Adoption and twin studies are able to clarify the extent to which similarity among family members is due to shared genes versus shared environment. In their simplest form, adoption studies involve comparing the extent to which adoptees resemble their biological relatives, with whom they share genes but not family environment, with the extent to which adoptees resemble their adoptive relatives, with whom they share family environment but not genes. Adoption studies have been pivotal in advancing our understanding of the etiology of many disorders and drawing attention to the importance of genetic factors. For example, Heston’s historic adoption study was critical in dispelling the myth of schizophrenogenic mothers in favor of a genetic transmission explaining the familiality of schizophrenia (Heston & Denney 1967). Furthermore, adoption studies provide a powerful method of detecting gene-environment interactions and have been called the human analogue of strain-by-treatment animal studies (Plomin & Hershberger 1991). The genotype of adopted children is inferred from their biological parents, and the environment is measured in the adoptive home. Individuals thought to be at genetic risk for a disorder, but
