If you had a brain condition so that you had no - here it's a little vague - consciousness, or moral sense, or free will, or, well, something - then it would be cruel to punish you for your crime.
If people in your pre-crime condition would commit certain crimes regardless of people in your pre-crime condition generally being punished for committing such crimes, punishing people post-crime does not deter others in such a position from committing such crimes.
Harming anyone who injures is still retributive.
Prison may sometimes serve to rehabilitate or improve the criminal, regardless of deterrence or responsibility.
Punishment may still be useful to some entities other than criminals, particularly if it is difficult to determine who is undeterrable. E.g., punishing the probably innocent as a warning to others, or strict liability even for the demonstrably innocent lest society spend resources determining truth. In extreme cases, the utility of prison or punishment may be dissociated from crime, e.g., society has decided Mr. J.J.J.S. is most useful in a chain gang and sends him there, or it is easier to send him to prison than admit according to its values society should pay for more expensive mental health treatment.
It's important to remember this about forbidden things: things are forbidden because if not for the law or taboo, people would find it useful to do them. The American Constitution forbids punishing relatives as a deterrent, i.e. "corruption of blood". To be able to credibly threaten relatives would often be useful, but we don't allow it regardless.
Rather than give up on the benefits of harm as it has given up on blackmailing relatives, our society tries to keep the useful practice of punishment by restricting it to a narrow group of people it defines as guilty. Not surprisingly, it goes too far and punishes with harm far more than is useful.
As an analogy, to how we ought to consider cases in which we think we get utility from harming others, consider the following. We know that many people who were wrong to think (metaphorically) that to save five they must kill one. From the inside, we can expect that the many of our beliefs that we must damage to save are in fact false beliefs. An unbiased being should always kill one to save five; a biased being must judge its internal belief that to save five it must kill one as likely false, all else being equal.
A society that aimed to determine (as straw-man act utilitarians do), for every suspect, the sentence most beneficial to society, the result would be a broken society. Instead, a commitment is made to only exploit the power of intentionally inflicted harm in a limited set of cases, those where we have reason to believe the criminal himself or herself was a member of the class of people deterrable by punishing people like him or her for crimes like his or hers. Society has correctly concluded that utilitarian calculations beyond that are both distorted views by minds prone to inappropriately value harm to others and sub-optimal to "rule utilitarianism" (which really telescopes back into sophisticated act utilitarianism).
If society is to continue down the road of replacing swathes of straw-man act utilitarianism with patches of rule utilitarianism to make sophisticated telescoping act utilitarianism, it will further restrict the scenarios in which we harm convicts.
Why do those words go together?
Society - and for once, I'm using this term universally - teaches that, if you committed a crime, you should be punished.
But in some societies, we have an insanity defense. If you had a brain condition so that you had no - here it's a little vague - consciousness, or moral sense, or free will, or, well, something - then it would be cruel to punish you for your crime. Instead of going to prison, you should be placed somewhere where you can't hurt anybody, where professional physicians and counselors can study your case and try to reform you so that you can rejoin society.
Wait - so that isn't what prison is for?
No. Prison is to punish people. Is it any wonder that prison doesn't reform people, when we don't want it to reform them? Most people would be upset if prisoners could go in on Friday, and emerge, rehabilitated, on Sunday. When people say, "It would be cruel to punish people who aren't responsible for their actions", they are implicitly saying, "Prison is necessarily cruel; and that's good, because we should be cruel to criminals who are responsible for their actions."
But the more we learn about psychology and neuroscience, the further responsibility recedes into the distance.
Outcome-based justice argues that we should give up playing the blame game, because neuroscience keeps finding more and more proofs that things are "not our fault". Instead, we should write laws that deter crime.
You might think this is what we already try to do. But it isn't! Witness this confused article from the Brookings Institute, Cognitive Neuroscience and the Future of Punishment by O. Carter Snead. Snead objects to outcome-based justice. He summarized all of the arguments for it, yet managed to completely miss their point, concluding where he started from, saying that outcome-based justice is obviously bad because it could lead to being cruel to people who didn't deserve it. (Instead of only being cruel to the people who do deserve it, which is obviously what we want to do.)
Snead understands that outcome-based justice deters crime:
You might expect that Snead goes on to explain why these laws are bad things. But he doesn't! He assumes we can all see that these are obviously bad things.
The Wrath of Kahneman describes a study which asked whether people punish others in order to deter crime, and concluded, No. People are doing something else.
One theory is that people are trying to be fair. Everyone should get the same chances; everyone should get the same punishment for the same crime. John Rawls argues this explicitly in Justice as Fairness: Justice should not be utilitarian, but should instead be fair.
I believe Rawls' view is also the popular view of what "justice" means. And, I will argue in a later post, it is part of a pattern showing a deep divide between two different ways of using the word "ethics".
ADDED: Constant made the point that, while one part of outcome-based justice is preventing future harm from the criminal on the dock, another part is deterring harm by other criminals. This latter part does not benefit from punishing criminals who cannot be deterred. Thus, to optimally punish both criminals who can and cannot be deterred, the law requires a concept of moral culpability, and should punish criminals who can't be deterred more lightly. This is a better origin story for the linking together of morality and free will than the just-so story I'd come up with, so I plan on stealing it for my next post. (SilasBarta may have been trying to make the same point, but I found his comments impenetrable.)
(This post is laying groundwork for two other posts that will go in different directions, neither of which concerns justice.)