In Florida a criminal with an IQ of 70 or more is deemed competent to be executed. Pat Sullivan / AP Photo

A Florida inmate is challenging the state's rigid use of IQ to determine mental competency, in a case that spotlights the issue of the intellectually disabled on death row.

Nearly 12 years after barring the death penalty for the intellectually disabled, the Supreme Court on Monday will hear the appeal of Freddie Lee Hall. If he loses, the 68-year-old, who has spent 35 years on death row and has an IQ of 60 to 80, is likely to be executed.

IQ scores of 90 to 110 are considered average on most accepted intelligence scales.

In June 2002 the high court barred states from executing intellectually disabled inmates but until now has let states determine the level of mental competency. In Florida a criminal with an IQ of 70 or more is deemed competent to be executed.

Hall, convicted for the rape and murder of a pregnant woman, has been declared "mentally retarded" since childhood. In nine tests from 1968 to 2008, Hall scored as low as 60 and as high as 80, with his most recent IQ 69 to 74, according to the state.

In his appeal, his lawyers argue that Hall falls within a five-percent margin of error cited by psychiatrists and experts when assessing IQs.

"No IQ test is a perfect measure of intellectual ability," his written appeal says.

"The best that any test can do, even when correctly administered, is to provide a certain level of confidence, as a statistical matter, that a person's true IQ score ... is within a particular range."

It adds, "The predictable consequence of Florida's rule is that persons with mental retardation will be executed. Without this court's intervention, that will happen here."

Hall's lawyers argue that the rigid threshold is inconsistent with the commonly agreed-upon definition of mental disability, embraced by the Supreme Court in its 2002 decision, which most U.S. states use.

John Blume, a professor of Law at Cornell University, wrote on an American Constitution Society blog, "A ruling in Hall's favor will not affect many states — or for that matter many cases ... Nevertheless, the case is still important, not only to Hall — whose life literally hangs in the balance — and other Floridians with an intellectual disability, but also to make clear that states cannot narrow a categorical ban created by the Supreme Court intended to protect a vulnerable group from wrongful execution."

Florida Attorney General Pamela Jo Bondi said in court papers that for death row inmates "the risk of overdiagnosis of mental retardation is particularly pronounced."

"(They) have every incentive to secure such a diagnosis," she said.

Florida officials have also argued that there's no national consensus about a strict IQ limit and no reason for the court to impose one.

The outcome of Hall's case is unlikely to affect the busiest death penalty state, Texas, which does not impose a rigid IQ criterion to assess mental disability.

In Georgia, Florida's neighbor, death row inmates need to prove intellectual disability "beyond a reasonable doubt." Warren Hill, who has an IQ of 70, had his application for a stay dismissed in October by the Supreme Court.

Al Jazeera and wire services