Zimbardo realized that rather than a neutral scenario, he created a prison much like real prisons, where corrupt and cruel behavior didn't occur in a vacuum, but flowed from the rules and principles of the institution to the people who carried out those principles. The behavior of the guards and prisoners wasn't dictated by some inherent internal trait, but by the situation they were in. The theory that external circumstances are the primary drivers of human behavior is known as situationist theory. Zimbardo didn't form the theory, but his experiment and later writings helped popularize it.
Experimental ethics are also an issue to consider. The Stanford Human Subjects Review Committee and Zimbardo's superiors approved the experiment — another layer of authority complicit in the experiment's outcome (it's like situationist "Inception") — but experimental ethics are more rigorous today. A modern institutional review board would likely never approve such an experiment without major modifications. Zimbardo has said that he feels the initial experiment was ethical (all the participants understood what they'd signed up for and consented), but that he suffered an ethical lapse when he allowed it to continue beyond the first emotional breakdown of a prisoner [source: Stanford Prison Experiment].
The idea that humans have an inherent tendency toward abuse of authority and submission to authority is not ruled out by the experiment, however. The Stanford Prison Experiment is closely related to another psychological experiment that's as infamous: Stanley Milgram's obedience to authority experiment. In fact, Zimbardo and Milgram were high school classmates and colleagues at Yale University. In the obedience experiment, volunteers were directed to press buttons delivering increasingly powerful, and eventually fake lethal shocks to another person at the direction of a researcher. A large percentage of volunteers went along with the researcher's demands. However, like the Stanford Prison Experiment, the ethics, methodology and conclusions of Milgram's experiment have been called into question recently. And both experiments influenced changes in the regulation and ethical guidelines of studies with human subjects [sources: Zimbardo et al., Defiesta].
Zimbardo's conclusion was that we are not so much inherently "evil," but that we will commit heinous acts if encouraged to do so by systems that enable or encourage them. He took his results to the U.S. House of Representatives shortly after the experiment ended, testifying before a subcommittee on prison reform. His primary argument was that given the power institutions have to dictate the behavior of the people within them, it's necessary to reform those institutions to avoid those abuses. He suggested better training and pay for guards, better protection for prisoners' human rights, and specific training programs that could include role-playing scenarios to help guards learn to deal humanely with prisoners (and weed out the most sadistic among guards) [source: House of Representatives]. The 2003 scandal surrounding prisoner treatment at the Iraqi prison Abu Ghraib, which mirrored the actions of Zimbardo's guards in disturbing ways, suggests that the experiment is not a relic of the past but still relevant to the way people are treated in modern civilian and military prisons.
Zimbardo's testimony also reflected his belief that researchers should not remain impartial observers, but should engage in social and political ways to act on the discoveries they make and seek ways to improve the world [source: Zimbardo et al.].