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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.
The first adoption study on schizophrenia published in 1966 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. This discovery was done through a process of interviews of both a child that was adopted along with their biological mother who contained the schizophrenia gene, and of adopted child along with their mother who did not contain the schizophrenia gene. The experiment was conducted more than once on various families and continued resulting on the schizophrenia child inheriting the gene from his mother. This supports the theory that it doesn't matter what specific environment a child is raised in; if its parent or parents suffer from a mental disorder, the risk for suffering from the same disorder will be equal regardless of if the child was raised with its biological parents or with its adoptive parents (Plomin et al., 1997).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. Recent studies has shown that childhood disorders are not only genetic, but form in more children that are adopted vs children that are not adopted. Many researchers of this topic believed the disorder developed over the time the child was adopted. With further research being done, results have shown that some of the adoptees had been already diagnosed with the disorder before they were even adopted. Researchers concluded the disorders are caused by the way a child is raised and also from the genes of their birth parents. The other few may have developed the disorders after being adopted due to curiosity and trouble finding their true identity. Parents that are willing to adopt are always advised to be aware of the phenomenon that a child that is to be adopted may need help on dealing with psychological issues.
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. Correlation should also be dependent on the schooling factor for the adoptee since home-based schooling and public schooling would each have its own statistics associated with IQ development.
One of the most influential and widely cited adoption study on criminality was conducted by Sarnoff A. Mednick and Karl O. Christiansen in Denmark, where relevant data was relatively easily available that demonstrated that criminality has a significant genetic component that interacts with environmental risk factors. Adoption studies that followed have had similar results. The twin, adoption, and family studies have been a common method to test on criminals to determine if there is a correlation. When studying the criminals that are adopted children, their biological and adoptive parents are used for the experiment as well. The first study involving adopted children dealt with the genetics of criminal behavior that proved the children which were more likely to become criminals were biologically related to offenders. This study demonstrates that crime is correlated to be a hereditary factor. The study took place in the 1980s in Iowa and used 52 adopted children which were born into female offenders. Results showed seven of the fifty two children had committed a criminal act and only one of the adoptive parents had been accused of a crime. The relationship between crime and adoption study has shown evidence to property crime and not to violent crime in adopted vs not adopted individuals and their adoptive parents or biological ones. Other factors such as age and where the child was moved during adoption affect the accuracy in the conclusions. Even though the adoption study dealing with crime is performed all over the world, possible information has been declined, obtained from deficient adoption centers, and not conducted face to face. The study has been proven to be influenced by genetic factors, but statistics are not 100% defined.
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