AD peaks later in development. We note that Kendler et al. (2011) examined the same maximal drinking (drinking frequency × drinking quantity) measure across development. Therefore, comparisons between the current study and the previous study (Kendler et al., 2011) should be interpreted with caution. As mentioned above, the low base rate of ASPD in the parent sample (<1% of parents met criteria for ASPD) was expected. However, this low base rate may affect the interpretation of these results in that the EXT-GR is largely influenced by the co-twin’s ASPD criteria and less so by the parent’s ASPD criteria. Finally, while internalizing disorders (e.g. anxiety and depression) clearly influence alcohol-related problems across adolescence and young adulthood (Wellman et al., 2014), evidence regarding the genetic influences shared amongst internalizing and alcohol-related behaviors is mixed, with a recent study finding evidence that the phenotypic comorbidity between intoxication frequency and internalizing disorders among adolescence is due to shared environmental risk factors (Edwards et al., 2011). Therefore, we chose to focus on the established shared genetic covariance between substance use disorders and externalizing spectrum disorders.
