The most humane penal system I can imagine would be an imaginary jail. Under the pretend jail system, convicted criminals would simply disappear in a government-marked van, leaving the citizens to speculate about their fate. Is the government killing these convicts? Are they being tortured?
Imagine a system in which convicted criminals are secretly relocated to distant countries and given jobs, with the cooperation of the receiving government. I would think it's cheaper to relocated, train, and employ a criminal in a third-world country than it would be to incarcerate him in the United States.
Obviously this is a good deal for convicted criminals. A new life is better than decades in jail. And a society with an imaginary jail wins too because potential repeat offenders would be offshore. As a bonus, it might be cheaper for the government than building new jails.
Potential criminals would still fear getting caught because the unknown is scary. People would imagine the worst. Why else would the system be so secret? Is the government doing medical experiments? The deterrent effect of pretend jail might actually be stronger than today. Jail is probably worse than a potential criminal imagines whereas pretend jail might be far worse in the imagination than in reality. Deterrents are psychological in nature, so the unknown might be the scariest possible scenario.
The pretend jail system has some challenges, of course. It would be hard to keep the whole thing secret, for starters. And it might be hard to find a country that wants to accept criminals in return for cash. But what fascinates me about this thought experiment is that it feels so unsatisfactory that the criminals are not punished. Why should I care about punishment as long as society's goal of reducing crime is satisfied? We can't go back in time and undo the crime. I should only care about the future, not the past.
If you knew for sure that the pretend jail system was the best way to keep both crime and taxes low, would you agree to such a system? Or would you insist on punishing criminals even if it means higher taxes and higher crime in your neighborhood?
How much is revenge worth to you?