Also nobody ever proofed that they serve one of the promoted purpose (to change the criminals for better).
They actually only worsen the situation in this regard.
It is generally very ineffective system (as all state solutions...)
Discussion
Oh. See, that's not what prison should be for, at all. It should be part of a punishment and also an isolation for those unfit to be left loose with the greater public.
Prison as a reform methodology is a scam and, IMO, also exceedingly inhumane.