Two hours before he was supposed to be executed in Texas Tuesday night, a federal appeals court granted Robert James Campbell a stay of execution after deciding that new evidence of intellectual disability was “more than sufficient” to merit further consideration. Campbell, now 41, arrived on death row at age 19 after he was convicted of abduction, rape, and murder in 1991. His lawyers say that state files recovered in recent weeks include IQ tests indicating an IQ of 68 when Campbell was tested as a child and 71 at the time when he arrived on death row. When Campbell’s lawyers gave him an IQ test last month, he scored a 69.

In 2002, the Supreme Court ruled in Atkins v. Virginia that executing intellectually disabled individuals violated the Eighth Amendment’s ban on cruel and unusual punishment. The Court said that an IQ of under “approximately 70” demonstrates disability. However, the Atkins decision left states to define what it refered to as “mental retardation.” (Though clinicians now use the term “intellectually disabled,” the courts still use this older term.)

Most states have opted to use the clinical definition of intellectual disability, which is based on three factors: limitations in intellectual functioning; limitations in adaptive behavior; and the onset of these limitations before age 18 (after age 18, it is considered dementia). Like the Supreme Court, the American Association of Intellectual and Developmental Disabilities uses IQ to measure intellectual ability, with a score of around 70 and “as high as 75” qualifying as limited. The adaptive behavior standard is key because it distinguishes intellectually disabled individuals from those who are able to meet the demands of everyday life despite a low IQ.

But a number of states have established their own definitions, so that prisoners who test as intellectually disabled in one state could be eligible for execution in another. Texas, for example, uses a set of guidelines known as the Briseño factors, which consider whether people who knew the individual as a child think he was intellectually disabled and “act in accordance with that determination”; whether the individual carried out formulated plans or conducted himself impulsively; whether the individual can lie effectively; and whether his offense required forethought, planning, and complex execution, among other considerations. The Briseño factors, which were written by the Texas Court of Criminal Appeals, ask Texas citizens to compare the inmate to the character of Lennie from Of Mice and Men. “Most Texas citizens might agree that Steinbeck’s Lennie should, by virtue of his lack of reasoning ability and adaptive skills, be exempt [from execution],” they read. By implication, an individual who seems less impaired than the fictional character would not be exempt. The Briseño factors are not recognized by a single clinical or scientific body.

In 2012, Texas executed Marvin Wilson, a convicted murderer who had undergone five IQ tests that determined his IQ was 61. The only psychological expert to express an opinion at his trial said he was intellectually disabled. Wilson read at around a five-year-old’s reading level and sucked his thumb into adulthood. The Supreme Court declined to issue a stay.

Two years prior, Texas tried to execute Daniel Plata, another intellectually disabled inmate, but Plata’s life was spared after a state district judge found problems with the evaluation methods psychologist George Denkowski used to determine Plata’s intellectual ability. Judge Mark Kent Ellis noted that “it is not generally accepted practice within the field of psychological assessment to obtain an IQ score, declare it invalid, and then estimate an IQ score with numbers,” as Denkowski had done, meaning the psychologist used his own, non-standard questions and estimations to calculate Plata’s IQ.

A few states draw a hard line on IQ scores without any consideration of adaptive behavior: Alabama, Kentucky, Virginia, Idaho, and Florida all say any prisoner who scores above 70 on an IQ test is not intellectually disabled, according to NPR. In these five states, only two appeals on the grounds of of intellectual disability—or about 2 percent—have been successful since the Atkins verdict. Other death penalty states have seen a 28 percent success rate.

One case in Florida is asking the Supreme Court to revise the Atkins definition of “mental retardation.” Since being sentenced to death in 1981 for murder, Freddie Lee Hall has made multiple attempts to appeal his death sentence based on his low IQ, which ranges from 71 to 80 in the multiple IQ tests he has taken. Hall reads at less than a third grade reading level and a court psychologist found he functions at a mental age level of thirteen years. The Florida courts, however, determined he was eligible for the death penalty despite his limits in intellectual functions and adaptive behavior because his IQ is higher than 70. The Supreme Court has yet to make a decision in Hall’s case.