Adoption study
Adoption studies are one of the classic research methods of behavioral genetics. The method is used alongside twin studies to identify the roles of genetic and environmental variables.

Study design
There are two standard ways in which adoption studies are carried out; the adoptee's study method and the adoptee's family method. 

The adoptee's study method compares adoptee's similarity to their biological and adoptive parents. Similarity with the biological parent is expected to be due to genetics, while similarity with the adoptive parent is due to home-environment, which is referred as the shared environmental effect.

The adoptee's family 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 environmental effect, as the siblings are genetically unrelated but share the home environment.

Variation that cannot be accounted for by either genetics or home-environment is typically described as a non-shared environment.

Examples

Mental disorders
The first adoption study on schizophrenia published in 1966 by Leonard Heston demonstrated that the biological children of parents with schizophrenia were just as likely to develop schizophrenia whether they were reared by their parents or adopted and was essential in establishing schizophrenia as being largely genetic instead of being a result of child rearing methods. Analogous studies that followed have shown that mental disorders such as alcoholism, antisocial behavior and depression have a large genetic component that can interact with environmental risk factors such as family conflict, poor coherence, and deviant communication.

Cognitive ability
The most cited adoption projects that sought 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 and widely cited adoption study on criminality was conducted by Sarnoff A. Mednick and Karl O. Christiansen in Denmark. They argued that relevant data demonstrated that criminality has a significant genetic component that interacts with environmental risk factors. Adoption studies that followed have had similar results.

Alcoholism 
In the mid-1970s, adoption studies were conducted to investigate the effects on children of having a biological parent with alcoholism. The study found that sons whose biological father has alcoholism were four times more likely to develop alcoholism within their lifetime, but were not more likely to have other mental disorders. The study, was unable to conclude what the impact was on daughters. A more recent study was published in 2009 which compared the parental alcoholism of children in non-adoptive and adoptive families. This study found that the likelihood of children developing alcoholism later in life was associated more with genetic rather than environmental factors.

References

Sources
 
 
 
 
 Newman et al. 1937, Twins: A Study of Heredity and Environment
 Shields 1962, Monozygotic Twins: Brought Up Apart And Brought Up Together
 Juel-Nielsen 1980, Individual and Environment: Monozygotic Twins Reared Apart

Adoption research
Behavioural genetics
Genetics studies
Quantitative genetics
Human genetics