From about 1917 to 2006, large representative samples of American black adults have scored about one standard deviation below American white adults on the type of verbal and performance IQ tests first created for screening WWI recruits, and later borrowed by David Wechsler to use in his wildly popular scales; considered the gold standard in the field.
Although the black-white test score gap has shrunk somewhat on more scholastic tests where it used to be absurdly high, the longevity and consistency of the gap on the most conventional and respected of official IQ tests has led some to conclude that it is mostly or entirely genetic.
The single most powerful piece of supporting evidence for the genetic hypothesis is the Minnesota Transracial adoption study in which white, black and mixed-race kids were raised from early childhood in white upper-class homes. Although the adopted white and black kids scored well above the national white and black means (corrected for outdated norms) of about 102 and 86 respectively (U.S. norms) in childhood (though not at 17), large racial IQ gaps were found among the adopted kids at both ages.
However the study had a problem, as explained by its authors Scarr and Weinberg (1976):
It is essential to note, however, that the groups also differed significantly (p < .05) in their placement histories and natural mother’s education. Children with two black parents were significantly older at adoption, had been in the adoptive home a shorter time, and had experienced a greater number of preadoption placements. The natural parents of the black/black group also averaged a year less of education than those of the black/white group, which suggests an average difference between the groups in intellectual ability. There were also significant differences between the adoptive families of black/black and black/white children in father’s education and mother’s IQ.
Because the children with two black biological parents were adopted later than the children with only one black biological parent, it’s best to exclude them from our analysis and focus only the IQ gap between the adopted kids with two white biological parents and those with one black and one white biological parent. Not only were both these groups adopted early into white upper-class homes, but since both had white biological mothers, both enjoyed the benefits of a white prenatal environment. What the study found was that by age seven, the fully white kids average IQ 111.5 and the half-black kids averaged 105.4, a difference of 6.1 points (see chart above).
This difference may sound small, but keep in mind that we are not comparing full-blooded blacks to full-blooded whites, we are comparing half-African Americans to full-blooded whites. Also keep in mind that because everyone is being raised in the same social class, and social class independently explains such a large percent of the IQ variance at age seven, the entire IQ scale becomes compressed, so instead of the white standard deviation being about 14.5 (U.S. norms), it is only 11.3 in these adopted white kids. Thus a 6.1 point gap should be thought of as a 0.54 SD gap since 6.1/11.3 = 0.54.
So if kids with one black parent score 0.54 SD below white kids when both are raised in upper class homes and both have white prenatal environments, that 0.54 SD gap is arguably 100% genetic. And if having one black parent causes a 0.54 SD genetic drop in IQ, then having two black parents should cause a 1.08 SD genetic drop in IQ (note that the national black-white IQ gap in adults has been about 1 SD since WWI).
Failure to replicate
Now before HBDers get too excited, one should remember that the Minnesota transracial adoption study has never been replicated and that three other similar studies failed to find much of any black < white IQ gap, with some even showing the opposite pattern.
Tizard (1974) compared black, white and mixed-race kids raised in English residential nurseries and found that the only significant IQ difference favored the non-white kids. A problem with this study is that the children were extremely young (below age 5) and racial differences in maturation rates favor black kids. A bigger problem with this study is that the parents of the black kids appeared to be immigrants (African or West Indian) and immigrants are often hyper-selected for IQ (see Indian Americans).
A second study by Eyferth (1961) found that the biological illegitimate children of white German women had a mean IQ of 97.2 if the biological father was a white soldier and 96.5 if the biological father was a black soldier (a trivial difference). Both the white and mixed kids were raised by their biological white mothers. One problem with this study is that the biological fathers of both races would have been screened to have similar IQs because at the time, only the highest scoring 97% of whites and highest scoring 70% of blacks passed the Army General Classification Test and were allowed to be U.S. soldiers. In addition, 20% to 25% of the “black fathers” were not African-American or even black Africans, but rather French North Africans (dark caucasoids as we define them here).
A third study by Moore (1986) included a section where he looked at sub-samples of children adopted by white parents. He found that nine adopted kids with two black biological parents averaged 2 IQ points higher than 14 adopted kids with only one biological black parent. A 2 point IQ gap sounds small, but as I mentioned above, the IQ scale is compressed in kids when everyone is raised in the same social class (which might have been the case in this study), so a 2 point gap becomes 0.18 of the compressed white SD.
