Retributive punishment is punishment carried out because it is felt to be deserved, not because it rehabilitates, incapacitates or deters. In practice of course punishment is usually thought to have many purposes, and retribution will only be one. Many people argue though that a concern for retribution should -while being balanced against other concerns- play a role in decision making in the criminal justice system.
Imagine you operate the punishment engine as an agent of the judiciary. The punishment engine is a device which can make people experience a long subjective period of time in an unpleasant and boring simulated prison. After they experience this period -instantaneous in the real world- they wake up and are released. Further, suppose that the prisoners in this batch have all been convicted of pretty dreadful crimes.
You are about to press the button. At this point a genie appears and tells you that, due to a weird confluence of factors pressing the button or not will have no negative or positive effects, beyond the experience of punishment itself. No one will be deterred from future crimes, either among the punished or the general population, instead exactly the same number and grade of crimes will occur regardless. No one will be rehabilitated, or for that matter dehabilitated. Pressing the button will accomplish nothing, except the punishment itself.
In other words, let us imagine the only purpose of pressing the button in this particular case would be retribution, and the suffering that retribution consists in. While I don’t believe in retributive punishment, I can understand that some people might still have the itch in these circumstances to engage in it and press the button- these people have, after all, done terrible things, and the sense that punishment for the sake of punishment matters when people have done terrible things is not uncommon, though I don’t really share it.
But suppose the genie said, further, that, exactly one person of the hundred currently strapped to the punishment engine was innocent of any crime. The genie refused to tell you who it was, but wanted to know if you still wanted to go ahead with punishment for it’s own sake, knowing that you would effect at least one innocent person. The genie can arrange matters so that you can get away with not pressing the button, and no one else will press it. Under these circumstances would you press the button?(1)
I suspect that a lot of people under these conditions, even if they supported retributive punishment in principle, would say no. This could simply be a quirk of my psychology, but I think that while people might be willing to accept the accidental punishment of an innocent person for the purposes of a policy such as deterrence, very few people would have the stomach to make an innocent person suffer purely for the sake of making ninety-nine guilty people suffer. Or at least this is true if no more practical consideration were at stake than a kind of ethical-cosmic balancing.
Our whole argument then is based on an intuition that retribution is asymmetric with other motivations for punishment. Whereas other motives for punishment might be acceptable even if punishments motivated by them are occasionally against the innocent (and surely some will be) because accepting the risk of the punishing the innocent is necessary to defend society, retribution is different. Retribution for the sake of retribution makes no sense if there is even a chance of effecting an innocent person, and in practice there is always such a chance.
The policy implication is that, if the above is right, the justice system ought not make any sentence stiffer for purely retributive purposes, since it is absolutely certain at least some innocent people will be effected by a partially or wholly retributively influenced policy.
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(1) Some people might say yes, purely out of a respect for the will of the democratic state as manifested by the decisions of the courts. If this is you, abstract away from this point at the moment.