The abuse of licit and illicit drugs constitutes one of the leading public health problems in the world. There are tens of millions of alcohol or drug abusers and more than a billion smokers in the world (WHO, 2002). Each year, substance abuse results in the loss of tens of millions of dollars due to health-care costs and lost productivity (Cartwright, 2008; Rehm, Taylor, & Room, 2006). Substance abuse also shortens lives, increases risk for chronic disabling illness, and results in untold social costs in terms of broken families, ruined careers, and violent victimization (Goldman, Oroszi, & Ducci, 2005). While effective treatments for addiction exist, the amelioration of the societal burden of substance abuse will require more effective prevention. Behavioral genetic research in this area is part of a larger effort aimed at improving prevention and intervention efforts by bringing about a better understanding of the origins of substance use disorders (O'Brien, 2008).