Adoption studies are one of the classic tools of behavioral genetics. These studies are used to estimate the degree to which a trait varies in response to environmental and genetic variation. Adoption studies are typically used together with twin studies when estimating heritability. There are two adoption study designs. 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. 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 attri