Although behavioral problems associated with abuse of alcohol emerge during late adolescence and adulthood, some behavioral characteristics indicative of an increased risk of alcoholism may already be obvious during early childhood. Studies in several countries have demonstrated that children with high levels of novelty-seeking behavior and low levels of harm-avoidance behavior are more likely to develop alcohol-related problems during adolescence. Moreover, as early as age 3, children at high risk of future alcoholism because of a family history are more active, more impatient, and more aggressive than matched controls of low-risk children. Causal influences on the initiation of drinking must be distinguished from those that affect patterns of consumption once drinking is initiated. Studies of adolescent twins have demonstrated that initiation of drinking is primarily influenced by the drinking status of parents, siblings, and friends and by socioregional differences in the environments within which adolescent twins reside. The influence of genetic factors is negligible. Conversely, once initiated, differences in frequency and quantity of drinking are strongly influenced by genetic factors. However, these influences, too, are modulated by sibling and peer effects and by regional environmental variation.