To examine whether our GxE effects would persist when diagnostic DSM-IV AD was used, we computed MZ and DZ correlations for a diagnosis of AD in those with early versus later onset of drinking (using 14 years and younger to denote early drinking). While there was insufficient power to detect sex differences, we found MZ correlations to range between 0.50–0.52 and DZ correlations to range between 0.14–0.24 with somewhat lower DZ correlations in twin pairs concordant for onset of drinking prior to age 15. These findings suggest that either our increased heritability estimates in those with early age at 1st drink are due to the inclusion of non-diagnostic individuals in our measure of AD symptomatology (i.e. moderation occurs at sub-threshold levels) or due to increased power to detect GxE when using continuous indices of both outcome (AD symptoms) and moderator (age at 1st drink). Additionally, the secondary correlations also imply that shared environmental influences on AD symptomatology, if any, may be attributable to the inclusion of sub-threshold drinkers.
