>I always find it unbelievable that they let these kinds of hierarchies arise in prisons, especially in American prisons (from what I've heard, European prisons have it too, but to a much lesser degree). Someone told me they allow it on purpose to help keep order. Maybe it is also a consequence of overfull, understaffed prisons.

A few years back I took a tour of the main prison in Amsterdam. The warden actually talked about this and said they specifically organize their prisoners into as diverse blocks as possible to avoid any sort of gangs/groupings from starting. He claimed it made the prison significantly safer, as everyone was looking out for only their own wellbeing. The prison was safe enough for us to walk around among the prisoners (who were not cuffed or restrained in anyway), so I think it was working quite well for them.