Of the four standard justifications for punishing people, only one needs the metaphysics this course has been arguing about, and it is the one most legal systems say they are using.
The previous lesson isolated basic desert as the contested item: blame and punishment deserved just because of what someone has done, apart from any good it produces. Here that abstraction meets the institution that spends it, and the argument becomes partly empirical. Where numbers exist they are given, because a claim about deterrence is a claim about the world and not a matter of taste.
Four justifications, and which need what
Retribution holds that a wrongdoer deserves to suffer in proportion to the wrong, and that inflicting that suffering is intrinsically justified. Kant put the position at its starkest in the Metaphysics of Morals of 1797: if a civil society were to dissolve by common agreement, the last murderer in prison must first be executed, so that everyone receives what their deeds deserve. Nothing about consequences appears in that argument, and it is entirely dependent on basic desert.
Deterrence holds that punishment is justified by the offences it prevents, whether by dissuading the offender or others. Incapacitation holds that it is justified by preventing an offender from reoffending while confined. Rehabilitation holds that it is justified by changing the offender.
The last three are forward-looking and need no desert at all. They need punishment to work, which is an empirical question, and they need some constraint to stop them licensing outrages, which is a moral question. So a sceptic about free will does not have to abolish the criminal law. They have to give up one justification of four, and then explain how the remaining three are kept within limits that used to be supplied by desert.
That is the real practical disagreement, and it is narrower than the rhetoric on either side.
The classic objection to going forward-looking
The reason desert kept its place is that consequentialist justifications alone permit things nobody will accept.
The standard case, put by H. J. McCloskey in 1957, is the sheriff in a town on the edge of a riot. He knows that framing and hanging one innocent man will prevent a lynching in which several people would die. If punishment is justified by consequences, and the consequences of framing him are better, then he should frame him. Nearly everyone thinks he must not, and desert is the usual explanation: punishment must fall on the guilty because only the guilty deserve it.
Deterrence has a second problem of scale. If the point is to prevent offences, and severe punishments prevent more, the theory has no natural stopping point: execution for shoplifting would deter shoplifting. Desert supplies the ceiling, in the form of proportionality.
Any sceptic about basic desert has to answer both, and the leading attempt is worth taking seriously.
The quarantine model
Derk Pereboom, developed further with Gregg Caruso, argues that the right analogy for dealing with dangerous offenders is public health rather than deserved suffering.
We already detain people who have done nothing wrong when they carry a dangerous infection. The justification is the right of self-defence and the defence of others, which does not require the carrier to deserve anything. Four constraints come with the analogy, and they are not decorative. Detention is permitted only for genuine danger. It must use the least restrictive means adequate to the danger. It carries an obligation to rehabilitate or cure where possible. And it ends when the danger ends.
Applied to crime, the model gives a system that incapacitates the dangerous, works hard on rehabilitation, and inflicts no suffering beyond what the containment requires. It answers the sheriff case cleanly: the innocent man is not dangerous, so quarantine gives no ground whatever to detain him, and the analogy blocks the very move that embarrassed pure deterrence. It also implies a serious reduction in punitive severity, since nothing in the model justifies harsh conditions that do not reduce danger.
The strongest objection is about proportionality in the other direction. A persistent but minor offender, say a burglar with a high probability of reoffending, may be more dangerous than a man who killed his wife's lover in circumstances that will never recur. Desert says the killer should serve much longer. Quarantine says the opposite, and is committed to detaining the burglar for as long as the danger lasts, which could be indefinitely. Critics regard indefinite detention for minor offences as a decisive objection; Caruso replies that the least-restrictive-means requirement and the obligation to address the causes of the danger together prevent the worst outcomes, and notes that existing systems already do detain minor offenders repeatedly and for long total periods without admitting it.
A second objection is that the model still uses people: detaining a person for what they might do treats them as a hazard. The reply is that self-defence has always permitted this, and that the alternative, deliberate infliction of suffering on the ground that they deserve it, is not obviously more respectful.
Example. A man drives drunk, kills a cyclist, and is genuinely remorseful. Assess him under each of the four justifications.
