I don't have any sources handy, but I believe their is much evidence that prisoners do learn very good lessons about violence. Unfourtuantly, those lessons are in how to be violent, which (I think) results in prisons being the best way to make a non-violent criminal into a violent one.

A childhood friend was sent to juvenile detention on a statutory rape charge (not a rapist, they were both 14). After 2 years he emerged to be a worser human being with a huge, justifiable, chip on his shoulder. By 16, he was dealing in cocaine (not weed, cocaine, he gained the connections for that in JD), prostitution (addicts will do anything for blow) and weapons.

And Scarface, the movie character, became his idol. We used to be backpackers (the rapping kind, check urban dictionary) and he transformed into everything that was wrong with hip-hop. And I blame everything on how all the adults completely snowballed a 14yo.

Prisons are like professional conferences for committed criminals where they get to swap tips and tricks and network.