Adoption studies provide another means of teasing apart the effects of genes and environment. The basic method of the adoption study lies in comparing the rates of disorder in biological relatives and adoptive relatives There are three main types of adoption study.
1. The adoptee study: here the rate of disorder in the adopted-away offspring of affected individuals is compared with the rate of disorder in control adoptees whose biological parents are unaffected.
2. The adoptee's family study: in this design, the rate of disorder in the biological relatives of affected adoptees is compared with that among the adopted relatives.
3. The cross-fostering study: this allows us to examine gene-environment interaction by comparing the rate of disorder in adoptees who have well biological parents and affected adoptive parents with the rate of disorder in adoptees who have affected biological parents and normal adoptive parents.
Although adoption studies allow us to examine the effects of both genes and environment, there are several potential drawbacks to the method. First, adoption is in itself an unusual event and there is a tendency for higher rates of some psychiatric difficulties such as antisocial personality traits amongst adoptees. Second, adoptive placements are not random in that adoption agencies are likely to attempt to match adoptive and biological parents for physical, social, and other characteristics. Nevertheless, despite these difficulties, adoption evidence has given much support to the role of both genes and environment for traits and behaviours such as cognitive ability and criminality.
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