Figure 1. Multivariate biometrical models; Only Twin 1 depicted.
Note: Path diagrams of multivariate behavioral genetic models used to describe multivariate or longitudinal data. Figure 1a describes the Independent Pathway Model that consists of a common genetic and environmental factor that describes the covariance between substances, while the specific factors capture genetic and environmental influences that are unique to each substance. The Common Pathway Model, depicted in Figure 1b, is a restricted form of the Independent Pathway Model created by positing that the covariation between substances arises from a latent factor (referred to as Substance Dependence Vulnerability). Abbreviations: A – additive genetic effects; C – shared environmental effects; Common – identifies the portion of the model which estimates factor loadings common to all phenotypes; E – non-shared environmental effects; Specific – describes the portion of the model that identifies genetic or environmental effects that are unique to a particular phenotype, beyond that which may already be shared; Alc – Alcohol; Tob – Tobacco; Can - Cannabis.