The results of this study suggest that half-white kids are 0.18 SD genetically duller than black kids, which predicts that fully white kids are 0.36 SD genetically duller than black kids. One problem with this study is that the black kids would have had black prenatal environments while many, or all, of the half-white kids would have had white prenatal environments, but given the low birth weight of black babies, if anything this suggests the genetic IQ gap favoring blacks is even larger than 0.36 SD!
We have two quality studies: The Minnesota Transracial adoption study (when black kids are excluded because of confounds) and Moore (1986). The first study implies U.S. black genes reduce IQ by 1.04 SD in kids (-1.04 SD), while the second implies U.S. black genes increase IQ by 0.36 SD in kids (+0.36 SD). But the first analysis was based on comparing 55 mixed kids to 16 white kids (total n = 71), while the second analysis was based on comparing nine black kids with 14 mixed kids (total n = 23). The total n of both studies combined is 94, so the first study provided 76% of the total sample while the second study provided 24%, thus the best I can do is just weigh these two conflicting results by sample size:
Effect of black genes on childhood IQ = 0.76(-1.04 SD) + 0.24(+0.36 SD)
Effect of black genes on childhood IQ = -0.79 SD + 0.09
Effect of black genes on childhood IQ = -0.7 SD
What this suggests is that on a scale where the white genetic IQ is set at 100 with an SD of 15, the U.S. black genetic IQ is 90, at least in childhood (in adulthood it may be around 85 since some IQ genes might not exert influence until post-puberty). This is consistent with the fact that despite half a century of affirmative action, the average black IQ (when expressed with reference to white norms) remains below 90 in both children and adults (see charts below).
On the other hand, my estimate is based on only two studies with a combined sample of only 71 adopted kids and we can only assume (based on education when known) that the IQs of their biological parents are roughly racially representative. And although the black-white IQ gap in adults has apparently changed not at all since WWI, the environmental gap might not have changed that much either. Despite decades of affirmative action, the median wealth for white families in 2013 was around $141,900, compared to Hispanics at about $13,700 and blacks at about $11,000 so even in the age of a black President, environmental factors can’t be ruled out.
Black white IQ gap in the Wechsler Intelligence Scale for Children in the nationally representative samples used to norm each edition:
|white iq (u.s. norms)||black iq (u.s. norms)||white iq (white norms)||black iq (white norms)||black-white iq gap (u.s. norms)||black-white iq gap (white norms)|
|wisc-r (1972)||102.3 (sd = 14.08)||86.4 (sd = 12.63)||100 (sd = 15)||83 (sd = 13.46)||15.9||17|
|wisc-iii (1989)||103.5 (sd = 13.86)||88.6 (sd = 12.83)||100 (sd = 15)||84 (sd = 13.89)||14.9||16|
|wisc-iv (2002)||103.2 (sd = 14.52)||91.7 (sd = 15.73)||100 (sd = 15)||88 (sd = 16.25)||11.5||12|
|wisc-v (2013)||103.5 (sd = 14.6)||91.9 (sd = 13.3)||100 (sd = 15)||88 (sd = 13.66)||11.6||12|
Black white IQ gap in the Wechsler Adult Intelligence Scale in the nationally representative samples used to norm each edition:
|white iq (u.s. norms)||black iq (u.s. norms)||white iq (white norms)||black iq (white norms)||black-white iq gap (u.s. norms)||black-white iqgap (white norms)|
|wais-r (1978)||101.4 (sd = 14.65)||86.8 (sd = 13.14)||100 (sd = 15)||85 (sd = 13.45)||14.6||15|
|wais-iii (1995)||102.6 (sd = 14.81)||89.1 (sd = 13.31)||100 (sd = 15)||86 (sd = 13.48)||13.5||14|
|wais-iv (2006)||103.4 (sd = 14)||87.7 (sd = 14.4)||100 (sd = 15)||83 (sd = 15.43)||15.7||17|
Sources for charts:
WISC-R, WISC-III, and WISC-IV U.S. norms, from pg 27 (Table A1) of Black Americans Reduce the Racial IQ Gap: Evidence from Standardization Samples by William T. Dickens & James R. Flynn
WAIS-IV U.S. norms from pg 190 of WAIS-IV, WMS-IV, and ACS: Advanced Clinical Interpretation edited by James A. Holdnack, Lisa Drozdick, Lawrence G. Weiss, Grant L. Iverson
WISC-V U.S. norms from page 157, table 5.3 of WISC-V Assessment and Interpretation: Scientist-Practitioner Perspectives By Lawrence G. Weiss, Donald H. Saklofske, James A. Holdnack, Aurelio Prifitera