A multivariate genetic analysis of the use of tobacco, alcohol, and caffeine in a population based sample of male and female twins.

Numerous epidemiologic studies in the past few decades have consistently demonstrated positive associations between the use of various psychoactive substances, both licit and illicit. This association could be due to shared genetic and/or shared environmental risk factors. This study uses multivariate structural equation modeling to determine the sources of covariation between the use of tobacco, alcohol, and caffeine, the three most commonly consumed psychoactive substances. In particular, we wish to clarify the extent to which genetic and environmental risk factors are shared across these three substances versus are substance specific in their effect. The sample, consisting of data collected from members of the population-based Virginia Twin Registry, consists of 774 monozygotic and 809 dizygotic male and female twin pairs. Our results demonstrate that genetic and individual specific environmental factors that are shared between these three substances account for a modest proportion of the total variance. For example, shared genetic risk factors across the three substances in males and females account for between 7 and 28% of the total variance in liability and 12-56% of the genetic variance. Common familial environment appears to play little or no role. Underlying genetic and individual environmental risk factors produce liability to (poly)substance use in general; substance specific factors also play an important etiologic role.