IQ in early adulthood and later risk of death by homicide: cohort study of 1 million men.

BACKGROUND Risk factors for homicide are emerging; however, the predictive value of IQ, for which there is a strong prima facie case, has yet to be examined. AIMS To examine the association between IQ and risk of death by homicide. METHOD A cohort of 968 846 men, aged 18-19 years, were administered an IQ test battery at military conscription and then followed for mortality experience over two decades. RESULTS There were 191 deaths due to homicide during follow-up. In age-adjusted analyses, a high total IQ test score was associated with a reduced rate of homicide (hazard ratio (HR) per standard deviation increase in IQ score=0.49, 95% CI 0.42-0.57). A step-wise gradient was apparent across the three IQ groups (P-value for trend <0.001). After adjustment for indicators of socio-economic position and illness at conscription, this gradient was marginally attenuated (HR=0.57, 95% CI 0.49-0.67). CONCLUSIONS High IQ test scores in early adulthood were associated with a reduced risk of death by homicide.