Studies of Adopted Children
The practice of adoption makes possible other kinds of studies that, in principle, might be able to unravel the combined effects of heredity and environment on IQ. A number of interesting and relevant questions can be asked about the IQs of adopted children. We might ask, first, do adopted children tend to have normal IQs? The answer is clearly no: The average IQ of adopted children is distinctly superior.
This tends to be the case even when the biological parents of the adopted children have very low IQs. For example, 100 adoptees in Iowa had an average IQ of 117 (Skodak & Skeels, 1949). The biological mothers of the same children had an average IQ of only 87. We can safely conclude that the source of the superior IQs of the adopted children must have been the excellent environment that most adoptive parents give their children.
Those families that choose to adopt children-and that are selected by adoption agencies as suitable parents-tend to be highly advantaged, Thus they can provide environments that foster the development of high IQ in their children.
The beneficial effect of an adoptive environment has been dramatically illustrated in a recent French adoption study (Schiff, Duyme, Dumaret, & Tomkiewicz, 1982). The authors located a number of children whose biological parents we’re unskilled workers and who had been adopted shortly after birth into upper-middle-class families, The adopted children, however, had biological siblings or half-siblings who were reared by their natural parents. There is no reason to expect any systematic genetic difference between the adopted children and their siblings or half-sibs.
Nevertheless, the adoptees, at school age. had an average IQ 14 points higher than that of their sibs. Perhaps of more social importance, the adopted children had had to repeat one or more school grades only one-fourth as often as did the sibs reared by their own parents. These facts tell us that environment can have a large effect on IQ and school performance. They tell us little however about the relative importance of heredity and environment. The results do tell us that we could reduce the school-failure rates and increase the IQs of the children of unskilled workers-by providing them with the kinds of environments typical of middle-class adopting families.
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