An important consideration in the study of gene-environment interaction is the nature, or shape, of the interaction that one hypothesizes. Figure 1 illustrates the two primary types of interactions. One type of interaction is the fan-shaped interaction (Figure 1a). In this type of interaction, the influence of genotype is greater in one environmental context than in another. This is the kind of interaction that is hypothesized by a diathesis-stress framework, whereby genetic influences become more apparent, i.e., are more strongly related to outcome, in the presence of negative environmental conditions. There is a reduced (or no) association of genotype with outcome in the absence of exposure to particular environmental conditions. The literature surrounding depression and life events would be an example of a hypothesized fan-shaped interaction: When life stressors are encountered, genetically vulnerable individuals are more prone to developing depression, 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
