The Stanford Prison Experiment: Replications, Disputes and Different Results

The Stanford Prison Experiment is one of the best-known and most controversial experiments in the history of psychology. It was conducted in 1971 by the psychologist Philip Zimbardo, with the aim of studying the psychological effects of imprisonment and how authority can change people’s behaviour. This experiment was repeated many times over the following decades, with different results each time. Let us analyse the original experiment, its subsequent replications and what might explain the differences in the results.

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The Original Stanford Experiment (1971)

The original experiment took place in the basement of the psychology building at Stanford University. Twenty-four students took part in the experiment; they were randomly selected to play the roles of «prisoners» or «guards» in a simulated prison. The experiment was intended to last two weeks, but it was halted after just six days due to the violent and inhumane behaviour exhibited by the guards and the severe psychological distress suffered by the prisoners.

The guards quickly became authoritarian and violent, whilst the prisoners began to show symptoms of anxiety, depression and despair. The experiment demonstrated how power and roles can significantly influence behaviour, even in ordinary people without any particular influence from their external environment.

The conclusion reached was that people do not necessarily realise which actions are good and which are not. They distinguished between what was good and bad based on the emotion they felt during or after the act. For example, following an act of abuse of power by a group of people (guards), they experienced positive emotions and faced no consequences whatsoever. Just as they considered the experiment to be “fun”, without any punishment for their violent actions.

Repeats of the Experiment and Different Results

The Stanford experiment has been the subject of controversy and criticism over the years. The experiment has been replicated many times, with some studies reproducing the original conditions and others introducing modifications.

Repeat Experiment by the BBC (2001)

One of the best-known replications of the experiment was carried out in 2001 by the BBC, known as “The Experiment”. In this version, researchers Alex Hazlam and Stephen Richter undertook to repeat the experiment with stricter ethical guidelines and greater transparency. Interestingly, the results of this experiment were very different from those of the original Stanford experiment.

In the BBC experiment, the participants did not display the same degree of violent and authoritarian behaviour. The prisoners were able to organise a form of resistance, whilst the guards did not show the same inclination to abuse their authority. The results of this replication study showed that behaviour is not shaped solely by assigned roles, but also by group dynamics, personal identity and social norms.

Possible Explanations for the Differences

The differences between the original experiment and its replicates can be explained by various factors:

1. Ethical Standards and Awareness: Following the Stanford experiment, ethical standards for psychological experiments became much stricter. Participants in subsequent experiments were better informed and better prepared for what to expect, which may have led them to behave in a more conscious and controlled manner.

2. Specific Instructions and Roles: In the original experiment, the instructions given to the guards were vague, allowing them to act arbitrarily. In the repeat experiments, the guards were usually given clearer instructions on how to behave, reducing the likelihood of violence.

3. Social and Cultural Changes: Social attitudes towards power, violence and justice have evolved since the 1970s. Participants in subsequent experiments may have held different social values that influenced their behaviour.

4. Psychological Preparation and Support: In the follow-up experiments, participants often received psychological support and counselling, which protected them from developing extreme behaviours.


The Stanford experiment, despite the controversies and criticisms, remains one of the most significant experiments in the history of psychology, as it offers a deep understanding of how roles and authority can influence behaviour. However, replications of the experiment have shown that human behaviour is complex and cannot be predicted solely by social roles. Cultural changes, social norms and moral conscience also play an important role in how people behave when assigned positions of authority.

A comparison of the different findings highlights the complexity of human nature and underlines the need for a careful and ethical approach to the study of psychology.