There is a general recognition that genes and environment work together—often in complex ways—to produce wide variations in behavior and psychological function. G × E interaction, by definition, is a statistical term indicating that genetic effects on a given phenotype depend upon environmental factors or vice versa. Gene expression, for example, can be moderated by an individual's experiences or exposure to certain environments. Likewise, various individuals may respond differently to the same environmental exposure because they have different genotypes. Such genetic sensitivity to the environment has been demonstrated extensively in plant and animal species for a variety of traits. But even though the importance of G × E interactions in human behavior has long been considered (Eaves, 1984; Mather and Jinks, 1982), G × E interactions have been rarely reported in human traits until relatively recently. The failure to find G × E interactions in studies of human characteristics may be due to a number of factors. One likely explanation is related to statistical power. In general, it is difficult to detect G × E effects due to their low
