effects, and interactions between genetic and individual-specific environmental factors are confounded with estimates of individual-specific environmental effects (Eaves, Last, Martin, & Jinks, 1977; Heath & Nelson, 2002). If the twin model is expanded by including explicit measures of the environment or additional types of relatives, variance due to GxE can be separated from the variance due to latent genetic and environmental main effects (Purcell, 2002). GxE is indicated if the variation in a behavior attributable to genotype differs across environmental conditions. Sometimes, depending on the study methodology, this is indicated by differences in heritability (proportion of total variance due to genetic influences) across environmental conditions, although later we note some methodological concerns about this practice.
