Evidence for a genetic influence on substance dependence has been provided by many family, twin, and adoption studies. Family members of alcohol-dependent individuals have a higher probability of suffering from alcohol dependence (54). In a study of families severely affected by alcohol-abuse disorders, approximately 50% of brothers and 22%–25% of sisters of an alcohol-dependent proband were alcohol dependent (15). Similarly, siblings of marijuana-dependent, cocaine-dependent, or habitual-smoking probands were at increased risk (approximately 1.7-fold higher) of developing marijuana dependence, cocaine dependence, or habitual smoking compared with siblings of nondependent individuals (15). Studies with large twin cohorts have shown that the risk of alcohol dependence in the co-twin of an affected monozygotic twin is significantly higher than the risk in the co-twin of an affected dizygotic twin pair, which is similar to that of full siblings of the affected individual (117). In adoption studies, children of alcoholics adopted by nonalcoholics who grow up in a nondrinking environment have a higher risk of becoming alcoholic than do children of nonalcoholics adopted by the same parents; children of alcoholics raised by their alcoholic father
