I recall meeting at some point with Dr. Zimbardo and asking about what to do if I wanted to quit the experiment. I was told that I could not quit. I had agreed to take the job and had to stay until the experiment ended. At that point I felt that I had actually made myself a prisoner. I was told the only way I could leave before the experiment ended was by becoming ill or being paroled.
Source: exhibits.stanford.edu/spe – Image