The experiment was perceived by many to involve questionable ethics, the most serious concern being that it was continued even after participants expressed their desire to withdraw. Despite the fact that participants were told they had the right to leave at any time, Zimbardo did not allow this.
Since the time of the Stanford experiment, ethical guidelines have been established for experiments involving human subjects. The Stanford Prison Experiment led to the implementation of rules to preclude any harmful treatment of participants. Before they are implemented, human studies must now be reviewed and found by an institutional review board (US) or ethics committee (UK) to be in accordance with ethical guidelines set by the American Psychological Association. These guidelines involve the consideration of whether the potential benefit to science outweighs the possible risk for physical and psychological harm.
A post-experimental debriefing is now considered an important ethical consideration to ensure that participants are not harmed in any way by their experience in an experiment. Though Zimbardo did conduct debriefing sessions, they were several years after the Stanford Prison Experiment. By that time numerous details were forgotten; nonetheless, many participants reported that they experienced no lasting negative effects. Current standards specify that the debriefing process should occur as soon as possible to assess what psychological harm, if any, may have been done and to rehabilitate participants, if necessary. If there is an unavoidable delay in debriefing, the researcher is obligated to take steps to minimize harm.