Bad science behind unconstitutional US executions


Life 15 August 2012

FEW topics invoke such passionate feelings in the US as the death penalty, especially when applied to convicts with the intellectual capacity of children.

In 2002, the US Supreme Court ruled that executing people with an intellectual disability was unconstitutional, citing a “national consensus” against it. Unfortunately the ruling failed to end the practice, in that it left it up to individual states and courts to decide how to assess intellectual disability.

Those assessments are often flawed. Many courts fail to accept scientific facts on the limitations of IQ testing, such as inevitable measurement errors. Inmates are also on death row because their IQ happened to be assessed using old tests (see “Death by IQ: US inmates condemned by flawed tests“).

Convicts pleading intellectual disability must also prove that they have significant deficits in basic life skills, such as dealing with money. Assessment of this is, if anything, a bigger problem.


Texas, which executes more people than any other state, is home to some particularly egregious practices. For example, though validated tests of life skills exist, Texan courts favour standards that have no scientific basis. These include vague considerations such as the ability to tell lies and whether people who know the defendant think he or she is “mentally retarded”. This is absurd, turning a complex clinical judgement over to witnesses with no specialist training.

Some Texan courts also accepted expert witness testimony from psychologist George Denkowski, who argued that life-skills scores for people from impoverished backgrounds should be adjusted upwards because they were brought up in an environment in which skills such as hygiene were not valued.

Denkowksi was stopped from making assessments in April 2011, but by then 14 of the men he assessed were on death row and two had already been executed.

The final injustice is that only people who had the deficits from childhood are exempt from execution. A convict mentally disabled by a brain injury suffered as an adult cannot seek clemency.

It’s time the Supreme Court was presented with a case allowing it to clean up this mess of its own creation.

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