The baseball statistics analyst Bill James writes in his new book Popular Crime
Build smaller prisons.
... Large prisons become "violentocracies" — places ruled by violence and by the threat of violence. In a violentocracy, the most violent people rise to the top.
In any prison of any size, the prisoners are going to be pushed toward the level of the most violent persons in the facility. ... In a prison 3,000 people, the entire prison is pushed toward the level of violence created by the five most violent people in the joint. The most violent person finds the second-most violent person and the third-most violent person, and they form an alliance to exploit the weak. Everyone else is compelled to avoid looking weak. ...
Large prisons promote paranoia in the prisoners. You never know who in here is waiting for you with a homemade knife.
... A prison of 20 people is, by its very nature, extremely different. You know who is in there with you; you know who you have to stay away from. ... Plus, if you have many small prisons, you can contain the violent people in a limited number of those prisons, the preventing their violent tendencies from infecting the rest of the prison population.
... What you would do, with a network of small prisons, would be to place each prisoner in a facility that is appropriate to the threat that he represents. You grade the prisoners on the threat of violence that they represent, one through ten. You put the tens with the tens and the ones with the ones.
Plus, when you move to a system in which some prisoners have more rights and live in more humane conditions, you create a powerful incentive to get into one of the less restrictive prisons..
In a large and horrible prison, the new prisoner thinks "I've got to show everybody here how tough and vicious I really am, so that nobody will mess with me." But when you put a new prisoner in a 24-man prison with 23 other tought guys, and he knows that there are other prisons that are not like this, his natural thought is "I've got to get out of here. I've got to show these people that I am not a crazy, vicious sociopath, so they will move me to some other facility that is not populated by crazy, vicious sociopaths."
My vague impression is that some of the supersoft Norwegian prisons
you read about are actually reward prisons for good behavior on the part of inmates at tougher prisons.