Zimbardo's own recordings and instructions to guards, later examined by historian Thibault Le Texier through the Stanford archives, show guards were directly coached toward the study's expected conclusions before the results were interpreted as a spontaneous emergent effect; the original study also had no control condition and was never formally peer reviewed, making it difficult to separate genuine situational influence from guided performance.
What would resolve this A controlled replication with genuinely blind, uncoached participant instructions and pre-registered analysis, which the ethical concerns the original study itself raised make difficult to conduct today.
OpenSocial psychology, research methodology, and the history of science.Wikipedia: Stanford prison experiment