The above sections have focused on candidate genes for specific addictions and several of the confirmed genetic findings support that specific genetic variants contribute to specific substance dependence risk. For instance, variants in the alcohol metabolizing genes specifically contribute to differences in alcohol consumption and alcohol dependence, but not to other addictive behaviors. Similarly, the variation in nicotine metabolizing genes contributes to smoking behavior and cigarettes smoked per day, but not alcoholism or other drug addiction. Yet data from family and twin analyses also support the idea that there is a strong contribution from common genetic factors to the development of dependence on various classes of drugs (Bierut et al., 1998; Merikangas et al., 1998; Tsuang et al., 1998; Kendler et al., 2003). In fact, twin studies have convincingly shown that most of the genetic variation to addiction is shared across the liability to develop nicotine, alcohol, and illicit drug addiction (Tsuang et al., 1998; Kendler et al., 2003). As a result, once an association is identified, the next step is to test whether this genetic variant influences multiple drug dependencies.
