Adoption study

Adoption studies typically compare pairs of persons, e.g., adopted child and adoptive mother or adopted child and biological mother, to assess genetic and environmental influences on behavior. These studies are one of the classic research methods of behavioral genetics. The method is used alongside twin studies to identify the roles of genetics and environmental variables that impact intelligence, and behavioral disorders. Adoption studies differ from twin studies in that adoption studies do not necessarily need to use twins; instead, they compare the traits of children to their parents, whether those are the adoptive parents or biological parents.

Adoption studies typically compare pairs of persons, e.g., adopted child and adoptive mother or adopted child and biological mother, to assess genetic and environmental influences on behavior. These studies are one of the classic research methods of behavioral genetics. The method is used alongside twin studies to identify the roles of genetics and environmental variables that impact intelligence, and behavioral disorders. Adoption studies differ from twin studies in that adoption studies do not necessarily need to use twins; instead, they compare the traits of children to their parents, whether those are the adoptive parents or biological parents.

Study designs and methods

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 1966¹ 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. The study in Minnesota was conducted by Bouchard. It had an emphasis on identical twins that were separated and raised by different families.

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. "The Stockholm adoption study (2000 adoptees) found that criminality alone was not transmitted from biological parents to adoptees, but did find elevated rates of criminality in adopted-away offspring of biological parents with alcohol use disorders (AUD) alone, or with both AUD and criminality (Bohman, 1978)."

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

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