One obscene result of this paradigm is that you sacrifice the actual lives of individuals for merely potential benefits for society. The literary example is Jean Val Jean, who of course ends up spending a lifetime in prison for stealing a loaf of bread.
This actually happens in American justice. Those harshly punished for minor offenses are not only not rehabilitated, they are turned into hardened criminals by association with the low-lifes who truly deserve to be in prison.
> It's interesting that you bring up the example of Anders Breivik. How would your opinion of him change if, hypothetically, it were later shown that he had a brain tumor? For me, the justification for retribution evaporates.
To extend your hypothetical, say we can conclusively prove that the universe is deterministic and there is no such thing as free will. Then is no one responsible for anything they do?
I contend, in the absence of contradicting evidence, we must live as if we have free will, because (ironically) we have no other choice. So if Breivik's lawyers/doctors demonstrate a brain tumor affecting his behavior, we treat him differently than we do the garden-variety psychopathic mass-murderer he appears to (and purports to) be.
The moral of logarithmic growth is clear: "If you are gonna do the crime, make it worth the time!"
The reason for this is simple: Sentence length isn't just retribution for the crime. It can be for rehabilitation as well (at least, in other countries). Prison sentences also try to have a deterrent effect. Thieves aren't caught nearly as often as murderers. If the punishment for theft was repayment and a fine, we'd see a lot more theft. It's a similar story with drunk driving. Most of the time, people get away with driving drunk. Nobody gets hurt, nobody gets pulled over. But we want to discourage this behavior since it does kill people. So we make harsh punishments for even a first-time offense.
It's interesting that you bring up the example of Anders Breivik. How would your opinion of him change if, hypothetically, it were later shown that he had a brain tumor? For me, the justification for retribution evaporates. Sure, lock him up forever; he's still dangerous. But if we can trace someone's actions back to causes outside of their control, doesn't that obviate the need for punishment? Then, all we can do is try to rehabilitate them and/or keep them from harming others.
I think this argument applies to the vast majority of people in prison. We don't choose our genes. We don't choose our parents. We don't choose the household or culture we grow up in. We don't choose how much lithium is in our tap water.[1] The vast majority of our personalities and behaviors are shaped by forces completely outside our control, often without our knowledge. We're nowhere close to fully understanding the interactions between genes and environment necessary to diagnose each person's violent behavior, but that's no excuse for exacting retribution. Right now our society is effectively punishing people for having brain tumors. I think focusing on rehabilitation would be much more effective and fair, even for some of the most horrific crimes.
1. Yes, this actually matters: http://www.gwern.net/docs/nootropics/1990-schrauzer.pdf