whereas in the absence of life stressors, these individuals may be no more likely to develop depression. In essence, it is only when adverse environmental conditions are experienced that the genes “come on-line.” Gene-environment interactions in the area of adolescent substance use are also hypothesized to be fan-shaped, where some environmental conditions will allow greater opportunity to express genetic predispositions (allowing for more variation by genotype as in the right side of Figure 1a), and other environments will exert social control in such a way as to curb genetic expression (Shanahan & Hofer 2005), leading to reduced genetic variance (as on the left side of Figure 1a). Twin analyses yielding evidence of genetic influences being more or less important in different environmental contexts are generally suggestive of fan-shaped interactions. Changes in the overall heritability do not necessarily dictate that any one specific susceptibility gene will operate in a parallel manner; however, a change in heritability suggests that at least a good portion of the involved genes (assuming many genes of approximately equal and small effect) must be operating in that manner for a difference in heritability by environment to be detectable. The diathesis-stress model has largely been the dominant model
