The guards used a variety of psychological tactics that lead to prisoners to express much distress. "Although it was clear to all subjects that experimenters would not permit physical violence to take place, varieties of less direct aggressive behavior were observed frequently (especially on the part of the guards)." (Zimbardo, 80-81). The guards made prisoners do many push ups if they had made any errors, which Nazi's had also done to punish those within internment camps. They would take away mattresses, leaving only concrete for the prisoners to sleep on. As a form of degrading the prisoners, they would be forced to give their uniforms and stand naked in cells. Other forms of punishment prisoners had to endure was not being allowed to use the restroom or only using a bucket to use in their cells. Sometimes as forms of punishment the guards would not allow the prisoners to empty the buckets, creating an unsanitary environment. "Throughout the experiment commands were the most frequent form of verbal behavior and, generally verbal exchanges were strikingly impersonable, with few references to individual identity." (Zimbardo, 80). Guards used deindividualization techniques by reinforcing the idea that prisoners were just numbers and not a person with a name. They eventually started to use this technique to harass them frequently. (Zimbardo)

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During the experiment, prisoners began to internalize their roles and began to believe this was more real than it actually was. Some of the prisoners even thought about accepting "parole" without their payment for the experiment, instead of simply quitting the study. "They talked about prison issues a great deal of time. They "told tales" on each other to the guards. They started taking prison rules very seriously, as though they were there for the prisoners' benefit and infringement would spell disaster for all of them. Some even began siding with guards against prisoner who did not obey rules." (Mcleod). As the experiment continued, prisoners began to act more erratically, express intentions of harm, and lost control of themselves. Thirty-six hours into the study, prisoner number 8612 began to scream and act irrationally to the point of possible emotional and psychological trauma that could have lasted long-term. The experimenters pulled out a total of five prisoners before the study ended in order avoid long-term damage. Prisoner number 416, was a stand-by participant, until he replaced another prisoner that had to leave because of possible psychological harm. When he saw how prisoners were treated, he decided to go on a hunger strike. The guards responded by putting him into solitary confinement and forcing the other prisoners to bang on the door and scream 416. The prisoners experienced intense psychological abuse from the participating guards, which lead to extreme measures. "The most dramatic evidence of the impact of this situation upon the participants was seen in the gross reactions of five prisoners who had to be released because of extreme emotional depression, crying, rage, and acute anxiety. The pattern of symptoms was quite similar in four subjects and began as early as the second day of imprisonment. The fifth was released after being treated for a psychosomatic rash which covered portions of his body." (Zimbardo, 81)

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The experiment ended six days into a fourteen day study. The guards were becoming more cruel as the experiment went along and displayed sadistic behaviors that could end up causing emotional and psychological trauma to other participants. "Despite the fact that guards and prisoners were essentially free to engage in any form of interaction (positive or negative, supportive or affrontive, etc.), the characteristic nature of their encounters tended to be negative, hostile, affrontive, and dehumanizing." (Zimbardo, 80). Despite the fact that there were many observers in the experiment, only one person questioned the morality and ethics of the experiment. "Professor Zimbardo's former graduate student (and future wife) Christina Maslach confronted him and said that by taking on the role of prison superintendent, he had become indifferent to the suffering of his participants." The guards thought they were not being watched by experimenters at night, so their actions were becoming more aggressive. This was also another reason the experiment ended early. (Zimbardo)
References:
McLeod, Saul. "Stanford Prison Experiment." Simply Psychology. N.p., 24 Sept. 2016. Web. 1 Oct.
2016.
Banks, Curtis. Haney, Craig. Zimbardo, Philip. "International Journal of Criminology and Penology." Intropersonal Dynamics in Simulated Prison. (1973) 69-97. Web. 1 Oct. 2016.
Breil, Jeff. Plous, Scott. Jensenius, David. "Prisonexp.org." Stanford Prison Experiment. Social Psychology Network, 2015. Web. 1 Oct. 2016.