There has been controversy over both the ethics and scientific rigor of the Stanford prison experiment since nearly the beginning, and it has never been successfully replicated. French academic and filmmaker Thibault Le Texier, in a 2018 book about the experiment, Histoire d'un mensonge ("Story of a lie"), wrote that it could not be meaningfully described as an experiment and that there were no real results to speak of. In response to criticism of his methodology, Zimbardo himself has agreed that the SPE was more of a "demonstration" than a scientific "experiment":
From the beginning, I have always said it's a demonstration. The only thing that makes it an experiment is the random assignment to prisoners and guards, that's the independent variable. There is no control group. There's no comparison group. So it doesn't fit the standards of what it means to be "an experiment." It's a very powerful demonstration of a psychological phenomenon, and it has had relevance.
In 2018, in response to criticism by Le Texier and other, Philip Zimbardo wrote a detailed rebuttal on his website. In his summary, he wrote:
I hereby assert that none of these criticisms present any substantial evidence that alters the SPE's main conclusion concerning the importance of understanding how systemic and situational forces can operate to influence individual behavior in negative or positive directions, often without our personal awareness. The SPE's core message is not that a psychological simulation of prison life is the same as the real thing, or that prisoners and guards always or even usually behave the way that they did in the SPE. Rather, the SPE serves as a cautionary tale of what might happen to any of us if we underestimate the extent to which the power of social roles and external pressures can influence our actions.
Treatment of "prisoners"
Some of the guards' behavior allegedly led to dangerous and psychologically damaging situations. According to Zimbardo's report, one third of the guards were judged to have exhibited "genuine sadistic tendencies", while many prisoners were emotionally traumatized, and three of them had to be removed from the experiment early. Zimbardo concluded that both prisoners and guards had become deeply absorbed in their roles and realized that he had likewise become as deeply absorbed in his own, and he terminated the experiment.
Ethical concerns surrounding the experiment often draw comparisons to a similar experiment, conducted ten years earlier in 1961 at Yale University by Stanley Milgram.
Reliance on anecdotal evidence
Because of the nature of the experiment, Zimbardo found it impossible to keep traditional scientific controls in place. He was unable to remain a neutral observer, since he influenced the direction of the experiment as the prison's superintendent. Conclusions and observations drawn by the experimenters were largely subjective and anecdotal, and the experiment is practically impossible for other researchers to accurately reproduce. Erich Fromm claimed to see generalizations in the experiment's results and argued that the personality of an individual does affect behavior when imprisoned. This ran counter to the study's conclusion that the prison situation itself controls the individual's behavior. Fromm also argued that the amount of sadism in the "normal" subjects could not be determined with the methods employed to screen them.
Coaching of "guards"
Carlo Prescott, who was Zimbardo's "prison consultant" during the experiment by virtue of having served 17 years in San Quentin for attempted murder, spoke out against the experiment publicly in a 2005 article he contributed to the Stanford Daily, after he had read about the various ways in which Zimbardo and others used the experiment to explain atrocities that had taken place in real prisons. In that article, entitled "The Lie of the Stanford Prison Experiment", Prescott wrote:
Ideas such as bags being placed over the heads of prisoners, inmates being bound together with chains and buckets being used in place of toilets in their cells were all experiences of mine at the old "Spanish Jail" section of San Quentin and which I dutifully shared with the Stanford Prison Experiment braintrust months before the experiment started. To allege that all these carefully tested, psychologically solid, upper-middle-class Caucasian "guards" dreamed this up on their own is absurd. How can Zimbardo and, by proxy, Maverick Entertainment express horror at the behavior of the "guards" when they were merely doing what Zimbardo and others, myself included, encouraged them to do at the outset or frankly established as ground rules?
Like Zimbardo, Prescott has spoken before Congress on issues of prison reform.
(Zimbardo, in his 2018 response, wrote that, though Prescott attached his name to the article, it was in fact written by Hollywood writer/producer Michael Lazarou, who had unsuccessfully tried to get film rights to the Stanford prison experiment story, and when he was turned down began to publicly criticize it.)
