The method is used alongside twin studies to identify the roles of genetics and environmental variables that impact intelligence, and behavioral disorders.
[6][7] 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.
[4] The most cited adoption projects that sought to estimate the heritability of IQ were those of Texas,[8] Colorado[9] and Minnesota[10] that were started in the 1970s.
[12] 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.
[15][16] 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.
This raised some social and moral issues some children were deemed unfit for adoption because of their low mental test scores.
The outcome of these selective adoption studies found that ultimately had no effect on the happiness or success of the children.