Familial aggregation of addiction has been recognized for decades. Rates of alcoholism in male and female siblings of alcoholic probands have been noted to be 50% and 24% respectively, compared with 20% (male) and 6% (female) of participants from the general population [6]. Rates of other addictions (cannabis, cocaine) and habitual smoking are also elevated, not only in alcoholic probands but also their first-degree relatives. Twin studies have revealed that genetic influences are primarily responsible for this familial aggregation of addiction (see [7] for a review). Regardless of substance, 50% of the variance in addictions can be attributed to those segregating genes that are shared by twin pairs [8]. Notably, such estimates are influenced by a variety of factors, including age and exposure. For example, heritability estimates of substance-related problems are as low as 0% during adolescence, and increase during adolescence and adulthood [9]. This has led to the broad consensus that while familial environment is more important for the early stages of experimentation and substance use, genetic factors exert a more pivotal role in later stages of addiction [10,
