The evening of Day 3. The Guards decide which Prisoner will be promoted to join their ranks. It is also made clear that there will be no more promotions or demotions. The system is now frozen. In technical terms, it is impermeable.
No matter how well the Prisoners behave or how hard they try, they cannot improve their lot. According to social identity theory, this produces a situation where they should begin to identify with their group and to act as a group.
In the prison it is as if a door has been slammed shut. The Prisoners can no longer advance through it individually. They can only batter it down together.
So, as Day 4 dawns, everything has changed in the prison. Those who, just the day before, were seeking to impress the Guards now join with others in poking fun at them. They draw a Hitler moustache and a swastika on the picture of one of the Guards. The inmates of one cell draw conspiratorially together. “You’ve got to start something in here mate,” says pJE who, previously, had aspired to the luxuries of a Guard. And with his cellmates – both of whom had just been turned down for promotion – he begins to plot ways of undermining the Guards' authority.