Harmful alcohol use is one of the leading causes of preventable death and disability in the world (Mokdad et al, 2004, WHO 2002), and hazardous use of alcohol and other psychotropic drugs are important contributors to other adverse social and economic outcomes (WHO 2002). In the United States, many studies have found that age at initiation of drinking is one of the strongest predictors of risk for alcohol and other substance use problems in adulthood (e.g., Robins and Przybeck, 1985, Grant and Dawson 1997, and Hingson et al, 2006). However, the causal relationship between age of drinking initiation and later alcohol use disorder remains controversial. On the one hand, early drinking may simply be a marker for genetic risk factors that increase risk of both early initiation of alcohol use and later substance dependence (e.g., Liu et al 2004; McGue & Ianocco 2008, Prescott & Kendler 1999). On the other hand, secular trends in adult alcohol dependence track secular trends in age of initiation across time periods that are too short to be explained by changes in the genetic composition
