This paper presents findings from the British Broadcasting Corporation (BBC) prison study - an experimental case study that examined the consequences of randomly dividing men into groups of prisoners and guards within a specially constructed institution over a period of 8 days. Unlike the prisoners, the guards failed to identify with their role. This made the guards reluctant to impose their authority and they were eventually overcome by the prisoners. Participants then established an egalitarian social system. When this proved unsustainable, moves to impose a tyrannical regime met with weakening resistance. Empirical and theoretical analysis addresses the conditions under which people identify with the groups to which they are assigned and the social, organizational, and clinical consequences of either doing so or failing to do so. On the basis of these findings, a new framework for understanding tyranny is outlined. This suggests that it is powerlessness and the failure of groups that makes tyranny psychologically acceptable.