Theoretically, the liability distribution in the population may range from the individual norms of reaction that do not include the disorder (at least, in the present range of environmental conditions) and thus totally resistant (resilient)—up to the phenotypes corresponding to the most severe fastest-developing disorder. The “gradations of normality” (the subthreshold liability phenotypes) correspond to variation in the risk (propensity), whereas “gradations of affectedness” (the suprathreshold phenotypes, which are likely to be assigned a clinical diagnosis) correspond to variation in severity, comprising the two parts of the liability distribution relative to the threshold. Applied to addiction (Conway et al., 2010; Vanyukov et al., 2003a), severity refers to the degree of maladaptive compulsive drug-seeking and using behavior displayed by an individual, and corresponds to variation in liability above the diagnostic threshold. Propensity refers to the probability of the disorder onset and corresponds to liability variation below the diagnostic threshold. Variation in propensity may manifest as psychological/behavioral precursors of addiction. These precursors have been conceptualized as problem behavior (Jessor and Jessor, 1977), overlapping with the CLA (Vanyukov et al., 1994, 1996; Vanyukov
