Background Recent reports have linked high childhood IQ scores with excess alcohol intake and alcohol dependency in adult life, but the relationship with illegal drug use in later life is relatively unknown.
Methods The authors used data from a large population-based birth cohort (1970 British Cohort Study) with measures of lifetime cannabis and cocaine use, parental social class and psychological distress at 16 years; cannabis, cocaine, amphetamine, ecstasy and polydrug use (more than three drugs) in the past 12 months; and social class, educational attainment and gross monthly income at 30 years. All members of the cohort with IQ scores at 5 or 10 years were eligible to be included in the analyses.
Results Of the 11 603 (at 5 years) and 11 397 (at 10 years) cohort members eligible, 7904 (68.1%) and 7946 (69.7%) were included in the analyses. IQ scores at 5 years were positively associated with cannabis (OR (bottom vs top tertile) =2.25, 95% CI 1.71 to 2.97) and cocaine use (OR 2.35, 95% CI 1.41 to 3.92) in women and with amphetamines (OR 1.46, 95% CI 1.03 to 2.06), ecstasy (OR 1.65, 95% CI 1.15 to 2.36) and polydrug use (OR 1.57, 95% CI 1.09 to 2.26) in men at 30 years. IQ scores at 10 years were positively associated with cannabis, cocaine (only at 30 years), ecstasy, amphetamine and polydrug use. Associations were stronger in women than in men and were independent from psychological distress in adolescence and life-course socioeconomic position.
Conclusion High childhood IQ may increase the risk of illegal drug use in adolescence and adulthood.
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Funding The 10-year follow-up was carried out by the Department of Child Health, Bristol University. The 30-year follow-up was carried out under the auspices of the Joint Centre for Longitudinal Research (comprising the Centre for Longitudinal Studies, Institute of Education, University of London, the International Centre for Health and Society, University College Medical School, London, and the National Centre for Social Research). The authors would like to thank the UK Data Archive, University of Essex, for providing the data. The original data creators, depositors or copyright holders, the funding agencies, and the UK Data Archive bear no responsibility for the analyses and interpretation presented here. The work was undertaken at The Centre for the Development and Evaluation of Complex Interventions for Public Health Improvement (by JW), a UKCRC Public Health Research: Centre of Excellence. Funding from the British Heart Foundation, Cancer Research UK, Economic and Social Research Council (RES-590-28-0005), Medical Research Council, the Welsh Assembly government, under the auspices of the UK Clinical Research Collaboration, and the Wellcome Trust (WT087640MA, by GDB) is gratefully acknowledged. GDB would also like to acknowledge the Centre for Cognitive Ageing and Cognitive Epidemiology, Department of Psychology, University of Edinburgh, Edinburgh, Scotland.
Role of the Sponsors The funding organisations played no role in the design and conduct of the study; collection, management, analysis and interpretation of the data; and preparation, review or approval of the manuscript.
Competing interests None.
Ethics approval Plewis I, Calderwood L, Hawkes D, Nathan G. National Child Development Study and 1970 British Cohort Study technical report: Changes in the NCDS and BCS70 populations and samples over time. London: Institute of Education.2004.
Provenance and peer review Not commissioned; externally peer reviewed.
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