The origins of the correlations between tobacco, alcohol, and cannabis use during adolescence.

Methods of structural equation modelling were used to analyse the correlations between reports of tobacco, alcohol, and cannabis use in a birth cohort of New Zealand children studied to the age of 16. This analysis produced three major conclusions: (a) the correlations between tobacco, alcohol, and cannabis use could be explained by a factor representing the individual's vulnerability to substance use; (b) predictors of vulnerability to substance use were the extent to which the individual affiliated with delinquent or substance using peers, novelty seeking, and parental illicit drug use; (c) in the region of 54% of the correlations between substance use behaviours could be predicted from observed risk factors and 46% was attributable to non-observed sources of vulnerability.