Inaccurate IQs could be a matter of life and death

by Jessica Griggs

Some people in the US may have been wrongly executed because of inaccuracies in the IQ tests used to assess them. But the inaccuracies may also have seen some escape execution if they scored lower on the tests than their real IQ.

Simon Whitaker, a psychologist at the University of Huddersfield, UK, searched the literature to identify sources of error in two of the most commonly used IQ tests: the third edition of the Wechsler Adult Intelligence Scale (WAIS-III), which can be used on people aged between 16 and 93, and the fourth edition of the Wechsler Intelligence Scale for Children (WISC-IV), suitable for children aged between 6 and 16.

In both cases, the test manuals state that you can be 95 per cent confident that a person’s true IQ lies within 4 points of their test score.

However, Whitaker found that for people with extremely poor WAIS-III scores, their actual IQ could be up to 16 points higher or 26 points lower than the score achieved. In the WISC-IV test, actual IQ may be up to 25 points higher or 16 points lower than the score achieved.

In 2002, the US Supreme Court ruled that the execution of any mentally retarded person violates the Eighth Amendment’s prohibition against "cruel and unusual punishments". An IQ of 70 is generally considered the threshold – making Teresa Lewis (IQ 72), who was executed in September last year, a borderline case.

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