The interesting thing is that longitudinal studies indicate that the relative importance of the nonspecific genetic predisposition that impacts alcohol outcomes via general behavioral disinhibition, and the part of the genetic predisposition that is specific to alcohol, changes across adolescence. Data from two independent twin studies indicate that alcohol use and problems are more strongly influenced by the general predisposition toward externalizing behavior early in adolescence, but by late adolescence and young adulthood, alcohol-specific genetic factors become more important (Meyers et al., 2014). Data from yet another twin study indicate that this pattern is not specific to alcohol, but rather, extends to marijuana and nicotine dependence symptoms as well (Vrieze et al., 2012). Adolescent substance use is influenced largely by nonspecific genetic factors that broadly impact externalizing outcomes, but as individuals age into adulthood, substance-specific genetic risk factors become more important. This is likely because early substance use is more experimental, and hence more strongly influenced by general disinhibitory, sensation-seeking factors; however, once substance use becomes more established and regular, genetic factors involved in drug response become increasingly important, moreso than the disinhibitory disposition that contributed to initial use patterns.