Retribution asks what he deserves for a serious wrong committed through culpable recklessness, and answers with a sentence proportional to the gravity of the harm and the fault, typically years. Deterrence asks what sentence will most reduce drink-driving by him and by others, and the empirical literature suggests the answer depends far more on the perceived probability of being caught than on the length of the sentence, so it favours breath testing over long terms. Incapacitation asks how dangerous he is now, and a remorseful first offender who has lost his licence is not very dangerous, which points to a short sentence or none. Rehabilitation asks what will change him, and answers with treatment for alcohol dependence if he has it. Only the first justification gives a reason for a long sentence, and noticing that is the point of the exercise: much of what our systems do is retributive in fact whatever they say in their sentencing guidelines.
Now you. Under the quarantine model, what would justify keeping this man in prison at all, and for how long?
Answer
Only his ongoing danger to others, and therefore for as long as that danger persists at a level that lesser measures cannot manage. For a remorseful first offender who has lost his licence and accepted treatment, the honest answer is probably little or no detention, with monitoring and a driving ban doing the work. Many readers will find that answer intolerable, and the reaction is worth examining rather than dismissing: it is the clearest possible evidence that our intuitions about sentencing are retributive, and that giving up desert would change the criminal law substantially rather than cosmetically. A sceptic should accept that consequence openly. A retributivist should notice that their position is now doing real work and must be defended, not assumed.
What the numbers say
Three empirical claims bear on the argument, and all three are better established than most of what is said about them.
The first concerns deterrence. Reviews of the evidence, notably by Daniel Nagin, converge on the finding that the certainty of apprehension deters, while the severity of the sentence has little detectable marginal effect. Increases in police presence and clearance rates reduce crime; increases in sentence length, at the margins tested, mostly do not. That result damages the deterrence justification for long sentences specifically, which is the part of the system that consumes most of the money.
The second concerns scale. The World Prison Brief puts the United States at roughly 530 prisoners per 100,000 residents and Norway at roughly 55, a factor of about ten. Norway's maximum determinate sentence is 21 years, with a separate preventive detention regime, forvaring, extendable in five-year increments for offenders judged still dangerous, which is close to a quarantine model operating inside an otherwise ordinary system. Anders Breivik, who killed 77 people in 2011, received exactly that: 21 years of preventive detention, extendable indefinitely.
The third concerns reoffending, and it is where care is needed. The United States Bureau of Justice Statistics followed 404,638 prisoners released in 30 states in 2005 and found that 67.8 percent were rearrested within three years and 76.6 percent within five. Norwegian figures are usually quoted at around 20 percent within two years. Those numbers are not comparable: the American figure counts rearrest, the Norwegian counts reconviction, the follow-up periods differ, and the underlying populations and policing differ enormously. The comparison is suggestive and it is not a controlled experiment, and anyone who cites it as one is overreaching. What can be said is that a system with a tenth of the incarceration and a much lower ceiling on severity does not display the catastrophic outcomes that a purely deterrent theory would predict.
Example. The United States imprisons about 530 people per 100,000 and Norway about 55. What can and cannot be inferred from the comparison?
The ratio is , so the American rate is nearly ten times the Norwegian one, and that much is a measurement rather than an inference. What cannot be read off it is a causal claim in either direction. The two countries differ in the prevalence of violent crime, in firearm availability, in inequality, in drug policy, in policing, in what counts as a prison sentence, and in a hundred other respects, and any of these could account for part of the gap. The honest use of the comparison is as an existence proof: a wealthy society can run a criminal justice system at a tenth of the American incarceration rate, with a 21 year ceiling on determinate sentences, without collapsing. That rules out the strongest version of the claim that severe punishment is necessary for social order, and it establishes nothing about what would happen if one country adopted the other's system.
Now you. What kind of evidence would settle whether increasing sentence length reduces crime?
Answer
Something with a control. The useful designs are natural experiments in which sentence length changes sharply for reasons unrelated to the offenders, such as a statutory threshold at a particular age or offence value, a sentencing reform applied on a fixed date, or a random assignment of cases to judges who differ in severity. Each of these compares similar offenders on either side of an arbitrary line, which is what a raw comparison between countries cannot do. That literature exists and its results are consistent: the effects of severity at the margins studied are small and often indistinguishable from zero, while the effects of the probability of apprehension are robust. This is the pattern to look for whenever a philosophical argument turns on an empirical claim, and it is why the deterrence branch of this dispute is closer to settled than any other part of the subject.
The law's own compromise
Legal systems have never waited for philosophers, and what they have built is a set of exemptions that track capacity rather than causal history.
