We examined the genetic and environmental influences on initiation of substance use and progression to heavier substance use (while controlling for the influences of initiation on progression). The pattern of results indicated that environmental factors (particularly common environment) were most important in the initiation of substance use. However, sibling interaction processes could, in part, explain this finding (i.e. with one twin's substance use influencing the substance use of the other twin and vice-versa [13]). In the progression to heavier/problem use, genetic and unique environmental influences appeared to be of greater importance. The exception in this study was cigarette use, for which genetic factors appeared equally important for initiation of use as common environmental influences.
