L.N. Legrand1, Matt McGue1, & W.G. Iacono1
Early initiation into substance use is a consistent, strong predictor of substance-related problems (e.g., D.B. Kandel, 1982, J. Amer. Acad. Child Psychiat., 21, 328-347). As delaying substance-use initiation may thus be an effective preventative measure, establishing the antecedents of early substance use and understanding their etiological components are important first goals. In the present study, we seek to determine the relative genetic and environmental influences upon the relationship between one such antecedent -- low academic performance -- and subsequent early initiation. As part of the Minnesota Twin Family Study, academic performance, intelligence, and level of substance use were assessed in 588 male twins (370 MZ; 188 DZ). Those boys who would go on to try tobacco, alcohol, or marijuana by age 14 had received significantly lower academic grades three years earlier. Effect sizes for academic grades, between nonusers and early users, ranged from .32 to .46 standard deviation units. This effect was independent of IQ. Univariate genetic analyses indicate that additive genetic factors account for 35.2% of the variance in academic performance at age 11 and 21.7% of the variance in substance use at age 14; shared environmental factors account for 17.0% and 40.0% of the variances, respectively. The results of bivariate analyses designed to determine whether the relationship between academic performance and early drug use is environmentally or genetically mediated will be presented. The implications of these results for effective, early intervention will be discussed.
Address: Department of Psychology, University of Minnesota, 75 East River, Road, Minneapolis MN 55455, llegrand@tfs.psych.umn.edu
1Department of Psychology, University of Minnesota, Minneapolis, MN 55455 2Supported in part by NIH grant DAO5147 and the Eva O. Miller Fellowship from the University of Minnesota.