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A few dropped marks in an IQ test is no big deal – unless you’re on Death Row. US federal law bans the death penalty for the “intellectually disabled”; several American states use an IQ score of 70 as the qualifying criterion, with inmates scoring above that facing execution (70 is two standard deviations below the mean score of 100). Recently, lawyers successfully challenged the use of a fixed IQ threshold to determine a convict’s fate, arguing that the science of intelligence testing is imprecise and that IQ tests alone could not determine mental competence.

The 5-4 ruling by the US Supreme Court centred on the case of Freddie Lee Hall, a double murderer who has been on Death Row for 36 years. After numerous tests yielding scores between 60 and 80, reported the journal Nature, Hall was finally assigned an IQ of 71. The state of Florida wanted to execute him; his lawyers argued, with backing from the American Psychological Association, that any score has an error margin of five points either way. The Supreme Court ruled in Hall’s favour, saying that Florida could not use IQ as the “final and conclusive” measure of intellectual ability. The ruling could prompt a flood of appeals.

James Harris, a psychiatrist at Johns Hopkins University in Maryland, told the journal that courts should scrap outdated IQ assessments in favour of tests for “adaptive functioning”, which measure impulse control, empathy and social skills. The APA is now advocating a new definition of intellectual disability that reflects current scientific thinking on intelligence.

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