One problem associated with testing for environmental moderation of genetic risk is that many so-called environmental risks do not occur at random to people. People often seek out or create their environments, possibly based upon genetically predisposed characteristics. This is often referred to as genotype-environment correlation (rGE) (Plomin and Bergeman, 1991; Scarr and McCartney, 1983), although again, it may be inaccurate to assume that the environmental component is purely environmental. In biometrical analyses of moderation, it is often difficult to disentangle GxM from rGE. For example, the higher genetic variance for alcoholism in adults from urban communities than those from rural communities (Rose et al., 2001) may occur because urbanicity influences the expression of genetic risk (GxM), or because people who are genetically predisposed to alcoholism are more inclined to seek out environments in which alcohol is more readily available (rGE). Therefore, without controlling for the existence of rGE, one cannot attribute evidence for moderating effects to GxM.
