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Intelligence in Early Adulthood and Subsequent Risk of Assault: Cohort Study of 1,120,998 Swedish Men

From the MRC Social and Public Health Sciences Unit (E.W., G.D.B.), University of Glasgow, Glasgow, UK; Centre for Cognitive Ageing and Cognitive Epidemiology (G.D.B., I.J.D.), Department of Psychology, University of Edinburgh, Edinburgh, UK; The George Institute for International Health (G.D.B.), University of Sydney, Sydney, Australia; MRC Epidemiology Resource Centre (C.R.G.), University of Southampton, Southampton, UK; and Department of Public Health Sciences (P.T., F.R.), Karolinska Institute, Stockholm, Sweden.

Address correspondence and reprint requests to Finn Rasmussen, PhD, MD, Child and Adolescent Public Health Epidemiology Group, Department of Public Health Sciences, Karolinska Institute, SE-17176 Stockholm, Sweden. E-mail: finn.Rasmussen{at}ki.se

Objective: To examine the association between low intelligence (IQ) and increased risk of assault. Previous studies have been relatively small, have not adjust for socioeconomic status, and have not examined method-specific assaults.

Methods: Cox proportional hazards regression was used to explore IQ associations with assault by any means and by four specific methods in a large prospective cohort of 1,120,988 Swedish men. Study members had IQ measured in early adulthood and were well characterized for socioeconomic status in childhood and adulthood. Men were followed-up for an average of 24 years, and hospital admissions for injury due to assault were recorded.

Results: A total of 16,512 (1.5%) men had at least one hospital admission for injury due to assault by any means during follow-up. The most common assault was during a fight (n = 13,144), followed by stabbing (n = 1,211), blunt instrument (b = 352), and firearms assaults (n = 51). After adjusting for confounding variables, lower IQ scores were associated with an elevated risk of hospitalization for assaults by any means (hazard ratio per standard deviation decrease in IQ, 1.51; 95% confidence interval, 1.49, 1.54) and for each of the cause-specific assaults: fight: 1.48 (1.45, 1.51); stabbing: 1.68 (1.58, 1.79); blunt instrument: 1.65 (1.47, 1.85); and firearms: 1.34 (1.00, 1.80). These gradients were stepwise across the full IQ range.

Conclusions: Low IQ scores in early adulthood were associated with a subsequently increased risk of assault. A greater understanding of mechanisms underlying these associations may provide opportunities and strategies for prevention.

Key Words: IQ • assault • socioeconomic status • cohort

Abbreviations: IQ = intelligence; SES = socioeconomic status; ICD = International Classification of Diseases; HR = hazard ratio; CI = confidence interval; SD = standard deviation.