The identification of one genetic factor whose effects decline steadily into young adulthood and another whose effects increase during this time corresponds with some prior evidence of developmentally specific genetic effects. A study using the current sample (Kendler et al. 2011a) examined the genetic risk for externalizing disorders and alcohol-use disorders, which were derived based on co-twin and parental phenotypes, and their respective relationships to alcohol intake. Results indicated that genetic risk for externalizing is more strongly related to intake during adolescence, while genetic risk for alcohol problems is more strongly related to intake beginning in early adulthood. Other studies have found that externalizing disorders and alcohol problems are genetically correlated during adolescence (Button et al. 2007; Hicks et al. 2007; Legrand et al. 2008), though the covariance in phenotypes might also be due to environmental factors (Rose et al. 2004). Such findings raise the possibility that the adolescent-limited genetic factor identified in the current report is actually capturing liability to general externalizing behaviors (which would encompass alcohol use) during that period, rather than liability to alcohol use per se.
