Stanford Prison Experiment

Many people cite this study from the seventies - for those of you who've never heard of it, a psychology professor simulated a prison situation with eighteen student volunteers. Half were chosen to be prisoners and half guards. It's important to understand that the assignments were random and arbitrary. The researcher halted the experiment less than half-way through its originally-intended two-week duration because of the effects it was having on both sets of volunteers. The experiment has its own website here: link - read the whole thing and you might have a better appreciation for what's going on in a lot of prisons.