Multivariate assessment of factors influencing illicit substance use in twins from female-female pairs.

Although familial factors have been shown to influence drug use, abuse, and dependence, little is known about the common and specific factors that influence polysubstance use and misuse. Our objective was to assess whether there are genetic and environmental factors specific to each substance or whether there are factors that predispose an individual to use of illicit substances in general. Twins from female-female pairs from the Virginia Twin Registry were interviewed by phone to assess lifetime nonmedical use of cannabis, sedatives, stimulants, cocaine, opiates, and hallucinogens. Multivariate, biometrical model-fitting was applied to the data using the Mx computer package. In the best-fitting model, use of all classes of drugs was influenced by a single general genetic factor (common to all substances) and a general familial environmental factor. The magnitude of influence of the general genetic factor ranged from 3% of the variance for opiates to 59% of the variance for cannabis. Some differences were seen from the univariate results, indicating some of the parameter estimates were unstable due to small numbers of concordant pairs. However, generalizations could be made. In women, the substances examined share genetic and familial environmental factors which contribute to the vulnerability to use. Degree of influence of the factors differs for the substances examined. However, no specific genetic or familial environmental factors were found to contribute significantly to use of any of the illicit substances.