Marvin Wilson, aged 53, has an IQ of 61 and still sucks his thumb - and is set to die by lethal injection at a Texas prison next week.
He was convicted of a 1992 murder which he says he did not commit - and many activists also argue that his execution conflicts with a Supreme Court ruling which forbids states from putting mentally retarded people to death.
His supporters believe that his exceptionally low IQ qualifies him to be put in that category.
‘This case really does very much push the line,’ Paul Campos, a professor at the University of Colorado Law School told the Huffington Post. ‘We're talking about a mental child.’
According to the Post, in the state of Texas individuals having an IQ of 70 or below are deemed mentally retarded. In many other U.S. states the boundary is 75 or lower.
But Texas has found that despite his low IQ level he is not mentally slow enough to qualify for the Supreme Court's ruling that would spare his life.
That 2002 ruling, Atkins v. Virginia, states that 'the mentally retarded should be categorically excluded from execution' due to 'their disabilities in areas of reasoning, judgment and control of their impulses.'
'I couldn't believe it when I saw him still sucking his thumb when his son was born,' Wilson’s younger sister Kim Armstrong recalled in a 2003 affidavit. 'Marvin was in his twenties.'
Growing up in Southern Texas, Wilson was enrolled in special education classes, but still performed very poorly.
He failed the 7th grade - barely passing the second time with F's and D's - and twice only advanced to a higher grade level through social promotion, according to a 2004 report by neuropsychologist Dr Donald Trahan with the Center For Behavioral Studies in Texas.
Lewis is currently serving a life sentence for Williams' murder.
'The most shocking aspect of this case is that the state of Texas has never even bothered to present any evidence contesting the defence’s claim that Wilson is mentally retarded,' Campos wrote in an article for Salon.com on Thursday.
The 2002 Supreme Court ruling allowed states to determine how they choose which individuals are mentally retarded or only claiming to be to escape execution.
The court has since been asked to review their ruling on those state rights.
Wilson's attorneys have filed an appeal with the Supreme Court and have also requested a stay of execution by Gov Rick Perry and the Texas Board of Pardons.
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