In complementary analyses, we addressed the effect of alcohol use on P3 by incorporating ERP measures and longitudinal self-reports of alcohol use from these Finnish twins into HMR analyses. The HMR analyses offer additional evidence that co-twin similarity for novel P3 is modulated by the twins' alcohol use and abuse. Some of the HMR results suggest three-way interactions in which these associations are modified by genetic similarity. But because our laboratory sample of twins was selected, pairwise, for the co-twins' concordance/discordance for adolescent alcohol problems, inferences of genetic effects from this sample must be made with caution.
