In the Philadelphia project
(69), 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.