Even after controlling for the influences of individual factors, such as gender, birth cohort, and other drug dependence, however, evidence indicated a familial transmission of a drug-specific risk. Thus, habitual smoking in the probands specifically increased the risk of habitual smoking in their siblings, and alcohol dependence in the probands specifically increased the same diagnosis in their siblings. These findings are consistent with the presence of both common and drug-specific influences that are transmitted in families and combine to determine a person’s risk for developing alcohol dependence or habitual smoking. These influences likely include genetic factors, which are discussed in the following sections.
