The Stanford prison experiment was a landmark study — and likely a deeply flawed one

In 1971, the psychologist Philip Zimbardo conducted the Stanford prison experiment: a simulated prison in the basement of Stanford's psychology building, in which student volunteers were randomly assigned to be 'guards' or 'prisoners.' The study was meant to run two weeks, but it was terminated after just six days, as guards reportedly became abusive and prisoners became distressed.

For decades the experiment was taught as a landmark demonstration of the power of situations: that ordinary people, placed in a role of authority, could become cruel. It became one of the most famous studies in psychology, cited in textbooks and popular culture as evidence that the situation, not the person, drives behavior.

But in later years, serious doubts emerged. Reporting and archival research revealed that Zimbardo and his team had coached the guards on how to behave, that some participants said they were acting, and that the study lacked proper controls — problems known as 'demand characteristics,' where subjects do what they think the researcher wants. The Stanford prison experiment became a cautionary tale not about human nature, but about research methodology: how a dramatic, compelling study can be built on shaky foundations and still shape a field for decades.