Cognitive ability and risk for substance misuse in men: genetic and environmental correlations in a longitudinal nation-wide family study.

AIMS: To investigate the association in males between cognitive ability in late adolescence and subsequent substance misuse-related events, and to study the underlying genetic and environmental correlations. 

DESIGN: A population-based longitudinal study with three different family-based designs. Cox proportional hazards models were conducted to investigate the association at the individual level. Bivariate quantitative genetic modelling in (1) full brothers and maternal half-brothers, (2) full brothers reared together and apart and (3) monozygotic and dizygotic twin brothers was used to estimate genetic and environmental correlations. 

SETTING: Register-based study in Sweden. 

PARTICIPANTS: The full sample included 1 402 333 Swedish men born 1958-91 and conscripted at mean age 18.2 [standard deviation (SD) = 0.5] years. A total of 1 361 066 men who had no substance misuse events before cognitive assessment at mandatory military conscription were included in the Cox regression models, with a follow-up time of up to 35.6 years. 

MEASURES: Cognitive ability was assessed at conscription with the Swedish Enlistment Battery. Substance misuse events included alcohol- and drug-related court convictions, medical treatments and deaths, available from governmental registries. 

FINDINGS: Lower cognitive ability in late adolescence predicted an increased risk for substance misuse events [hazard ratio (HR) for a 1-stanine unit decrease in cognitive ability: 1.29, 95% confidence interval (CI) = 1.29-1.30]. The association was somewhat attenuated within clusters of full brothers (HR = 1.21, 95% CI = 1.20-1.23). Quantitative genetic analyses indicated that the association was due primarily to genetic influences; the genetic correlations ranged between -0.39 (95% CI = -0.45, -0.34) and -0.52 (95% CI -0.55, -0.48) in the three different designs. 

CONCLUSIONS: Shared genetic influences appear to underlie the association between low cognitive ability and subsequent risk for substance misuse events among Swedish men.