Most prisoners believed that the subjects selected to be guards were chosen because they were bigger than those who were made prisoners, but actually, there was no difference in the average height of the two groups. What do you think caused this misperception?

This question is incomplete, here´s the complete question.  

In 1971, Phillip Zimbardo set up the Stanford Prison Experiment in which 18 young, middle-class white men were randomly allocated to the role of guards and prisioners for the next six days. The experiment was prepared as if it were a play or a film, but te dramatization quickly became believable: the perceived roles were played out as if for real. When the prisoners rebelled, the guards´ perception of the prisoners changed, so that it was no longer only an experiment. The guards saw the prisoners as troublemakers and ´out to get them´. The guards imposed severe penalties on the prisoners. One prisoner broke down after 36 hours and another after 48 hours, despite being ratified as psycologically healthy beforehand; this meant that the experiment had to be cut short. The experiment demonstrated how people were liable to both vivious aggression and self-confirming victimhood.

Most prisoners believed that the subjects selected to be guards were chosen because they were bigger than those who were made prisoners, but actually, there was no difference in the average height of the two groups.

What do you think caused this misperception?

I think the misperception about the guars is part of the process of self-confirming victimhood and the effect of the poor treatment the perceived prisoners got during the experiment.

Explanation:

Having people dressing as guards may have already provided them with an aura of power, related to the image we have collectively of policemen and guards being strong and big. Furthermore, considering the psychological consequences of the violent manner those guards behaved, it´s no surprise that the prisoners begun to see them as powerful, and accordingly "bigger".