Most prior GWAS have focused on alcohol outcomes in mature adults, and less is known about the transition period between adolescence and young adulthood. Alcohol use increases across adolescence, with consumption, risky drinking, and alcohol use disorders all peaking in young adulthood (SAMHSA, 2014). Longitudinal studies indicate that heritability estimates for alcohol use and problems also increase across adolescence and generally reach the levels observed in adults during emerging adulthood (Rose et al., 2001; Palmer et al., 2013; Samek et al., 2013), though there is some suggestion from cross-sectional and/or retrospective studies that heritability may continue to increase slightly across adulthood (Bergen et al., 2007; Kendler et al., 2007; Hansell et al., 2008). During emerging adulthood there is also a shift in the nature of genetic influences on substance use outcomes, as it is during this time frame that genetic factors become more substance-specific (Vrieze et al., 2012) and less related to overall externalizing behavior (Kendler et al., 2011a; Edwards and Kendler, 2013; Meyers et al., 2014). Thus, emerging adulthood is a critical time frame for clarifying alcohol use etiology.
