Peter Gray publicly stated why he hasn’t included the SPE in his introductory psychology textbooks, explaining that by remaining silent when the guards abused the prisoners, Zimbardo was approving of their methods of intimidation. And the guards who were less assertive were instructed to be ‘tougher’- For example, Warden Jaffe (one of Zimbardo’s student research associates) said to one of the ‘nicer’ guards,“The guards have to know that every guard has to be what we call a ‘tough guard.’ The success of this experiment rides on the behavior of the guards to make it seem as realistic as possible” (pp. 65)There was direct instruction to behave more assertively, and approval of the behaviour of the 'tough guards'. 