of Dutch twins, found that while the total heritability of these symptoms varied from the age of 15–17 years to the age of 30–32 years, this variation was only quantitative in nature: a single genetic factor accounted for heritable influences (van Beek et al. 2012). Another study, which used the adult twin sample examined in the current analyses, reported that alcohol consumption in adolescence and adulthood were differentially predicted by genetic risk for externalizing versus for alcohol problems, as indexed by parental and co-twin phenotypes: while a familial liability to externalizing predicted alcohol use in adolescence, the alcohol-specific familial liability predicted later levels of alcohol consumption (Kendler et al. 2011a). This raises the possibility that qualitatively different genetic factors are relevant during different developmental periods, though such a hypothesis was not formally tested in the previous report.
