permutations of possible subpopulations increases the risk of Type I errors, making the criteria for replication more stringent. Well-validated and consistent measures of environmental factors are needed, as some G x E interactions has been shown to be artifacts caused by scaling of environmental measures. A lack of clear distinction between environmental and genetic influences is also an issue. For example, bad parenting as an aversive environmental risk factor is in part related to genetics as manifested by individual differences in personality traits. Other contextual variables, such as social cohesion in the local community, may also be needed in the analysis. For example, certain G x E interactions (such as gene-parenting interactions) may appear in certain cultures but not others due to the different context of drug use and abuse. Multi-level analyses may be one method by which to model these effects. Finally, the nature of genetic, environmental factors and their interactions is likely transient and their relative importance will vary according to the life stage of the individual. Thus, longitudinal and age-specific models may be important, particularly for drug addiction.
