Because it was a field experiment, it was impossible to keep traditional scientific controls. Zimbardo was not merely a neutral observer, but influenced the direction of the experiment as its "superintendent". Conclusions and observations drawn by the experimenters were largely subjective and anecdotal, and the experiment would be difficult for other researchers to reproduce.

One of the most abused prisoners, #416, and the guard known as "John Wayne", who was one of the most abusive guards, confront each other in an "encounter session" two months later.

Some of the experiment's critics argued that participants based their behaviour on how they were expected to behave, or modelled it after stereotypes they already had about the behaviour of prisoners and guards. In other words, the participants were merely engaging in role-playing. Another problem with the experiment was certain guards, such as "John Wayne", changed their behaviour because of wanting to conform to the behaviour that they thought Zimbardo was trying to elicit. In response, Zimbardo claimed that even if there was role-playing initially, participants internalized these roles as the experiment continued.

Additionally, it was criticized on the basis of ecological validity. Many of the conditions imposed in the experiment were arbitrary and may not have correlated with actual prison conditions, including blindfolding incoming "prisoners", not allowing them to wear underwear, not allowing them to look out of windows and not allowing them to use their names. Zimbardo argued that prison is a confusing and dehumanizing experience and that it was necessary to enact these procedures to put the "prisoners" in the proper frame of mind; however, it is difficult to know how similar the effects were to an actual prison, and the experiment's methods would be difficult to reproduce exactly so that others could test them.

Some said that the study was too deterministic: reports described significant differences in the cruelty of the guards, the worst of whom came to be nicknamed "John Wayne." (This guard alleges he started the escalation of events between "guards" and "prisoners" after he began to emulate a character from the Paul Newman film Cool Hand Luke. He further intensified his actions because he was nicknamed "John Wayne" though he was trying to mimic actor Strother Martin who played the role of the sadistic "Captain" in the movie.) Most of the other guards were kinder and often did favours for prisoners. Zimbardo made no attempt to explain or account for these differences.

Also, it has been argued that selection bias may have played a role in the results. Researchers from Western Kentucky University recruited students for a study using an advertisement similar to the one used in the Stanford Prison Experiment, with and without the words "prison life." It was found that students volunteering for a prison life study possessed dispositions toward abusive behavior.

Additionally, the sample size was very small, with only 24 participants taking part over a relatively short period of time. This reality means that it is difficult to generalise across a wider scale. Also, the sample selection only contained males, meaning that the sample was androcentric and thus exhibited a marked lack of representativeness.

Finally, the study was never published in a peer-reviewed journal. Lacking peer-review, it is difficult to interpret the meaningfulness of the results.

Haslam and Reicher

Alex Haslam and Steve Reicher (2003), psychologists from the University of Exeter and University of St Andrews, conducted a partial replication of the experiment with the assistance of the BBC, who broadcast scenes from the study as a reality TV program called The Experiment. Their results and conclusions differed from Zimbardo's and led to a number of publications on tyranny, stress and leadership (moreover, unlike results from the SPE, these were published in leading academic journals; e.g., British Journal of Social Psychology, Journal of Applied Psychology, Social Psychology Quarterly). While their procedure was not a direct replication of Zimbardo's, their study does cast further doubt on the generality of his conclusions. Specifically, it questions the notion that people slip mindlessly into role and the idea that the dynamics of evil are in any way banal. Their research also points to the importance of leadership in the emergence of tyranny (of the form displayed by Zimbardo when briefing guards in the Stanford experiment).^[4]