Finally, twin and adoption studies are quite different in their assumptions and potential biases. The estimation of genetic effects in these two kinds of studies derives from quite distinct kinds of relationships: monozygotic (MZ) v. DZ twins, and biological and adoptive relatives, respectively. Specifically, twin studies leverage the differential genetic relatedness between MZ and DZ twins with the major assumption that the rearing environments to which they are exposed are equally correlated. By contrast, adoption studies leverage the genetic or environmental similarity between biological or adoptive relatives with the major methodological concern being assortative placement – whether adoptees are placed randomly within adoptive homes. While there are many types of adoption designs, the literature on AUD has focused on the relationship between biological parents and their adopted away offspring. Previous research in other behavioral domains has suggested that estimates of genetic factors from adoption studies are smaller than the same estimates in twin studies (Loehlin, 1992; Rhee & Waldman, 2002). Would the estimated heritability of AUD from these two methods be similar?
