Studies have also indicated the familial transmission of illicit-substance-use disorders, with heritability estimates ranging from 30% to 80% (2, 83, 127). A recent meta-analysis of twin studies on marijuana use clearly indicated that vulnerability to both initiation and persistent use was significantly affected by both genetic and environmental factors (132). Genetic factors accounted for 48% and 40% of the total variance in initiation of marijuana use in men and women, respectively, and for 51% and 59% of the total variance in marijuana abuse in men and women, respectively.
