Low intelligence is an important predictor of offending, and it can be measured very early in life. In a prospective longitudinal survey of about 120 Stockholm males, (9) low IQ measured at the age of 3 years significantly predicted officially recorded offending up to the age of 30 years. Frequent offenders (with four or more offences) had an average IQ of 88 at the age of 3 years, whereas non-offenders had an average IQ of 101. All of these results held up after controlling for social class. Similarly, low IQ at the age of 4 years predicted arrests up to the age of 27 years in the Perry Preschool Study (!°) and court delinquency up to the age of 17 years in the Collaborative Perinatal Project. (!!)
In the Cambridge Study, twice as many of the boys scoring 90 or less on a non-verbal IQ test (Raven's Progressive Matrices) at the age of 8 to 10 years were convicted as juveniles as of the remainder. However, it was difficult to disentangle low intelligence and low school attainment. Low non-verbal intelligence was highly correlated with low verbal intelligence (vocabulary, word comprehension, verbal reasoning) and with low school attainment, and all of these measures predicted juvenile convictions to much the same extent. In addition to their poor school performance, delinquents tended to leave school at the earliest possible age (which was then 15 years) and to take no school examinations.
Low non-verbal intelligence predicted juvenile self-reported offending to almost exactly the same degree as juvenile convictions, suggesting that the link between low intelligence and delinquency was not caused by the less intelligent boys having a greater probability of being caught. Also, measures of intelligence and attainment predicted measures of offending independently of other variables such as family income and family size; 53 per cent of boys with low non-verbal intelligence (90 or less) at the age of 8 to 10 years were convicted up to the age of 32 years. Delinquents often do better on non-verbal performance tests, such as object assembly and block design, than on verbal tests, suggesting that they find it easier to deal with concrete objects than with abstract concepts.
Low IQ may lead to delinquency through the intervening factor of school failure; the association between school failure and delinquency has been demonstrated consistently in longitudinal surveys. In the Pittsburgh Youth Study, it was concluded that low verbal IQ led to school failure and subsequently to self-reported delinquency, but only for African-American boys. Another plausible explanatory factor underlying the link between low IQ and delinquency is the ability to manipulate abstract concepts. Children who are poor at this tend to do badly in IQ tests and in school attainment, and they also tend to commit offences, mainly because of their poor ability to foresee the consequences of their offending and to appreciate the feelings of victims.
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