The M'Naghten Rules, formulated by the English judges in 1843 after Daniel M'Naghten was acquitted of killing the Prime Minister's secretary, excuse a defendant who, through a defect of reason from disease of the mind, did not know the nature and quality of his act, or did not know that it was wrong. The American Law Institute's Model Penal Code of 1962, 119 years later, broadened this to a defendant who lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the law, which adds a volitional limb.
Notice that both tests are about capacity now, not about how the defendant came to have his character. That is a compatibilist structure, and it is what almost every criminal code in the world uses in practice.
The defence is also far rarer and far less successful than its cultural prominence suggests. An eight-state study by Callahan and colleagues published in 1991 found the insanity defence raised in about 0.9 percent of felony cases and successful in about 26 percent of those, which is roughly two acquittals per thousand felony cases. The political pressure has run in one direction: after John Hinckley's acquittal for the attempted assassination of President Reagan, the federal Insanity Defense Reform Act of 1984 narrowed the test and shifted the burden to the defendant, several states abolished the defence outright, and in Kahler v. Kansas in 2020 the Supreme Court held by six votes to three that the Constitution does not require a state to offer a moral-incapacity test at all.
Does believing this change behaviour?
One practical worry deserves an honest report, because it is often used as a reason not to discuss the subject: that telling people they lack free will makes them behave worse.
Kathleen Vohs and Jonathan Schooler reported in 2008 that subjects who read a passage denying free will subsequently cheated more on a task than controls. The finding was widely repeated and became a standard warning.
It has not held up well. Larger replication attempts, including work by Andrew Monroe, Garrett Brady and Bertram Malle in 2017 and by Damien Crone and Neil Levy in 2019 using substantially larger samples, failed to find the effect, and a 2023 meta-analysis by Oliver Genschow and colleagues put the average effect close to zero. The current state of the evidence is that reading a paragraph about determinism does not measurably make people cheat, which is what one might expect of a manipulation that mild.
That is a limited result, and it should not be inflated either. It says nothing about what a society organised around scepticism about desert would be like, since no such society has existed, and the honest position is that this is unknown. But the specific claim that the belief is dangerous, which has been used to close the discussion, is not currently supported.
Example. A retributivist argues that abandoning desert would make punishment unlimited, since a system aimed only at prevention has no ceiling. How should a sceptic reply?
By pointing out that the ceiling has to come from somewhere else, and then supplying it. The quarantine model's least-restrictive-means requirement is such a ceiling, derived from the same self-defence principle that licenses the detention in the first place: you may use the minimum force needed to avert a threat and no more, which is exactly why a person who has become harmless must be released. That is a genuine constraint with a familiar structure in law. Whether it is as protective as proportionality is arguable, and the honest sceptic concedes that it protects differently: it is more protective of the reformed serious offender and less protective of the persistent minor one. The reply is not that nothing changes. It is that the change is specific and defensible.
Now you. Which of the four justifications does a compatibilist of the reasons-responsive kind need, and does the argument of this course threaten it?
Answer
A reasons-responsive compatibilist can have all four, and the retributive one is the interesting case. They claim that an agent whose reasons-responsive mechanism produced the offence is a fit target of blame and can deserve punishment, so desert survives determinism. What the course threatens is not their consistency but the sufficiency of their criterion, and the pressure comes from the manipulation argument rather than from the Consequence Argument: an agent can meet the reasons-responsiveness test and still have had the mechanism installed by processes he did not choose. A compatibilist who wants desert therefore has to answer that argument, which is why the two lessons on sourcehood, rather than the ones on physics, are the ones that matter for punishment.
Where the practical argument stands
The gap between a retributivist and a sceptic about desert is smaller in institutional terms than in rhetorical ones. Both detain dangerous people. Both prefer prevention to cure. Both accept excuses and exemptions that track capacity. They differ on whether suffering beyond what containment requires is ever warranted, and on whether the length of a sentence should track the gravity of the past act or the size of the future risk.
That is a real difference, and it is measurable in years of human life. It is also, unusually for this subject, a disagreement where evidence helps: how much deterrence severity actually buys, what rehabilitation achieves, how accurately danger can be predicted. Those questions have answers, and the answers have been arriving steadily for thirty years.
Which leaves the reader with everything needed to take a position. The last lesson assembles the whole argument into one place, shows that every view is a choice about which premise to deny, and sets out how to defend the choice you make.