In contrast to the GH, which addresses only the order of drug use initiation, the concept of common (general) liability to addiction or SUD (CLA) involves mechanisms and biobehavioral characteristics that pertain to the entire course of development of the disorder and changes in the risk. The CLA concept also overlaps with the psychological and psychopathological constructs that have been previously used to explicate addiction and its mechanisms. Liability denotes a latent (unobservable) quantitative trait that, when measured, “would give us a graded scale of the degree of affectedness or of normality” (Falconer, 1965, p. 52). These two latter broad phenotypic categories are divided by a latent threshold on the liability axis. An individual's quantitative liability phenotype at any time point, above or below the threshold, represents a value within the norm of reaction of the genotype (Dobzhansky, 1951), the genetically determined distribution of all phenotypic values for a trait in an individual for all possible environmental conditions. A probability distribution that can be considered at each time point as well as within the entire developmental trajectory, the norm of
