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 inf luence polysubstance use and misuse. Our objective was to assess whether th ere are genetic and environmental factors specific to each substance or whe ther there are factors that predispose an individual to use of illicit subs tances in general. Twins from female-female pairs from the Virginia Twin Re gistry were interviewed by phone to assess life-time nonmedical use of cann abis, sedatives, stimulants, cocaine, opiates, and hallucinogens, Multivari ate, biometrical model-fitting was applied to the data using the Mx compute r package. In the best-fitting model, use of all classes of drugs was influ enced by a single general genetic factor (common to all substances) and a g eneral familial environmental factor. The magnitude of influence of the gen eral genetic factor ranged from 3% of the variance for opiates to 59% of th e variance for cannabis, Some differences were seen from the univariate res ults, indicating some of the parameter estimates were unstable due to small numbers of concordant pairs, However, generalizations could be made. In wo men, the substances examined share genetic and familial environmental facto rs 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. Am. J. Med. Genet. (Neuropsychiatr. Genet,) 96:665-670, 2000, (C) 2000 Wiley-Liss, Inc.