& 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 of the population (Grucza et al, 2008), It is also possible that early drinking is a non-causal marker for other behaviors or environmental factors that increase the risk of later substance use disorders. These might include the intensity or frequency of adolescent drinking (rather than initiation per se), or cultural factors shaping the role of alcohol in social settings across the life cycle. Whether or not variations in age of initiation across adolescence in is a causal risk factor itself, or a marker for other environmental factors governing adolescent drinking, there are several lines of evidence suggesting that habits acquired in adolescence might have more persistent effects than habits acquired at older ages. For example, although alcohol and nicotine are legally sanctioned only for adults in the United States, the onset of alcohol and nicotine dependence is concentrated in adolescence and young adulthood and does not increase with increasing age beyond the early 20’s (Grant et al, 2004), and imaging
