A death row prisoner who has been medically diagnosed as "mentally retarded" and therefore exempt from execution is set to die on Tuesday in Texas, a state that rejects scientific consensus and instead applies its own definition of learning difficulties based on a character in a John Steinbeck novel.

Barring a last minute intervention by the courts, Marvin Wilson, 54, will be put to death by lethal injection even though he has been subjected to scientifically-recognised tests that show him to be intellectually disabled – or "mentally retarded" as the US legal system still calls the condition.

In 2002, the US supreme court banned executions for all such prisoners under the Eighth Amendment of the constitution that prohibits excessive punishment. The 2002 ban, in Atkins v Virginia, is categorical: individuals with mental retardation cannot be put to death. The court allowed some discretion on the part of individual states to devise procedures for administering the injunction, but no right to ignore it.

Texas took that discretion to mean – wrongly in the view of many lawyers and mental health experts – that it could set its own definition of retardation.

Instead of a clinical or scientific approach, based on widely recognized tests set out by the American Association on Intellectual and Developmental Disabilities, Texas decided to go its own way.

It came up with a set of seven criteria, known as "Briseno factors" after the decision that announced them, to determine which prisoners with learning difficulties should live and which should die.

The determinants were posited around the character Lennie Small in Steinbeck's 1937 novel Of Mice and Men.

"Most Texas citizens," the argument ran, "might agree that Steinbeck's Lennie should, by virtue of his lack of reasoning ability and adaptive skills, be exempt" from execution. By implication anyone less impaired than Steinbeck's fictional migrant ranch worker should have no constitutional protection.

"If Wilson is executed on Tuesday, Texas will be rendering the US supreme court's Eighth Amendment prohibition on the execution of mentally retarded prisoners a prohibition in name only," said Lee Kovarsky, Wilson's lawyer.

Kovarsky will be petitioning the supreme court on Monday for a stay of execution to give time for Texas's out-lying approach to intellectual disability to be confronted. The supreme court has a growing problem in this area with states coming close to open defiance of the will of the highest judicial panel in the land in relation to the execution of people with learning difficulties. Last month Warren Hill came within 90 minutes of execution in Georgia.

The prisoner had been diagnosed by the state's own courts as being "mentally retarded" in all probability, but Georgia is the only state in the union that insists on a standard of proof of intellectual impairment that is "beyond a reasonable doubt".

Texas's unique system for defining 'retardation'

Marvin Wilson was put on death row for the 1992 murder of a police drug informant in Beaumont, Texas. The circumstances of the crime had all elements that make death sentences for people with learning difficulties problematic: Wilson was one of two perpetrators, leaving him vulnerable to his more sophisticated accomplice, and the main witness against him was that accomplice's wife who claimed she heard him confess to pulling the trigger.

Wilson was subjected to a raft of tests to determine his intellectual abilities, using internationally recognised clinical procedures. The tests were carried out by Dr Donald Trahan, a neuropsychologist who has evaluated more than 500 patients with learning difficulties. Trahan personally administered to Wilson the TONI-II, the Raven Standard Progressive Matrices, the Peabody Individual Achievement Test-Revised, the Wide Range Achievement Test-3rd, the Language Assessment Battery, the Orientation Evaluation, the Verbal Selective Reminding Test, the Visual Reproduction Subtest and the Remote Sensory Evaluation.

In addition, Trahan interviewed Wilson for eight hours and analysed his past school records that showed he had a reading and writing level of a seven-year-old.

The young Wilson was placed in special education classes, where he was bullied by other kids who called him "stupid", "dummy" and "retarded".

He was deemed unable to manage his own money and was incapable of self-direction.

He could not, for instance, cut the grass or use a ladder on his own, or dress himself properly with matching socks and buttoned up shirt. The tests gave Wilson an IQ score of 61 – putting him in the lowest percentile of the population.

After all that evidence gathering, Trahan diagnosed the prisoner as having "mental retardation". Though it was mild, the condition still clearly fell under the US supreme court's prohibition. Texas, by contrast, applying its Lennie Small criteria, carried out not a single cognitive assessment of Wilson. It adduced no evidence and requested no testimony.

Experts in intellectual disability have warned that Texas's unique system for defining "retardation" puts at risk many people with learning difficulties who should be covered by the constitutional ban.

As the American Association on Intellectual and Developmental Disabilities has put it in another capital case, Texas's "impressionistic 'test' directs fact-finders to use 'factors' that are based on false stereotypes about mental retardation that effectively exclude all but the most severely incapacitated."