Originally posted on the Nature news blog

When deciding whether a defendant is too intellectually disabled to receive the death penalty, courts must take into account inherent variability in IQ scores, the US Supreme Court ruled today.

In its 5-4 decision, the court said that it is unconstitutional for states like Florida to use an IQ score of 70 as a cutoff above which a defendant is considered to be intelligent enough to understand the consequences of his or her actions.

The plaintiff in the case, Freddie Lee Hall, has been on death row in Florida for 35 years after being convicted of murdering two people in 1978. He has taken multiple IQ tests, yielding scores ranging between 60 and 80, and testimony from people who knew him suggest that he has been intellectually disabled his entire life. But under Florida law, an IQ score above 70 disqualifies a defendant from being spared execution on the basis of intellectual disability, and Florida’s Supreme Court ruled in 2012 that Hall’s scores were too high to qualify for this reprieve.

But the American Psychological Association (APA) and the American Association on Intellectual and Developmental Disabilities hold that IQ tests have an error margin of about ten points. Consequently, Hall’s lawyers argued that IQ tests are too imprecise to determine whether his score falls on one side or the other of this cut-off.

“Florida’s rule disregards established medical practice in two interrelated ways,” Justice Anthony Kennedy writes in the court’s majority opinion. “It takes an IQ score as final and conclusive evidence of a defendant’s intellectual capacity, when experts in the field would consider other evidence. It also relies on a purportedly scientific measurement of the defendant’s abilities, his IQ score, while refusing to recognize that the score is, on its own terms, imprecise.”

The Supreme Court sent Hall’s case back to Florida’s court for a reassessment. It is not yet clear what Florida, and as many as eight other states with similar laws, will adopt in lieu of the IQ threshold. But the court’s decision compels states to incorporate additional evidence if a defendant’s scores fall within the range of error.

James Harris, a psychiatrist at Johns Hopkins University in Baltimore, Maryland, and expert on intellectual disability, is pleased with the decision. “The Supreme Court validates professional practice in measurement,” he says. “They confirm the dignity of the process and the dignity of the people with intellectual disability who are being served by the process.”

But Harris would have liked to have seen the ruling go further in emphasizing the importance of the test for adaptive functioning — a person’s ability to function in society — which is another factor that the APA uses to diagnose intellectual disability. This factor, he contends, is often more relevant to a case than an IQ score, which mainly tests academic ability.

Although the APA has held for decades that IQ scores have a margin of error, Justice Samuel Alito worries that the ruling opens a can of worms, as the guidelines of professional societies change over time. Tying the law to these views will “lead to instability and continue to fuel protracted litigation,” he writes in the minority opinion. Alito adds that the court’s decision “adopts a uniform national rule that is both conceptually unsound and likely to result in confusion.”

This article is reproduced with permission from the Nature news blog. The article was first published on May 27, 2014.