To be sure, the externalizing–internalizing classification can be reduced to more elemental traits. This classification also does not entirely cover the domains of phenotypic and mechanistic variation contributing to individual differences in addiction liability. For instance, the mechanisms of attachment and bonding – an important component of socialization – potentially play an important role in addiction liability (Panksepp, 1998) and overlap at the neurobiological and genetic level (Insel and Young, 2001; Maher et al., 2011). Phenotypically, they share in common characteristics of dependence. It is hardly a coincidence that drug use is usually initiated in adolescence, when affiliation is relocated from parents to peers.
