Explainer: The Stanford Prison Experiment – what happened and why it still matters?

What shapes human nature? Why do some people act with compassion while others turn cruel? For centuries, thinkers — from philosophers and playwrights to theologians and psychologists — have grappled with these timeless questions, searching for what lies at the heart of good and evil.

American psychologist Philip Zimbardo was one of the questioners, too. 

In August 1971, Zimbardo led a study at Stanford University to examine how social roles influence behaviour. Funded by the U.S. Office of Naval Research, the experiment created a simulated prison in the basement of a campus building and assigned 24 healthy college students to act as either guards or prisoners, according to Britannica.

The aim was to observe the psychological effects of perceived power. The experiment was scheduled to last two weeks, but was abruptly ended after just six days due to escalating psychological abuse and emotional breakdowns.

How was it set up?

More than 70 students applied to take part in the study. 

Those selected were paid $15 per day and randomly assigned roles as guards or prisoners. The "prisoners" were mock-arrested by real police, blindfolded, and delivered to the faux prison. 

There, they were stripped of personal identity, dressed in smocks, given ID numbers, and had one foot chained.

The "guards" wore khaki uniforms and mirrored sunglasses to avoid eye contact. 

While they were instructed not to use physical violence, they were given authority to maintain order. The entire environment was designed to simulate the oppressive conditions of a real prison.

What went wrong?

By the second day, prisoners rebelled. 

Guards quickly developed a system of punishments to maintain control. Over the next few days, their behaviour became increasingly cruel, ranging from psychological manipulation to dehumanisation. 

Prisoners experienced emotional distress; three had to be released early due to breakdowns.

Zimbardo, who also acted as prison superintendent, did not intervene—until a visiting researcher objected. 

Shocked by what she saw, she urged Zimbardo to end the study, which he did on the sixth day.

Why is it controversial?

The Stanford Prison Experiment drew immediate criticism for its ethical shortcomings. Participants suffered real emotional trauma, and critics argued that the conditions were poorly controlled and that Zimbardo had allowed the abuse to escalate.

Zimbardo later claimed that the guards' behaviour was driven by the situational pressures and lack of accountability—what psychologists call deindividuation, where individuals lose their sense of personal responsibility in group settings.

However, other researchers argued the experiment was flawed from the start. 

Some pointed out that the call for volunteers attracted people with authoritarian tendencies. 

In 2002, the BBC conducted a similar experiment where participants resisted tyranny rather than embraced it.

What is deindividuation?

Deindividuation theory suggests that in groups, people are more likely to act impulsively or aggressively when they feel anonymous and unaccountable. 

Zimbardo and others argued that anonymity, diffused responsibility, and heightened arousal—such as being in a group—can lower self-awareness and reduce concern for consequences.

Later research refined this idea, showing that deindividuated people don’t always become violent—rather, they tend to conform more strongly to group norms, whether positive or negative.

What impact did the experiment have?

Despite ethical backlash, the Stanford Prison Experiment remains one of the most cited studies in social psychology. 

It influenced how psychologists and ethicists think about power, identity, and institutional settings. It also prompted tighter regulations on human research ethics in the U.S.

The study inspired books, documentaries, and films—including Das Experiment (2001), its American remake The Experiment (2010), and The Stanford Prison Experiment (2015), which featured Zimbardo’s input.

Why does it still matter?

The experiment continues to shape discussions on institutional abuse, especially in prisons, the military, and law enforcement. It serves as a cautionary tale about unchecked power, situational influence, and the ease with which ordinary people can commit extraordinary acts under the right conditions.

Zimbardo himself later reflected, “The line between good and evil is permeable. And almost anyone can be induced to cross it when pressured by situational forces.”

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