Zimbardo was a highschool contemporary of the late great Stanley Milgram, one of the fathers of Modern social Psychology and main movers of Conformity Psychology (that is, the study of how we influence each other). Milgrim's most famous experiment (from 1974 onwards) is normally taken to demonstrate how easily individuals can be persuaded to do bad things by authority figures. A 'teacher' is encouraged to give a 'learner' (ever increasing) electric shocks when he/she gives a wrong answer. Roughly 2/3rds of us will comply even as the shocks reach fatal levels (more here). Milgram's own musings - in the light of the My Lai massacre - on how easy it is to make good people do bad things is
Zimbardo's most famous experiment was the infamous.'Stanford Prison Experiment' where volunteers (randomly assigned roles of guard or prisoner) quickly adopted brutally oppressive or victim behaviour. You may remember the BBC's recreation a few years ago, which had to be stopped early...
Aside from geopolitical considerations about how the likes of Abu Graibh could be happen and more recent problems with British football fans being involved in rioting (even the mighty Spurs'...), Zimbardo's great gift to us is not in explaining how bad things get done by good/morally neutral people (this is the shock story that fits with our taboo of crowds and crowd behaviour, even NS fell for this line!!!) but rather in demonstrating the situational aspect to the forces shaping human behaviour.
You see, too many folk like to look inside the invididual - at their personality, their traits, their cognition, their judgements or even the underlying neurology - to explain why they do certain things. Zimbardo's contribution is to show the context around the individual - largely made up of other people.
And, of course, as Milgram also insisted, as agents ourselves we're often not aware of this other influence.
We'd like to think this was just for bad things, but the truth is it's for everything, good, bad, big and small.
And yet the idea that each individual does what they do independently of those around him/her is still the working model we marketers and managers carry around in our heads.