The present study investigated the methodological/statistical implications of adoptive parent knowledge for adoption methods used to estimate genetic and environmental contributions to individual differences. The sample used consisted of adoptees (n = 497) whose biological parents had been identified as either alcoholic, anti-social, or as a member of a control group in which neither alcoholism nor anti-social personality were diagnosed. The original purpose of the data set was content related in that it was intended to investigate genetic and environmental determinants of alcoholism and anti-social personality, as well as the interaction between genetic and environmental factors. The present study represents a methodological approach in that the data was used, not to predict alcoholism and anti-social personality, but rather, to determine whether estimates of genetic and environmental influences can be biased, either upward or downward, depending on whether the adoptive parent was cognizant of biological parent characteristics;Baron and Kenny's (1986) models of moderation, mediation, and mediated moderation were used to identify potential pathways through which adoptive parent knowledge might bias the estimates of the relative contribution of genes and environment to individual differences in adoptee outcomes. Adoptee outcomes were represented by both adaptive and maladaptive behaviors and were as follows: child conduct disorder symptoms, child attention deficit disorder symptoms, aggression symptoms, adolescent self-esteem, adoptee alcohol use symptoms, and adoptee adult anti-social personality disorder;The results showed that bias does exist if one does not include estimations of adoptive parent knowledge. For females, environmental influences were underestimated, whereas for males genetic influences were underestimated. The findings were discussed within the context of already existing confounds identified in behavioral genetics research (e.g., selective placement, assortative mating, additive-nonadditive genetic effects, twin assimilation) and the potential bias these confounds might produce in estimating genetic and environmental main and/or interaction effects.