the evil that good people can be readily induced into doing to other good people within the context of socially approved roles, rules, and norms, a legitimizing ideology, and institutional support.
In other words, a person's goodness or evilness can be dramatically influenced by situation.
The first thing you should notice: my phrasing. "...the point of which was to show X." Not to study X, not to determine if X was true.
Subjects were randomly given a guard or prisoner role in a fake jail. It was supposed to last 2 weeks. He aborted it in 6 days because "too many normal young men were behaving pathologically as powerless prisoners or as sadistic, all powerful guards."
Second thing to notice: his phrasing. "...normal... behaving pathologically... powerless... sadistic..."
About his conclusions, they may imply that men are born innately good, but can be changed; or born innately bad, or born as blank slates-- but how they're born doesn't seem to matter. They can be changed, and, apparently, without too much effort. No Milgram authority figure; simply the right circumstances, $15 and a uniform.
You may want to ponder what this implies about the trivial characteristics we long to be genetic and fixed-- extroversion, anxiety, the tendency towards spending sprees-- when one's entire moral compass can be spun full around in the course of a Hampshire Halloween.
The essential conclusion to be drawn from this study-- the one everyone draws all the time-- is that this can happen to anyone. "Normal" people were incited towards evil. 75 people applied; Zimbardo chose the 24 for the study who were "judged to be the most stable (physically and mentally), most mature and least involved in antisocial behavior." Also, they were Stanford students, right? Not from ASU. (zing.) Didn't matter. It's probably not necessary to point out how important this study is in psychology and the conventional wisdom. You can't have discussion about a group atrocity without this study being invoked.
There may be another explanation, and as soon as I start to write it you'll guess the rest. Zimbardo recruited subjects through a newspaper ad that said
male college students needed for a psychological study of prison life. $15/day for 1-2 weeks
It's a legitimate question: what kind of a nut signs up for that?
There's an answer. In a follow up experiment in 2007 designed specifically to answer that question, two ads were placed in newspapers, one recruiting "male college students needed for a psychological study. $70/day for 1-2 weeks" and the other, slightly different ad recruiting for "a psychological study of prison life. $70/day for 1-2 weeks."
The subjects weer screened with personality inventories, and, surprise, "prison study" recruits scored significantly higher on narcissism, social dominance, aggression, Machiavellianism and authoritarianism (but especially the first three.)
When you do a study, you get what you pay for.
Zimbardo thought he was showing how a "normal" person might be made evil. He could have asked what might make a "normal" person become a passive, beaten victim-- it was the same experiment. Presumably the same forces are at work, but it is easier to believe that the distance between normal and victim is shorter than normal to evil.
But if you accept, or at least seriously consider, that there was a selection bias in the recruitment of Zimbardo's study, then there's a new finding to ponder, and I don't really know what it means.
No surprise that a group of aggressive/narcissistic people who decide they want to be part of a prison experiment need little prompting to turn into viscous guards-- but, apparently, these same aggressive/narcissistic people can just as easily be made into submissive prisoners. You might think they'd rebel more, fight back more, but apparently not.
Maybe the the real conclusion isn't how easily people with those characteristics can be pushed into an aggressor or victim status, but their tendency to identify with a group, whatever it may be.