Psychological Medicine

Original Article

Genetic and environmental contributions to cannabis dependence in a national young adult twin sample

M. T.  LYNSKEY  c1 a1, A. C.  HEATH  a1, E. C.  NELSON  a1, K. K.  BUCHOLZ  a1, P. A. F.  MADDEN  a1, W. S.  SLUTSKE  a1, D. J.  STATHAM  a1 and N. G.  MARTIN  a1
a1 From the National Drug and Alcohol Research Centre, University of New South Wales, Sydney, NSW and Queensland Institute of Medical Research, Brisbane, Queensland, Australia; and Missouri Alcoholism Research Center, Department of Psychiatry, Washington University School of Medicine, St Louis and Missouri Alcoholism Research Center, Department of Psychology, University of Missouri, Columbia, MO, USA


Background. This paper examines genetic and environmental contributions to risk of cannabis dependence.

Method. Symptoms of cannabis dependence and measures of social, family and individual risk factors were assessed in a sample of 6265 young adult male and female Australian twins born 1964–1971.

Results. Symptoms of cannabis dependence were common: 11·0% of sample (15·1% of men and 7·8% of women) reported two or more symptoms of dependence. Correlates of cannabis dependence included educational attainment, exposure to parental conflict, sexual abuse, major depression, social anxiety and childhood conduct disorder. However, even after control for the effects of these factors, there was evidence of significant genetic effects on risk of cannabis dependence. Standard genetic modelling indicated that 44·7% (95% CI = 15–72·2) of the variance in liability to cannabis dependence could be accounted for by genetic factors, 20·1% (95% CI = 0–43·6) could be attributed to shared environment factors and 35·3% (95% CI = 26·4–45·7) could be attributed to non-shared environmental factors. However, while there was no evidence of significant gender differences in the magnitude of genetic and environmental influences, a model which assumed both genetic and shared environmental influences on risks of cannabis dependence among men and shared environmental but no genetic influences among women provided an equally good fit to the data.

Conclusions. There was consistent evidence that genetic risk factors are important determinants of risk of cannabis dependence among men. However, it remains uncertain whether there are genetic influences on liability to cannabis dependence among women.

c1 Address for correspondence: Dr Michael Lynskey, Department of Psychiatry, Washington University School of Medicine, 40N Kingshighway, Suite One, St Louis, MO 63108, USA.