It is unconstitutional to execute the mentally disabled. Warren Hill was almost certainly mentally disabled, but "almost certainly" was not good enough for the State of Georgia. So on Tuesday night, Georgia executed Hill for murdering his cellmate in 1990. The Supreme Court denied Hill’s petition to hear his case and refused to halt his execution, after a lengthy and hard-fought appeals process, with only Justices Stephen Breyer and Sonia Sotomayor voting for a stay.

All seven medical experts who examined Hill—including the three experts appointed by the state—found him to be intellectually disabled. Hill’s lawyers claimed their client had “the emotional and cognitive ability of a young boy” and that his execution would violate the Eighth Amendment. The American Bar Association, the ACLU, the Georgia chapter of the NAACP, the Vatican, the European Union, and mental-health organizations sided with Hill during his four-year legal battle. Even the family of Hill’s victim and some jurors from his trial opposed his execution.

“This execution is an abomination,” Brian Kammer, Hill’s attorney, said in a statement. “Like the execution of Jerome Bowden in 1986, the memory of Mr. Hill’s illegal execution will live on as a moral stain on the people of this State and on the courts that allowed this to happen.”

Warren Hill in an undated photograph
(Georgia Dept. of Corrections/Reuters)

As far back as medieval England, courts recognized exemptions for people who suffered from disabilities or mental illness. They could not be charged with crimes, let alone be executed for them. Lord Blackstone, whose writings on the English common law tradition influenced the Founding Fathers, argued that their diminished mental capacity prevented them from adequately defending themselves in court. “In criminal cases therefore,” he wrote in the language of his time, “idiots and lunatics are not chargeable for their own acts, if committed when under these incapacities: no, not even for treason itself.” The Supreme Court quoted Blackstone when it forbade executions of the insane in 1986; the Court later banned executions of the intellectually disabled in the 2002 case Atkins v. Virginia.

But what constitutes intellectual disability? The Atkins ruling crucially left that determination to the states themselves. Georgia requires defendants to prove their disability “beyond a reasonable doubt.” This is the highest legal burden of proof short of absolute certainty. It exists to protect the people from the malice or incompetency of the state during a criminal trial. Prosecutors must convincingly prove that defendants committed every element of a crime to satisfy it.

Georgia turns this shield into a sword against death-row inmates with intellectual disabilities. Instead of requiring that the state prove these inmates’ mental competence beyond a reasonable doubt before depriving them of their lives, Georgia insists that the inmates prove their disabilities beyond all reasonable doubt in order to live. Two lower courts determined that Hill was intellectually disabled under the lesser “preponderance of the evidence” standard. In another state, Hill likely would have received a life sentence at that point. But this was insufficient to meet Georgia’s high threshold.

Debating degrees of certainty here might seem esoteric, but even slight shifts in them can mean the difference between life and death for a human being. When a divided Georgia Supreme Court upheld Georgia’s reasonable-doubt threshold in 2012, the court’s then-chief justice enunciated what this threshold meant in practice in her dissent:

Despite the federal ban on executing the mentally retarded, Georgia’s statute, and the majority decision upholding it, do not prohibit the state from executing mentally retarded people. To the contrary, the State may execute people who are in all probability mentally retarded. The State may execute people who are more than likely mentally retarded. The State may even execute people who are almost certainly mentally retarded. Only if a mentally retarded person succeeds in proving their retardation beyond a reasonable doubt will his or her execution be halted.

The state of Georgia, for its part, told the Supreme Court on Tuesday that its procedural protections for intellectually disabled defendants actually surpass those of other states. Defendants in Georgia can call as many expert witnesses to prove their disability, they told the justices in their brief, and can use intellectual disability as a mitigating factor in sentencing even if it wasn’t proven during the trial. While generous, these factors don't substantially alter the burden of proof threshold itself.

In Hall v. Florida last year, the Court signaled that Atkins did not give the states carte blanche when crafting their intellectual-disability standards. Florida’s statute allowed the execution of inmates with an IQ of 70 or lower without considering other factors. The justices found this “bright line” rule insufficiently protective and struck it down as unconstitutional. “Intellectual disability is a condition, not a number,” wrote Justice Anthony Kennedy for the majority. Hill’s lawyers hoped the Hall decision would give them an opening to renew their challenge of Georgia’s reasonable-doubt threshold at the Supreme Court and save Hill from lethal injection. It did not.