In 2018, digitized recordings available on the official SPE website were widely discussed, particularly one where "prison warden" David Jaffe tried to influence the behavior of one of the "guards" by encouraging him to "participate" more and be more "tough" for the benefit of the experiment. In his 2018 response, Zimbardo wrote that the instructions they gave to the guards were "mild compared to the pressure exerted by actual wardens and superior officers in real-life prison and military settings, where guards failing to participate fully can face disciplinary hearings, demotion, or dismissal."
Implied demands by Zimbardo
The study was criticized in 2013 for demand characteristics by psychologist Peter Gray, who argued that participants in psychological experiments are more likely to do what they believe the researchers want them to do, and specifically in the case of the Stanford prison experiment, "to act out their stereotyped views of what prisoners and guards do." Gray stated that he did not include the experiment in his introductory textbook, Psychology, because he thought it lacked scientific rigor.
"John Wayne" (the real-life Dave Eshelman), one of the guards in the experiment, said that he caused the escalation of events between guards and prisoners after he began to emulate a character from the 1967 film Cool Hand Luke. He further intensified his actions because he was nicknamed "John Wayne" by the other participants, even though he was trying to mimic actor Strother Martin, who had played the role of the sadistic prison Captain in the movie. As he described it:
What came over me was not an accident. It was planned. I set out with a definite plan in mind, to try to force the action, force something to happen, so that the researchers would have something to work with. After all, what could they possibly learn from guys sitting around like it was a country club? So I consciously created this persona. I was in all kinds of drama productions in high school and college. It was something I was very familiar with: to take on another personality before you step out on the stage. I was kind of running my own experiment in there, by saying, "How far can I push these things and how much abuse will these people take before they say, 'knock it off?'" But the other guards didn't stop me. They seemed to join in. They were taking my lead. Not a single guard said, "I don't think we should do this."
In his 2018 rebuttal, Zimbardo wrote that Eshelman's actions had gone "far beyond simply playing the role of a tough guard", and that his and the other guards' acts, given "their striking parallels with real-world prison atrocities", "tell us something important about human nature".
Reporting and interpretation of outcome
Two students from the "prisoners" group left the experiment before it was terminated on the sixth day. Douglas Korpi was the first to leave, after 36 hours; he had a seeming mental breakdown in which he yelled "Jesus Christ, I'm burning up inside!" and "I can't stand another night! I just can't take it anymore!" His outburst was captured by a camera, and has become, in one commentator's words, "a defining moment" of the study. In a 2017 interview, Korpi stated that his breakdown had been fake, and that he did it only so that he could leave and return to studying. (He had originally thought that he could study while "imprisoned", but the "prison staff" would not allow him.) Zimbardo later stated that participants only had to state the phrase "I quit the experiment" in order to leave, but transcripts from a taped conversation between Zimbardo and his staff show him stating "There are only two conditions under which you can leave, medical help or psychiatric." In the 2017 interview, Korpi expressed regret that he had not filed a false imprisonment charge at the time.
In his 2018 rebuttal, Zimbardo noted that Korpi's description of his actions had changed several times before the 2017 interview, and that in Zimbardo's 1992 documentary Quiet Rage Korpi had stated that the experiment "was the most upsetting experience of his life".
Small and unrepresentative sample
Critics contend that not only was the sample size too small for extrapolation, but also having all of the experimental subjects be US male students gravely undercut the experiment's validity. In other words, it is conceivable that replicating the experiment using a diverse group of people (with different objectives and views in life) would have produced radically distinct results; that is, had the test subjects come from divergent socio-economic and psychological groups, different experimental results may well have resulted.
Researchers from Western Kentucky University argued that selection bias may have played a role in the results. The researchers recruited students for a study using an advertisement similar to the one used in the Stanford Prison Experiment, with some ads saying "a psychological study" (the control group), and some with the words "prison life" as originally worded in Dr. Zimbardo's experiment. It was found that students who responded to the classified advertisement for the "prison study" were higher in traits such as social dominance, aggression, authoritarianism, etc. and were lower in traits related to empathy and altruism when compared to the control group participants.