lacks major internal validity
can't be generalised - only done with males, very young, all emotionally stable
a very conformist and rebellious time - likes temporal validity
Zimbardo was too close and part of the experiment
mock prison was not a realistic setting (lacks mundane realism)
some guessed what the study was about so acted the way they thought they should (demand characteristics)
individual differences
willing to do it and volunteered
controlled participant observation