As mentioned in the previous chapter, the term ecogenetics refers to the study of the interplay of genes and environment (Motulsky, 1977), the application of which to depression is relevant though still in the initial stages (Van Os & Marcelis, 1998; Malaspina et al., 1999).
The models of gene–environment relationships presented below all assume that genetic and environmental factors increase the risk for depression, rather than reducing it. However, in the case of protective effects the underlying principles are the same, although some extensions of the models and their mathematical representations are sometimes necessary. Interaction between genes and environment means more than simply stating that both are involved in disease aetiology. There are several biological plausible mechanisms by which genes and environment can co-influence disease outcome (Plomin et al., 1977; Kendler & Eaves, 1986; Khoury et al., 1993; Ottman, 1996; Van Os & Marcelis, 1998).
Correlation: genes influence environmental exposure
A gene may increase the likelihood that a person becomes exposed to an environmental risk factor, which in turn increases the risk for depression. For example, liability to experience life events is influenced by genetic factors, especially controllable or dependent life events, that are within the influence of the person (Thapar & McGuffin, 1996; Kendler & Karkowski-Shuman, 1997; Saudino et al., 1997).