Adoption Study UP174
Adoption studies are one of the classic tools of behavioral genetics. These studies are used to estimate the degree to which variation in a trait is due to environmental and genetic influences. Adoption studies are typically used together with twin studies when estimating heritability. There are two types of adoption study designs; the Adoptee's method and the Familial method. The most powerful form of adoption studies compare pairs of genetically-identical monozygotic identical twins who are adopted into different families (MZA); however, historical changes in adoption practices to try to keep twin pairs together make these now the rarest kind of adoption studies. Adoption studies have pinpointed that some traits are linked to genetics, for example, Schizophrenia, IQ and criminality, however other compound factors (such as age and environment) can have an impact too.
Firstly, the adoptee's method investigates similarities between the adoptee and their biological and adoptive parents. Similarity with the biological parent is expected to be heritable genetic effect, while similarity with the adoptive parent is associated with home-environment, called the shared environmental effect. Secondly, the familial method compares non-biological siblings who are reared in the same household. Similarity to non-biological siblings raised in the same household is attributed to shared environment effect, as the siblings are biologically unrelated but share the home environment. Variation that cannot be accounted by either genetics or home-environment is typically described as non-shared environment. Adoption studies are meant to evaluate genetic and environmental influences on phenotype.