Adoption study

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 adoptees method investigates similarities between the adoptees 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 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 one of the key tools in behavioral genetics and are typically used in combination with twin studies when making heritability estimates.

Examples

Mental Disorders

The first adoption study on schizophrenia in 1968 by Leonard Heston demonstrated that the biological children of schizophrenic parents were just as likely to develop schizophrenia whether they were reared by the parents or adopted and was essential in establishing schizophrenia as genetic instead of being a result of child rearing methods. Similar studies that followed have shown that mental disorders such as alcoholism, antisocial behavior, depression and schizophrenia have a large genetic component that interacts with environmental risk factors such as family conflict, poor cohesion and deviant communication. .

Cognitive Ability

The most cited adoption projects that seeked to estimate the heritability of IQ were those of Texas, Colorado and Minnesota that were started in the 1970s. These studies showed that while adoptive parents IQ does seem to have a correlation with adoptees IQ in early life, when the adoptees reach adolescence the correlation has faded and disappeared. The correlation with the biological parent seemed to explain most of the variation. In 2015 an adoption study that compared Swedish male-male full-sibships in which at least one member was reared by one or more biological parents and the other by adoptive parents was published. Parental education level was rated on a 5 point scale and each additional unit of rearing parental education was associated with 1.71 points higher IQ. The results were replicated with 2 341 male-male half-sibships, controlling for clustering within families, each additional unit of rearing parental education was associated with 1.94 IQ units.

Crime

One of the most influential studies and widely cited adoption study on criminality was conducted by Sarnoff A. Mednick and Karl O. Christiansen in Denmark, were relevant data was relatively easily available that demonstrated that criminality has a significiant genetic component that interacts with environmental risk factors. Adoption studies that followed have had similar results.