Zimbardo and his colleagues made an advert in the Palo Alto City newspaper for a study of the psychological effects of prison life on college students. The participants were to be paid 15$ a day. They did psychological tests and interviews to make sure that the participants were psychologically healthy, with no history of crime or drug abuse or any medical disabilities. The 24 participants were randomly assigned to either be a ‘guard’ or a ‘prisoner’ based on a coin flip. 