when both video and audio tapes were deposited in Zimbardo’s online archive at Stanford University. It includes 994 items of which 49 are videos and 54 are sound recordings. Together, the materials show that the textbook account is even more misleading than we imagined.

The startling new evidence tells a tale of the experimenters treating the Guards effectively as research assistants. It reveals how disturbed the Prisoners were when Zimbardo told them they could not leave the study. It raises profound intellectual, moral and even legal questions about what went on in that Stanford basement in the summer of 1971.

The most thorough analysis of the archive, by French author Thibault Le Texier is labelled Histoire D’Un Mensonge – which translates to “History of a Lie”. However, even if Zimbardo’s account of what happened in the SPE is, at best, economical with the truth, that still leaves the question of why some of his Guards turned brutal.

We have had the same burning question since we tried to replicate aspects of the SPE in the BBC Prison Experiment some 20 years ago and found that participants were most reluctant to adopt their assigned roles.

There are clearly many reasons why a research finding might fail to replicate. But we had always suspected that Zimbardo’s unusual leadership role in the SPE was central to understanding the results of his study. And in this respect, one item in the archive – a recorded meeting between Zimbardo’s Warden, David Jaffe and a ‘reluctant’ Guard, John Mark – proved to be conclusive, offering hard evidence that our suscipions were right.

You can listen to this interview – start after 8.38 minutes. The tape shows the leadership of the experimenters was at the core of the SPE. More specifically, it provides evidence of identity leadership. That is, Zimbardo and his colleagues sought to ensure conformity amongst the Guards by making brutality appear necessary for the achievement of worthy ingroup goals, namely science that would make the case for prison reform. “What we want to do”, Zimbardo’s Warden told the Guard, “is be able to go to the world with what we’ve done and say “Now look, this is what happens when you have Guards who behave this way … But in order to say that we have to have Guards who behave that way.”

But you don’t need to take our word for it – we strongly advise that you listen to the tape and decide for yourself whether this supports Zimbardo’s argument that Guards slipped naturally into role, or whether it supports our argument that leadership was necessary to turn the Guards into brutes.

We suspect that, after hearing the tape, you will never think about the SPE in the same way again. We suspect that you will also see how important it is for the experimenters to acknowledge their role as the leaders of a toxic social system. But just to make sure, we are currently writing up our analysis of the Jaffe-Mark meeting for publication in an academic journal to highlight the importance of an identity leadership analysis of the SPE.

The tape joins other evidence which indicates that the experimenters intervened to shape the study more than they acknowledged. For instance, there was video evidence of Zimbardo addressing his Guards before the study started. Then there were letters, magazine articles, interviews with past participants and Zimbardo’s associates (notably his ‘consultant’ Carlo Prescott) which made stronger claims about intervention. But they were little more than hearsay, rumour. Until now, these seemed of inadequate weight to overturn one of the ‘monuments’ of our discipline. Zimbardo could swat them away like gnats on the back of an elephant. But the new evidence from the archives will finally allow readers to listen to the evidence themselves, rather than rely on the experimenter’s story.

How has Zimbardo responded this time? By reasserting that 'none of these criticisms present any substantial evidence that alters the SPE’s conclusion'. And at the same time that he berates his critics (without engaging with their arguments), he reworks his story to now say that, yes, Guards were told to be tough, but not how to be tough. For Zimbardo, then, this is all just fake news. Except that it plainly isn’t.

This might have worked in the past, but now the necessary evidence is available to anyone who wishes to spend a few minutes listening to it. There is no longer any excuse for repeating a story which is so deeply flawed. We need to get busy rewriting our texts and revising our lectures.

Stephen Reicher (University of St Andrews)

S. Alexander Haslam (University of Queensland)

Jay Van Bavel (New York University)

EDITOR'S NOTE: A different version of this letter was published in the August print edition, under the headline 'Breaking free from Stamford'. You can find this in the PDF version below.

Further reading

Griggs, R.A. (2014). Coverage of the Stanford Prison Experiment in introductory psychology textbooks. Teaching of Psychology, 41, 195-203.