While there is no single “substance abuse personality”, a certain proportion of variation in the liability to SUD is shared in common with personality/behavioral phenotypic variation predating substance use initiation. Initiation, however, is a necessary but not sufficient condition for addiction development. Individual phenotypic characteristics prevail over access to drugs in the determination of variation in propensity to addiction, particularly taking into account phenotype (and genotype)–environment correlations, whereby individuals with certain characteristics are more likely to attain environment facilitating access to drugs (Kirillova et al., 2008). This likelihood further grows once drug use commences and becomes habitual, regardless of the drug used. In other words, the ontogenesis of the addiction liability phenotype further strengthens the effect of the genotype and of the initial/prior phenotype due to compounding of the phenotype–environment correlation over time. Growth in heritability with age (Hicks et al., 2007) is likely caused by that compounding.
