The classical twin model can also be extended to examine the role of environmental influences on estimates A, C and E. This is done using a set of 4 parameters: B, which represents the influence of age at 1st drink on mean alcohol dependence symptoms (i.e. controlling for gene-environment correlation, or the possibility that earlier age at 1st drink is correlated with increasing AD symptoms) - if the mean for AD symptoms is μ, and M is the moderator (in our case, age at 1st drink), then the mean may be modified from μ to (μ+B*M). B represents control for gene-environment correlation – its magnitude and statistical significance also denote the extent to which a change in age at 1st drink is associated with phenotypic increase or decrease in AD symptomatology (i.e. do individuals with earlier age at 1st drink report greater AD symptomatology?).3 parameters, X, Y and Z, which represent the change in A, C and E respectively as a function of changing age at 1st drink. If the effect of X is significant (represented by a Δχ2 greater
