From The New Yorker:
“Since the Supreme Court reinstated the death penalty in the United States, in 1976, Texas has been responsible for more than a third of the country’s executions—five hundred and thirty-eight out of a thousand four hundred and forty. The most egregious reason is the state’s unique and grudging approach in cases where the defendant claims intellectual disability.
In 2002, in Atkins v. Virginia, the Supreme Court reached the decision that, no matter how heinous the crime, an intellectually disabled person cannot be sentenced to death. Disabilities of reasoning, judgment, and control of impulses, the Court said, do not allow a person to “act with the level of moral culpability that characterizes the most serious adult criminal conduct.” Because offenders with intellectual disabilities are less blameworthy, the Court said, imposing the death penalty contributes neither to deterrence of capital crimes nor to retribution for them, and so it causes “purposeless and needless” pain and is cruel and unusual punishment.
The Court recognized that there was “serious disagreement” about which offenders were intellectually disabled. “Not all people who claim to be mentally retarded will be so impaired as to fall within the range of mentally retarded offenders about whom there is a national consensus,” the majority opinion said. (“Intellectual disability” has replaced “mental retardation” as the favored term.) The Court anticipated a variety of approaches to enforcing its prohibition, and left to the states “the task of developing appropriate ways to enforce the constitutional restriction upon its execution of sentences.”