In Chapter 4, we reported that violent offending has been consistently associated with intellectual deficits. Studies comparing violent to nonviolent offenders consistently report that violent offenders have lower intelligence scores than nonviolent ones, and incarcerated violent offenders have surprisingly low IQ scores. This was true for both verbal ability and performance IQ. Genetically-informed studies have concluded that about 50% of the variability in intelligence is probably due to genetics, but this leaves substantial variability due to environment. Some risk factors, such as neglect, have been inversely correlated with IQ scores at a greater than chance level.

The major implication of these findings is to address any environmental risks for low IQ and use interventions to enhance cognitive function. Intelligence levels do not usually change dramatically with intervention, so an emphasis on particular skill development and problem solving makes the best sense. Some interventions for children have caused marked improvements in IQ scores, and interventions for particular cognitive skills have significant potential. Interventions aimed at enhancing intelligence in young children should not be applied in a vacuum; they should emphasize the environmental causes of intelligence deficits such as living in a deprived environment or parental neglect, and these may exert additional effects through other routes. We can speculate that interventions designed to raise IQ scores per se carried out with those already prone to violence are less likely to be effective than those offered to young children.

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