The book Humankind by Rutger Bregman goes into the details and is a fascinating read. Psychologists Haslam and Reicher did a follow-up “BBC Prison Study” in 2002 to test some of Zimbardo’s findings, and they didn’t find any of the really problematic behaviors that Zimbardo found (many of which were more or less coached or coerced). So it’s not necessarily that the results were invalidated, per se, and more that Zimbardo’s conclusions are not as ironclad as he made them out to be in his original paper. They simply weren’t repeatable once basic ethical safeguards were put into place for the safety of the participants. It kinda speaks to the wild west era of psychological research in the mid 20th century where there were no rules and people were free to do all manner of fucked up things that researchers could never get away with today. In some ways that period is useful because they allowed us to test some of our more fundamental understandings without the limitations placed on us by modern liability and psychiatric/psychological protection, but it can’t really be overstated how much damage was done to some of the subjects of those studies. Our modern system has matured in such a way that findings can more systematically and rigorously be tested because standardized practices are the norm and study subjects have basic safeguards across various disciplines.
For what it’s worth, Haslam, Reicher, and Zimbardo put out a joint statement that addressed some of the controversy surrounding their more or less conflicting results which essentially boiled down to the conclusion that both experiments are valid, though each has significant differences and limitations.
My dad has had two stolen. He just puts another one up each time.