Low intelligence and low levels of achievement in school have consistently been found to be associated
with youth violence. In the Philadelphia project, poor intelligence quotient (IQ) scores in verbal and performance IQ tests at the ages of 4 and 7 years, and low scores in standard school achievement tests at 13–14 years, all increased the likelihood of being arrested for violence up to the age of 22 years. In a study in Copenhagen, Denmark, of over 12 000 boys born in 1953, low IQ at 12 years of age significantly predicted police-recorded violence between the ages of 15 and 22 years. The link between low IQ and violence was strongest among boys from lower socioeconomic groups.