Minnesota Transracial Adoption Study - Results

Results

The children were first tested in 1975 at age 7. In 1985, 196 of the original 265 children were retested at age 17.

The adopting parents of 12 of the interracial children wrongly believed that their adopted children had two black parents. The average IQ of these 12 children was not significantly different from the scores of the 56 interracial children correctly classified by their adoptive parents as having one black and one white parent.

The study found that age at adoption was significantly associated with measured IQs:

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, pointing out 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, the proportion of IQ variance associated with pre-adoption variables declined from .32 to .13 between ages 7 and 17. Lee further suggests that causality may run from IQ and other behavioral variables to differences in pre-adoption experiences rather than the other way around, and that race by itself as a visible characteristic may have affected pre-adoption experience.

The average difference in IQ scores between the testing at age 7 and testing at age 17, seen in all groups, may be due to the use of different IQ tests. The original study used Stanford-Binet Form L-M, WISC or WAIS tests, depending on age, while the follow-up used WISC-R or WAIS-R. Weinberg, Scarr and Waldman point describe the effect of this change in test:

Declines in IQ scores have been documented when individuals are retested on a revised form of an original measure, as well as when a test used at a first administration was normed earlier than a test used at a subsequent administration (see Flynn, 1984, for a review). For example, the decline in Full-Scale IQ score from the WAIS to the WAIS-R averaged 6.8 points across a number of studies (reviewed by Sattler, 1988) and was 7.5 points in a sample of 72 35-44-year-olds tested as part of the standardization of the WAIS-R (Wechsler, 1981). This is precisely the test combination used for adoptive parents in our study.

Furthermore the data needed to be corrected for the Flynn effect as stated by Ulrich Neisser:

Everyone involved in this debate is well-aware that such comparisons must be corrected for the Flynn effect: Mean scores on all standard IQ tests seem to rise steadily at about 0.3 points per year. 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. Until they have been made–Waldman et al. reported that they are in progress–raw figures like those above are relatively meaningless.

The data corrected for the Flynn effect was published in 2000 by John Loehlin in the Handbook of Intelligence.

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. They had difficulties becoming competent in both European and African-American reference group orientation but had stronger affinity with the European than with the African-American group. Stronger identification with one or the other group predicted better adjustment.