Another issue concerns interpretation of results from twin models that found evidence for alcohol-related GxE on the basis of a reduction in standardized variance proportions. The heritability estimate (proportion of total variance due to genetic influences) can differ across environments even when the genetic estimate (actual variance in raw units) is constant (see Visscher, Hill, & Wray, 2008). Accordingly, an interaction may incorrectly be concluded when results are presented in standardized form. Consider, for example, a twin study of drinking quantity that stratifies participants on levels of presumed stressful life event (SLE) exposure and finds the MZ:DZ ratio is similar but the groups have increasing variability across level of SLE. If the results are standardized, this will appear as a decrease in heritability across increasing levels of SLE exposure (i.e., genetic variance accounts for a smaller proportion of the variation for the higher SLE groups). Thus this ExE effect (latent E x SLE) would be erroneously interpreted as GxE (heritability x SLE). Only four of the 12 twin studies reviewed provided estimates in both standardized and unstandardized formats (Table 1).
