That aside, my personal experience leads me to believe that prison is as horrible as it is because of the people locked inside. Evidence for this can be inferred from the differing degree to which low security prisons are less harsh than higher security prisons.
Do you have a source for this claim?
As I understand it, this is flatly untrue. Jails were, up until quite recently, designed to hold people awaiting trial, not as punishment.
My belief is that imprisonment for decades as punishment is a 20th century phenomenon.
The only book I've read on this specific matter is "Discipline and Punish: The Birth of the Prison" by Michel Foucault, but that was a decade ago; I'm not sure if the details are crisp in my mind.
If you've read anything on it, exactly what were they using for punishment then? Other things I've read of in various places include execution, the stocks, other forms of physical punishments. IIRC, some Islamic societies still recommend cutting the hands off of thieves. All of this stuff makes imprisonment seem rather sanitary and lenient by comparison.
But one of the points in the Foucault book jMyles mentioned is that the leniency of imprisonment didn't come about because we're somehow morally superior than earlier societies, it's because it's the most efficient way so far devised for the state to exert its power over a human body.
Someone please correct me if I'm misinterpreting.
On the other hand, there are also people in prison who actually are dangerous criminals, and really need to be locked away from the general population. That is what we created it for in the first place, and why every society has some sort of prison. But nobody writes stories about those types of people, at least not in "intellectual" sites and magazines. When was the last time you read a story about a truly dangerous criminal and what their experience in the legal system has been?