The death chamber and the steel bars of the viewing room at the state penitentiary in Huntsville, Tex. (Handout/Reuters)

Carol Steiker is the Henry J. Friendly professor and special adviser for public service at Harvard Law School. Jordan Steiker holds the Judge Robert M. Parker endowed chair at the University of Texas School of Law and is the director of the school’s Capital Punishment Center. They are the co-authors of “Courting Death: The Supreme Court and Capital Punishment.”

Even as the United States remains the only Western democracy that executes its citizens, the Supreme Court has exempted certain categories of our most vulnerable individuals from execution, including those with intellectual disability. Texas, however, routinely fails to enforce this protection. An upcoming case gives the court the opportunity to bring Texas in line with constitutional requirements.

Texas leads the country in executions, having executed four times as many inmates as Oklahoma (the second leading executioner) since 1976. In fact, Texas is responsible for more than a third of the inmates executed nationwide over the past 40 years. One of the ways Texas manages to execute at such a fast clip is by ignoring basic constitutional safeguards in capital cases. Nowhere is its refusal to toe the constitutional mark more evident than in the state’s overzealous efforts to execute people whose intellectual deficits place them within the bottom 3 percent of the U.S. population.

In 2002, the Supreme Court declared in Atkins v. Virginia that the Constitution forbids the execution of people with intellectual disability (its opinion used the now-outdated term “mental retardation”). The court did so because of the “widespread judgment” that offenders with such disabilities are less culpable for their crimes, given their limitations in understanding and processing information, learning from their mistakes and avoiding impulsive behavior. The court reasoned that the death penalty is supposed to be confined to the most highly aggravated offenses and offenders, and people with intellectual disability simply are not the “worst of the worst.”

Almost immediately, Texas defied the Supreme Court’s decision. Its highest criminal court, the Court of Criminal Appeals, declared that it was unwilling to exempt everyone who satisfies traditional clinical criteria for intellectual disability, even though that was the obvious point of the Supreme Court’s ruling. Instead, the court invented its own approach to intellectual disability to identify offenders whom “Texas citizens would agree” should be exempted from the death penalty. As its guidepost, the court declared that John Steinbeck’s fictional character Lennie from “Of Mice and Men” was the type of offender “most” Texas residents would regard as deserving exemption. In the novel, Lennie loves soft animals, especially rabbits, but ends up accidentally killing them because he is unaware of his own strength, and he does the same with a woman whom he encounters.

The resulting approach to intellectual disability focuses on questions that no medical professional would deem appropriate in diagnosing intellectual disability, such as whether an offender’s family and friends thought he had intellectual disability, or whether the offender could respond “rationally” to questions, or whether the offender was capable of lying. Instead of relying on the same approach to intellectual disability that Texas uses in every other context (such as placement in special education or eligibility for disability benefits), the court sought to redefine the condition in the capital context so that only offenders who meet crude stereotypes about intellectual disability are shielded from execution. As a result, inmates who would have been found ineligible for execution in every other state have been denied relief in the Texas courts. Some have been executed.

The Supreme Court will now review the Texas approach. In a case that will be argued this month, Moore v. Texas, the Court of Criminal Appeals took its war with clinical standards a major step further by prohibiting the use of current medical standards in Texas courts as they evaluate disability in death penalty cases.

In a recent case from Florida, the Supreme Court said that states cannot arbitrarily depart from standard clinical practices as they implement the court’s ban on executing people with intellectual disability. Unfortunately, the Texas courts have not yet heard the message. Texas still executes individuals who would be deemed intellectually disabled for all other purposes, and the Texas courts still search for the mythical “Lennie” rather than follow well-established professional practices.

It is time for the Supreme Court to make clear (yet again) that the Constitution applies to Texas and that its decision exempting people with intellectual disability was not a suggestion but a constitutional rule.