Those acting as guards quickly became bullies, while their prisoners became distressed and emotionally disturbed by the way they were treated.
Since then Zimbardo has continued to study what drives ostensibly good people to behave badly but also what enables others to speak out and resist being drawn into the cruel acts of others around them.
First on Zimbardo's list is admitting to mistakes - recognising when things are not going well and change is required, rather than attempting to rationalise or condone bad behaviour, or taking the "I was told/ordered/everyone else was to doing it" line.
One of the crucial factors is refusing to be de-individualised.
In detention camps, one of the common acts is to turn those incarcerated from names to numbers. This removes the "human" from the person and allows those holding them to treat then as non-human, enabling cruelty to become routine.
We only need look to the offshore detention of asylum seekers across the Tasman to see an example of this.
Zimbardo acknowledges that the desire for group acceptance can be a powerful driver but independence is crucial. If one person in a group yells at a migrant and tells him/her to "Go back where you came from" then someone in that group must resist, knowing that if they do, others may find they too can resist.
Perhaps most relevant in this age of hi-tech surveillance of citizens is Zimbardo's call for people to not sacrifice personal or civic freedom for the illusion of security.
It may seem hard to understand how the population of Germany, under the leadership of Hitler, could be persuaded to condone the treatment of Jews. My understanding is that this was done in an incremental fashion, and was heralded by the Nazi rhetoric labelling Jews as the cause of the nation's problems.
Many people did not agree but accepted this as the "way things are". This was followed by orders that all Jews must wear a yellow star to mark them out. This was accepted as not good, but not too bad.
Jewish properties were attacked, their homes and businesses taken. This appeared to have been regarded as not good but, again, not too bad - and once it became clear that the plan was to transport Jews to camps and the gas chambers it was too late.
Zimbardo asks that we be aware of this gradual slide into terror and resist the blinding lies of leaders who would want us to be complicit in their schemes to demonise whole sections of society, simply to appease their desire for power.
■Terry Sarten is a writer, musician and social worker. Feedback: tgs@inspire.net.nz