Intrapersonal predictors of
Processes contributing to individual differences in drug use include physiological susceptibility as measured in studies of genetic heritability and neurobiological processes, personality traits, affective states, cognition, including expectancies as a motivation to engage in drug use behaviour, and memory processes. Relative to extrapersonal factors, intrapersonal factors likely play a more active role following initiation of drug use and help explain why some individuals who use do not go on to abuse while others do.
Research on the genetic heritability of numerous drugs of abuse is ongoing (for example opiates, marijuana: Kendler and Prescott 1998; Reich 2000), although a majority of this work has focused on genetic predisposition to alcoholism. Genetics may determine metabolic processes involving differential effects of drugs. Alternatively, genetics may indirectly influence drug abuse through its influence on such precursors as individual temperament or personality, such that one becomes vulnerable to alcohol or other drug abuse (for example Schuckit 1987). Although research suggests that genetic heritability may be a possible explanation for intergenerational familial alcoholism or other drug problems, it is difficult to disentangle genetic-environmental interactions (Sher 1993; Sher et al. 1997). In other words, individuals who seem genetically predisposed to drug abuse may also live in social environments that are conducive to drug abuse (for example have family members who use). It has been argued that adoption studies, in which genetic factors are investigated among family members who are reared in different social environments, may help control for non-genetic factors. Adoption studies reveal that approximately 30 per cent of sons of alcoholic fathers themselves