I felt that throughout the experiment, he knew what he wanted and then tried to shape the experiment—by how it was constructed, and how it played out—to fit the conclusion that he had already worked out.
From John Mark, one of the day guards.
A follow-up study in 2007 should not surprise anyone who knows a bit of psychology: when you ask for students to participate in a study about prison life, you get a different subset of people.
Which leads to some interesting problems for anyone who accepts the results but interprets them as self-selection - if 'power attracts the corruptible', then how do you fill positions of power?
This year is the 40th anniversary of the Stanford Prison Experiment. I found this [retrospective](http://www.stanfordalumni.org/news/magazine/2011/julaug/features/spe.html) interesting. What really caught my eye is that, to some degree, it contradicts the main lesson of the experiment -- that context more than character determines behavior. If David Eschelman is accurately/truthfully recalling his role, then it seems like his individual character actually did play a role in how quickly things spiralled out of control (though the willingness of the other guards to go along with him supports the original conclusion).