Because genotyped participants in this study were adopted, mostly by white adoptive parents, into American families, our inference regarding the phenotypic effects of variation in ALDH2 is enhanced. Caucasian adoptive parents do not carry the ALDH2*2 allele themselves, but in non-adoptive families of ALDH2*2 carriers, at least one biological parent must also carry ALDH2*2, and thus is likely to experience the protection against alcohol use associated with the allele. Therefore, a passive gene-environment correlation may develop when ALDH2*2 carrying individuals are raised by their biological parents, whereby the influence of parental ALDH2 genotype results in a family environment characterized by fewer models of adult drinking, and, to the extent that offspring drinking is environmentally influenced by parent drinking, this environmental influence may compound upon, and be confounded with, the purely biological effect of the offspring’s own ALDH2 genotype. Consequently, in families where ALDH2*2 carrying offspring are raised by ALDH2*2 carrying biological parents, the protective effect associated with the ALDH2*2 allele is likely to be amplified, because it will also include the environmentally-mediated influence of parental ALDH2*2 genotypes (although it has
