Moderation of a bivariate twin model: Whereas the univariate models effectively control for rGE so that no spurious GxE effects are detected, they do not explicitly model the inter-relationship between genetic influences on age at 1st drink and those on AD symptoms. In order to address this hypothesis, we fit a triangular decomposition (i.e. Cholesky) model (Commandant Benoit, 1924; Neale & Cardon, 1992) that examined the moderating influence of age at 1st drink while modeling the genetic correlation between age at 1st drink and AD symptoms. The model serves two important purposes: first, it models the extent to which common etiologic factors (genetic, shared and non-shared environment) contribute to the co-variation between age at 1st drink and AD symptoms – for instance, measuring common genetic influences on age at 1st drink and AD symptoms. This is the standard bivariate model (without moderation). Second, with the addition of moderation, the model allows an estimation of whether these common genetic influences, or those genes that influence the liability to AD symptoms specifically, are moderated by age at 1st drink – in other
