Another related study investigating social adjustment in a subsample of the adopted black children was published in 1996.
[7] Some hereditarians, however, suggested that this could be attributed to higher levels of European admixture among black children in the northern United States.
At the time of the study, the gap in average performance between the two groups of children was approximately one standard deviation, which is equivalent to about 15 IQ points or 4 grade levels at high school graduation.
"Trans racial adoption is the human analog of the cross-fostering design, commonly used in animal behavior genetics research....
The sample of adopted children was selected by eligible parents contacting the researchers for participating following a newsletter call.
[1] Some have suggested that differing pre-adoption experiences, including age at adoption, explain the racial patterns in the results.
Lee (2009) argues against this interpretation, saying that there is no evidence from other studies that variables such as age at adoption exert an effect on IQ lasting until late adolescence.
In the Minnesota study, where the tests used in the follow-up were generally not the same as those that had been given the first time, these corrections are complex and must be made on an individual basis.
The analysis of structured interviews at age 7 and 17 reported by (DeBerry, Scarr & Weinberg 1996) found that about half of the studied black adopted children had adjustment difficulties.
[3] Scarr & Weinberg (1976) interpreted the results from age 7 suggesting that racial group differences in IQ are inconclusive because of confounding in the study.
In support of this interpretation, they drew special attention to the finding that the average IQ of "socially classified" black children was greater than that of the U.S. white mean.
The follow-up data were collected in 1986, and Weinberg et al. published their findings in 1992; they interpreted their results as still supporting the original conclusions.
Both Levin[5] and Lynn[6] argued that the data support a hereditarian alternative - that the mean IQ scores and school achievement of each group reflected their degree of Sub-Saharan African ancestry.
[6] They noted that the data taken of adoption placement effects can explain the observed differences but that they cannot make that claim firmly because the pre-adoption factors confounded racial ancestry, preventing an unambiguous interpretation of the results.
They argued that, "contrary to Levin's and Lynn's assertions, results from the Minnesota Transracial Adoption Study provide little or no conclusive evidence for genetic influences underlying racial differences in intelligence and achievement, " and noted that, "We think that it is exceedingly implausible that these differences are either entirely genetically based or entirely environmentally based.
The true causes of racial-group differences in IQ, or in any other characteristic, are likely to be too complex to be captured by locating them on a single hereditarianism-environmentalism dimension.
"[14] In a 1998 article, Scarr wrote, "The test performance of the Black/Black adoptees [in the study] was not different from that of ordinary Black children reared by their own families in the same area of the country.