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IQ tests are intrinsically imprecise. On one, Freddie Lee Hall scored 71. On other tests, he's gotten various scores between 60 and 75.

The problem for Mr. Hall is that until today, scoring over 70 on even one IQ test gave Florida the green light to him. That one time score of 71 put Mr. Hall just over the line in the sand drawn by the state's legislature, after the 2002 Supreme Court decision finding that it violates the Eighth Amendment to kill people who are mentally disabled.

No one seems to dispute what the Florida Supreme Court 15 years ago, that "there is no doubt that [Mr. Hall] has serious mental difficulties, is probably somewhat retarded, and certainly has learning difficulties and a speech impediment."

And that's exactly the point – even if someone meets the other criteria for mental disability, Florida was clinging to an unscientific and outmoded cut-off number of 70 and using this erroneous indicator to draw a line between those killed and those sent to prison for life.

This has happened often, not just to Mr. Hall.

One group of scholars determined that of the defendants who lost mental disability challenges to their death sentences in Florida had lost because of the strict 70 IQ cut-off score. And a large minority of states also adopted schemes under which the difference of one or two IQ points demarcated who could be killed and who could not.

Now these states have to change their dubious and dangerous ways of determining mental disability, thanks to yesterday's favoring science over arbitrariness.

The strict IQ cut-off rests on the fiction that a score, whether it is 70 or 71, is the precise value of the person's intellectual functioning. Psychiatrists and mental health professionals are crystal clear: an IQ score is an approximation, and because of a host of factors that score must be understood within a standard error of measurement, most commonly understood as a range of plus and minus five points. In other words, we can be relatively confident that a defendant whose IQ score is reported as 71 has a true IQ between 66 and 76, not that his or her score was exactly 71.

The professional community of psychiatrists is unanimous that a strict 70 IQ cut-off score will erroneously exclude individuals who qualify as intellectually disabled. Given this "inherent imprecision," in the of Justice Anthony Kennedy, it is no longer constitutional to use the IQ cut-off method to determine who will live and who will die. In rejecting Florida's cut-off score of 70, the Court stressed repeatedly the importance of consistency with the definitions of intellectual disability adopted and understood by the "medical community" and "medical experts."

In the battle between expediency and science, this is a clear win for science.

While the Supreme Court's ruling today comes too late for the unknown number of intellectually disabled death row inmates who already have been executed under strict cut-off schemes, it takes a large step towards ensuring that the death penalty is not used against people with intellectual disabilities, in Florida and beyond.

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