We can also look at corrections between various types of siblings in these two studies. The families contain some pairs of biological siblings. That is, the parents have had two or more biological children of their own.
Within each of these families there are also one or more adopted children There are therefore two kinds of biologically unrelated sibling pairs. There are some pairs of genetically unrelated adopted children reared by the same parents; and there are some genetically unrelated pairs consisting of one biological child and one adopted child of the same parents. The correlations for all three types of sibling pairs, in each study. The samples are in some cases relatively small, and the correlations fluctuate somewhat. What is clear however is that there is no tendency for the biologically related pairs to be more highly correlated than the unrelated pairs. Within the Scarr and Weinberg study, the biological pairs are all white the adopted pairs are all black and the biological- adopted pairs consist of one white and one black child each.
The Texas Adoption Project was able to obtain the IQ scores of the biological mothers of the adopted children. Their average IQ was lower, by about six points, than that of the adoptive mothers Despite this, the adopted children and the biological children of the adoptive parents each had the same average IQ of 112. Thus it is clear that the adoptive parents were able to transmit high IQs equally to all their children-whether or not they shared genes with them. These IQ averages, according to the authors of the Texas Project suggest ”a heritability of IQ that is close to zero” (Horn, Loehlin, & Willerman, 1979). From a consideration of all the correlational data in their study, the same authors suggested that ”moderate heritabilities” are indicated.
Perhaps the best summary we can make of modern, well-designed adoption studies is that their data are not consistent with a very high heritability of IQ. Depending on which aspects of the data one chooses to emphasize (not all the data have been reviewed here), heritability appears to be somewhere between moderate and very low.
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