Seventy-five percent of U.S. adults over the age of 18 have consumed alcohol (National Center for Health Statistics, 2009). One in three of those individuals have met criteria for at least one dependence symptom over their lifetimes (McBride et al., 2009) and the evidence suggests there is a dose-response effect between levels of consumption and risk of dependence. Alcohol use disorder symptomatology is in fact so strongly associated with heaviness of use that integration of a quantifiable indicator of excessive consumption into diagnostic criteria has been proposed (Li, 2008; Li et al., 2007; O’Neill et al., 2001; Saha et al., 2007). Although less commonly used than alcohol, cannabis has long been the most frequently used illicit drug (Johnston et al., 2003; Substance Abuse and Mental Health Services Administration, 2002), with estimated prevalence of lifetime (i.e., ever) use of 20.5% to 42.8% for adults (Grucza et al., 2007) and 47.4% for 12th graders (Johnston et al., 2009) in the U.S. DSM-IV cannabis dependence criteria are met by 10 to 18% of cannabis users (Anthony et al., 1994; Hall and Pacula, 2003;
