Our next task was to decide who would be one of the 5 Guards and who would be one of the 10 Prisoners we needed for the study.
It was critical to ensure that the Guards and Prisoners were psychologically similar so that any differences that arose in their behaviour would need to be explained by their group position and not their individual characteristics.
This involved first dividing the 15 men into five groups of three who were very similar on key psychological dimensions such as social dominance and authoritarianism. Then we randomly chose one person from each group of three to be a Guard and the other two to be Prisoners.
This left us with two groups with the same psychological profile – at least at the start of the study. This meant that any differences that emerged subsequently would have to be a consequence of unfolding events.