Hamm v. Smith: The Limits of Legal Certainty when Science Evolves

Can states keep IQ testing people sentenced to death until they get the “right” score for execution? What is really at the heart of the Eighth Amendment’s prohibition on executing people who have an intellectual disability?

Last month, the Supreme Court considered these and other important questions when it heard oral arguments in the case of Hamm v. Smith. In Hamm, the defendant argues that the state of Alabama cannot execute him because he has an intellectual disability. Alabama argues that his multiple IQ scores above 70 mean that he does not have an intellectual disability.

As the Court grappled with definitions of intellectual disability and how courts should weigh multiple IQ scores, I observed a core tension between law and science. The law’s desire for predictable rules conflicts with the evolving nature of scientific knowledge.

Hamm is one of a long line of cases dating back to the Supreme Court’s 2003 decision in Virginia v. Atkins, which held that execution of a person with an intellectual disability violates the Eighth Amendment. Notably, the Atkins Court largely delegated the definition of intellectual disability to the states. 

Cases decided after Atkins, such as Hall v. Florida (2014) and Moore v. Texas (2017), have struggled to establish the constitutional baseline that state definitions of intellectual disability must adhere to. In these post-Atkins cases, the Court has stated that a single IQ score above 70 cannot outweigh evidence of adaptive functioning deficits, and a single IQ score below 70 cannot outweigh repeated scores above 70. 

Justice Kennedy, writing in Hall v. Florida, summarizes the throughline of these post-Atkins cases: “Intellectual disability is a condition, not a number.” Yet, despite this science-grounded view, the Court continues to allow states to define intellectual disability in different ways.

These cases highlight an important tension at the heart of Atkins: the inflexibility of legal doctrine struggles to adapt to changing science. The Court decided to leave defining intellectual disability to state governments, but this conflicts with the Court’s wish to tie Eighth Amendment protections to scientific standards.

Many psychologists have raised concerns about the continued use of IQ scores, given the potential impact of the Flynn effect. The Flynn effect refers to the observed steady rise in average IQ scores since the inception of IQ tests in the mid-20th century. Put simply, an IQ score of 75 in the 1940s likely suggests greater intellectual functioning than a person with an IQ score of 75 today.

Empirical analysis of court decisions on Atkins claims reveals inconsistent treatment of the Flynn effect. Some courts rightly account for the Flynn effect by recognizing that scores above 70 can still indicate intellectual disability. Thus, these courts do not treat a score of 70 as a bright-line cutoff and often mathematically adjust IQ scores to account for the Flynn effect. Other courts do not account for the Flynn effect at all.

The execution of Warren Hill in 2015, despite compelling evidence of an intellectual disability, demonstrates judicial divergence in deferring to psychologists and other experts. Both state and defense experts in Hill’s case testified that he had an intellectual disability, but the Georgia Supreme Court split 4-3, rejecting his Atkins claim.

One of the amicus briefs submitted to the Supreme Court in Hamm v. Smith attacks several professional psychological organizations’ changed guidance for practitioners evaluating Atkins claims (which they say expands diagnoses of intellectual disability while ignoring the risk of false positives). However, changes in scientific consensus are not evidence of opportunism; they are the product of sustained empirical research and improved understanding. The ability of science to change and adapt to new data only strengthens its validity.

Still, the critique of changing positions by professional scientific organizations taps into a frustration the legal field routinely faces when dealing with science beyond the area of intellectual disability. The law prefers stability and predictability: values potentially threatened by shifting standards. However, by tying ineligibility for execution to the scientific concept of intellectual disability, the Atkins Court necessarily required states to respond to new scientific understandings about the concept.

If our legal system rightly seeks to incorporate scientific and empirical understandings about the world into its decisions, it must recognize that those understandings can and will shift over time.

Another issue faced by legal standards that incorporate science is the difficulty posed by making judges evaluate scientific evidence. Courts evaluating the adequacy of a state’s definition of intellectual ability for Atkins claims necessarily must make judgments about the current state of psychological science. Just as Justice Kagan acknowledged that the members of the Supreme Court “are not, like, the nine greatest experts on the internet,” scholars debate how effectively judges can evaluate scientific evidence. 

If our legal system rightly seeks to incorporate scientific and empirical understandings about the world into its decisions, it must recognize that those understandings can and will shift over time. For centuries, courts routinely accepted eyewitness identification without question, but many courts rightly recognized and accounted for the fallibility of eyewitnesses revealed by modern memory and perception research. 

As neuroscientist Dr. Thomas Albright recently wrote, there will always be tension between the ideal of legal decisions based on the best standards of evidence possible and the reality of scientific understanding that is subject to change when faced with new data. However, ignoring or pushing back against the reality of changing science risks prioritizing total predictability over inaccuracy. A focus on predictability above all risks a “law trapped in amber,” adorned with the mere veneer of scientific validity. 

In the context of adjudicating a person’s Atkins claim, an overriding desire for certainty, such as that provided by IQ score cut-offs, risks turning intellectual disability into a number, not a condition. In deciding Hamm v. Smith, I hope the Court properly recognizes the nuances of IQ scores as an estimate of intellectual disability.

With life and death stakes, both the legal and scientific fields have a duty to continue to learn from and challenge each other as the fields keep evolving.