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.
Familial resemblance of behaviors is due to genetic and/or common familial environmental influences. Adoption studies provide a direct test of the role of both factors. This is possible by drawing comparisons between families that share genetic and environmental influences and families that share only genetic or environmental factors. Adoption creates two types of families. The “genetic family” consists of pairs of genetically related individuals who do not share a common family environment (e.g., biological parent and adopted-away child). The similarity between these pairs of relatives provides a direct estimate of genetic effects on behaviors. The second type family is the “environmental family,” which is made up of pairs of individuals who are not genetically...