Crime and Punishment and IQ

“If he has a conscience he will suffer for his mistake. That will be his punishment — as well as the prison.”

– Fyodor Dostoevsky, Crime and Punishment

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Why a nation punishes its citizens says much about its values. Even as sweeping social changes in Europe led its nations to reject capital punishment as inconsistent with their emerging “image[s] of moderation and rationality,” America took a different path, continuing to this day to justify the death penalty in terms ostensibly rooted in morality and science.

Again and again, the Supreme Court has upheld the constitutionality of the death penalty while, at the same time, denouncing the injustices of its administration. The Supreme Court has held death to be excessive punishment for certain crimes not resulting in death and has struck down statutes imposing mandatory death sentences. In such cases, the Court has turned to concepts like “moral culpability” and “just deserts,” concluding that the defendants before it do not warrant the law’s most severe penalty.

This year, the Supreme Court will decide Hamm v. Smith, a case which will clarify how states can define intellectual disability and the reach of the Eighth Amendment’s prohibition on executing intellectually disabled people recognized in Atkins v. Virginia. In Atkins, the Court reasoned that the two acceptable purposes of capital punishment were “retribution and deterrence of capital crimes by prospective offenders,” and that executing defendants with intellectual disabilities served neither purpose. However, the Court “le[ft] to the State[s] the task of developing appropriate ways to enforce the constitutional restriction.” 

Predictably, in the wake of Atkins state legislatures and courts have struggled over the definition of intellectual disability, engendering the current fight over what IQ threshold is appropriate and how rigidly those lines can be enforced. That is the question now before the Court in Hamm v. Smith. But how, exactly, does IQ bear on the purposes of punishment identified in Atkins — retribution and deterrence? Is IQ in fact a useful proxy for a person’s “moral culpability,” and thus for the punishment he deserves?

Early History of IQ

These conversations long predate Atkins. Sir Francis Galton, one of the first scientists to develop intelligence tests, believed a person’s physical features like breathing capacity and head size could predict both their intelligence and their criminality. Today, scholars laud Galton as a pioneer in the field of psychometrics and fingerprinting but repudiate him for using those techniques in founding the eugenics movement.

Galton’s work, intertwined with problematic science and troubling ideologies, eventually contributed to early versions of the modern IQ test. Those tests were quickly applied in the United States in disturbing ways, to screen immigrants at Ellis Island and justify the mandatory sterilization of “feeble-minded” individuals. In his infamous 1927 opinion in Buck v. Bell, Justice Oliver Wendell Holmes upheld forced sterilization laws on the supposed strength of eugenic “science” advanced by researchers like Galton.

The science of IQ has improved much since the 1880s. IQ is no longer treated as a self-sufficient measure of intellectual disability. Diagnostic frameworks that define intellectual disability and measure intelligence require attention to how individuals learn basic skills and adjust behavior to changing circumstances (“adaptive functioning”) as well, and the DSM-5 specifically warns against overemphasizing IQ scores. Recent research finds only a “moderate correlation between adaptive behavior and intelligence,” underscoring that IQ does not fully capture the practical and social capacities that matter in everyday life. The Court has recognized this shift, rejecting a rigid IQ cutoff for Atkins claims and requiring courts to consider adaptive functioning.

IQ and “Retribution”

But the Court’s justifications for capital punishment are not, at their core, just inquiries into a person’s ability to deal with the vagaries of life. It has reasoned that retribution, for example, requires an inquiry into moral culpability and whether a person understands their conduct, appreciates its wrongfulness, deliberates, and can fairly be held to account.

Current research doesn’t seem to support treating IQ as a proxy for morality, even when combined with adaptive functioning. A recent critical review of the intelligence-and-moral-development literature concluded that the field struggled with “methodological issues,” and neuroscience research indicates that “moral reasoning arises from complex social decision-making” that “var[ies] across different cultures and historical contexts.” These do not sound like things that can be reduced to a single number.

One might respond that IQ and adaptive functioning are the best proxies available. Perhaps. That is a serious counterargument. But the case for IQ looks weaker still when we turn to the deterrence rationale.

IQ and “Deterrence”

The deterrent effect of punishment depends on how salient it is to a potential offender. Modern research suggests that the key mechanism here is temporal discounting: the tendency of people to devalue future costs relative to immediate ones. The more steeply a person discounts future punishment, the less deterrent force that punishment has today. 

One might hope, then, that IQ would be a good proxy for a person’s discount rate. But behavioral research links steeper discounting to executive functioning, not raw intelligence. In one study of people with intellectual disabilities, participants’ performance on temporal-discounting tasks correlated with executive functioning, not with IQ. If that is right, IQ is practically irrelevant to the very mechanism on which deterrence depends. And yet it continues to anchor the doctrine.

To be sure, the Court’s use of IQ testing in Atkins and its progeny is more protective of vulnerable populations than it was in Galton’s times, as is true for IQ testing in society today more generally. Schools, for example, use IQ to identify students for special education and gifted programs, although testing for educational placement has its own dark history. But benevolent use of IQ has not stopped educators and psychologists from asking whether such use is scientifically justified.

Perhaps it is time for the Supreme Court, too, to reconsider what IQ can actually tell us about moral culpability and deterrence, the sole concepts that continue to justify capital punishment in the United States today.