So, dehumanization is a fascinating concept: by establishing differences such as race or gender or occupation, people will see others as less than human (or barely even human) in order to eliminate any guilt or sympathy, or to detach. This psychological tactic is common in criminals, war, and even hospital settings.
One of the most infamous experiments of dehumanization is Dr. Zimbardo's mock prison in 1971. Set up in the basement of Stanford University's Psychology building, Zimbardo and some peers rigged hidden microphones and cameras, and gathered 24 middle class Caucasian undergraduates. Randomly, these students were assigned as a prisoner or guard. 24 hours a day, the “prisoners” were required to remain in the prison. The guards would come and go in eight-hour shifts.
The experiment was supposed to run a total of 14 days, but due to intense criticism of the morality, it was terminated after six days. After “prisoners” began rebelling against the “guards” instructions, Turns out, they began enforcing aggressive behavior. Without supervision of the research staff, they assaulted the prisoners with fire extinguishers. It took some time for the researchers to realize that the experiment was actually causing the inmates to suffer.
The case was interesting, because it proved that you don’t have to tread too far in order to begin detaching yourself from another human being.
Psychological experiments now have way more restrictions as far as ethics go, as to not repeat the abuse that went down during the mock prison event.
This is a summary of this article from Psychology Today by Neel Burton, an M.D., from Oxford. You can also get more information from that link if you are interested.