The Stanford Prison Experiment

I just read Stanford psychologist Philip Zimbardo's 2007 book, The Lucifer Effect, which is the first book he's written on the 1971 Stanford Prison Experiment - one of the most famous experiments in psychology.
Zimbardo took 18 normal, healthy, middle class students, and made nine of them 'guards' and nine of them 'prisoners' in a mock prison set up in a basement in Stanford. The guards were given total control over the prisoners, although they weren't allowed to hurt them physically. The idea was to run the experiment for two weeks, to see how normal people respond to the situation of a prison (Zimbardo is a situationist psychologist, which means he thinks people often do things not because of their internal disposition or personality, but because of the pressures of the situation).
They had to stop the experiment after less than a week, when two of the nine prisoners had emotional breakdowns, and the guards became increasingly sadist, confining prisoners in solitary confinement for whole nights, stripping them naked, and even making them perform homoerotic acts - in a grim precursor to the abuse prisoners were subjected to at Abu Ghraib.
It is an astonishing book, one of the most interesting and shocking books of psychology I've read. You read Zimbardo's novelistic account of the experiment spiralling out of control, and you wonder to yourself, how would I have survived as a prisoner, how resilient would I be, how mentally strong. But you know, that's not really the question. The question is, how would I have survived as a guard? How brutal would I have been? Would I have done anything to stop the abuse?
Some of the 'guards' were less sadistic than others. But not a single guard did anything to stop the abuse...
You can watch an hour-long documentary about the experiment on YouTube. The first part of it is below.