This study suffers from a lack of ethical consideration - Zimbardo's participants failed to leave the study in the same way they entered. Furthermore, they were exposed to intense mental harm, shown as one participant had to be removed from thes tudy.
This study also suffers from a lack of ecological validity - despite Zimbardo going to lengths to making his study accurate to real life, but many aspects of real prison life were missed, for example the fear of longer sentences, lack of renumeration and the guilt of committing a crime.