I'm grateful to Ben McLeish for introducing me (and I suppose many others) to the Stanford Prison Experiment in his talk 'Out of the box' at The Friends Meeting House in Euston on Saturday 10/3/12. I will post more details of Ben's talk when available.
The experient used innocent volunteers to be both guards and inmates at a specially constructructed test prison. The outcomes astounded the experimenters and will astound you.
This link goes to the conclusions section of the experiment's website.