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.