Family studies are important for identifying genetic vulnerability for drug abuse; for example, studies that have investigated generational differences in the transmission of drug abuse revealed that drug use or abuse is elevated among siblings of drug abusers and that there is a direct relationship between parental drug use or abuse and offspring use or abuse (Merikangas et al., 1992). A number of studies have focused on the familial aggregation of alcoholism and illicit drug abuse (see reviews by Merikangas, 1990; Glantz and Pickens, 1992; Gordon, 1994). Sons and daughters of alcoholics demonstrate a three- to fourfold risk of developing alcoholism (Cotton, 1979; Schuckit, 1986). Differences in the risk of alcohol and illicit drug use among individuals with a parental history of alcoholism may emerge at the time of transition from late adolescence to early adulthood, which may be a critical period for the expression of drug use vulnerability (Pandina and Johnson, 1989). The high recurrence of alcoholism among offspring of parents with alcoholism demonstrates that family history is one of the most potent predictors of vulnerability to alcohol abuse, which results to some extent from genetic factors (Merikangas, 1990; Pickens et al., 1991). However, the mechanism through which the family confers an increased risk is unknown. In addition to the contributions of genetic and biological factors to individual vulnerability for drug abuse, both transmitted and nontransmitted family factors, as well as unique environmental factors, appear to be involved in the vulnerability for drug abuse (Pickens et al., 1991). Family studies by themselves, however, cannot definitively determine the effect of genetics versus the environment on the development of alcoholism or drug abuse.
Twin Studies A traditional study paradigm used to identify the role of genetic factors in the etiology of a trait or disorder is the study of twins. Typically, a comparison is made between the prevalence of a disorder among twin pairs who possess identical genes (monozygotic or identical twins) and twin pairs who have only half of their genes in common (dizygotic or fraternal twins). For any disease, if the environment has no influence, monozygotic twins would always be concordant (similar) with respect to the disease. However, because both genetic and environmental factors play a role, it is generally not possible to discriminate among the many possible influences. Additionally, monozygotic twins are often raised in similar environments (e.g., they are dressed alike, often share friends, and copy each other's behaviors) and often share environmental factors to a greater extent than dizygotic twins, which makes it difficult to