Looking beyond the data

 

General intelligence (g factor) as a function of alleles associated with educational attainment (Education polygenic score). (Fuerst et al. 2021, p. 165)

Among non-Hispanic European Americans, cognitive ability shows a positive correlation with Amerindian admixture. The reason is to be found in the history of European settlement.

 

 

We know that cognitive ability differs among human populations, but are those differences innate? Or are they purely cultural? The question is difficult to answer because a purely cultural difference can, over time, become innate. If you are better able to meet the demands of your culture, you will probably live longer, have more offspring, and pass on many of your characteristics. Thus, over succeeding generations, those heritable characteristics will become more and more widespread in the gene pool, and they will increasingly determine certain abilities that were initially created by culture.

 

This is a recurring problem when we try to distinguish between cultural and genetic determination. The two often run parallel to each other, and we can seemingly rule out the existence of genetic determination by showing that cultural determination runs in the same direction.

 

But there is another recurring problem in our efforts to distinguish between culture and genetics. We lack the proper tools. For a long time, we could only infer genetic influences by using twin studies or adoption studies. 

 

Things have changed with the advent of a new tool: genomic data. Specifically, we can now:

 

·         Measure ethnic ancestry in mixed populations, as opposed to using self-report or inferring from skin color.

·         Measure the genetic component of cognitive ability, by using genetic variants associated with educational attainment. Although these variants explain only 11-13% of the variance in educational attainment among individuals, they explain a much higher percentage of the variance among populations (Piffer 2019). This is because genetic variants within the same population are exposed to the same pressure of selection and will thus vary in the same direction. They act, so to speak, as “weathervanes” that tell us the strength and direction of selection in that population.

·         Measure skin color, by looking at the relevant genes. We can thus control for the effects of “colorism” in mixed populations, i.e., discrimination in favor of lighter-skinned individuals.

 

In my last post, I described how Bryan Pesta used these tools to understand differences in mean cognitive ability between African Americans and European Americans (Lasker et al. 2019). To that end, his research team looked at cognitive ability among African Americans in relation to European admixture and in relation to genetic variants associated with educational attainment.

 

They made several findings: 1) among African Americans, cognitive ability correlates with European admixture; 2) the correlation is modestly reduced, but not eliminated, when controlled for parental education; 3) controlling for skin color has no effect; and 4) the correlation seems to be largely explained by genetic variants associated with educational attainment.

 

The same data source was then used by Fuerst et al. (2021) to investigate cognitive ability not only in European Americans and African Americans but also in Hispanic Americans. The research team thus looked at cognitive ability in relation to Amerindian admixture, and not just in relation to European and African admixture.

 

Most of their findings are similar to those of the first study:

 

·         Among Hispanic Americans, cognitive ability shows a positive correlation with European admixture and a negative correlation with African admixture and Amerindian admixture.

·         Among Hispanic Americans, the correlations are reduced but not eliminated by controlling for parental education. Controlling for skin color has no effect.

·         The above correlations are partially explained by variants associated with educational attainment, but not by skin color.

·         Among non-Hispanic European Americans, cognitive ability shows a positive correlation with Amerindian admixture.

 

The last correlation may seem curious. Keep in mind that the data came from residents of Pittsburgh and that the native peoples of the Eastern U.S. intermixed mostly with early settlers of British, Dutch, or French origin. There is much less Amerindian admixture among the descendants of later immigrants from southern and eastern Europe. The correlation may thus be due not to Amerindian admixture per se but rather to variation in cognitive ability among Europeans.

 

Until the eleventh century, mean IQ was relatively low throughout Europe, perhaps hovering in the low 90s. It then rose during late medieval and post-medieval times through the expansion of the middle class. There was in fact a broad mental and behavioral change: "Thrift, prudence, negotiation, and hard work were becoming values for communities that previously had been spendthrift, impulsive, violent, and leisure loving" (Clark 2007, p. 166; see also Clark 2007, 2009a, 2009b). More people could better understand probability, cause and effect, and another person’s perspective, whether real or hypothetical (Rinderman 2018, pp. 49, 86-87; Oesterdiekhoff 2012). As the "smart fraction" grew in size, a point was reached when intellectuals were no longer voices crying in the wilderness. They were now numerous enough to form learned societies and collaborate in projects of various sorts (Frost 2019b, pp. 175-176).

 

Western Europe was where the middle class began to expand, and that was where the expansion would have its greatest impact, not only demographically but also behaviorally and cognitively. Gregory Clark (2009a) has shown that the English, even in the lower classes, are largely descended from people who were middle-class several generations earlier. The same is likely true elsewhere in Western Europe. We should therefore see a cognitive gradient between the Western European core and its periphery, as can indeed be seen between northern and southern Italy. When Piffer and Lynn (2022) looked at genomic data from that country, they found a north-south gradient in alleles associated with educational attainment. That difference corresponds to historical differences in economic development. By the 18th century, the South had already fallen behind the North; its middle class had remained small and economic relations were still structured by paternalism and familialism (De Rosa 1979).

 

All of that leads to an interesting corollary: the IQ gap used to be smaller between Europeans and sub-Saharan Africans. On the one hand, European mean IQ had probably remained in the low 90s until late medieval times. On the other hand, mean IQ may have been in the upper 80s among those Black African groups that Europeans had first encountered, particularly the Nubians. By the time of Classical Antiquity they had reached a high level of material culture, social complexity and State formation.

 

A smaller IQ gap would be in line with an observation by Jason Malloy. He noted that blacks were often described in the ancient world as having large penises but not as being less intelligent. Indeed, I have found only two Greco-Roman texts in which the writer disparaged Black Africans as being unintelligent. One of them is of doubtful authenticity, and both come from Late Antiquity (Frost 2019b). By then, blacks in the Roman world were increasingly slaves who came from farther within the African interior. Thereafter, a stereotype of low intelligence is regularly attested in Middle Eastern and European sources.

 

 

References

 

Clark, G. (2007). A Farewell to Alms. A Brief Economic History of the World. Princeton University Press: Princeton and Oxford.

 

Clark, G. (2009a). The indicted and the wealthy: surnames, reproductive success, genetic selection and social class in pre-industrial England.  http://www.econ.ucdavis.edu/faculty/gclark/Farewell%20to%20Alms/Clark%20-Surnames.pdf     

 

Clark, G. (2009b). The domestication of man: The social implications of Darwin. ArtefaCTos 2: 64-80. https://www.researchgate.net/publication/277275046_The_Domestication_of_Man_The_Social_Implications_of_Darwin

 

De Rosa, L. (1979). Property Rights, Institutional Change, and Economic Growth in Southern Italy in the XVIIIth and XIXth Centuries. Journal of European Economic History 8(3): 531-551.

 

Frost, P. (2019a). The Original Industrial Revolution. Did Cold Winters Select for Cognitive Ability? Psych 1(1): 166-181. https://doi.org/10.3390/psych1010012   

 

Frost, P. (2019b). Why that stereotype and not the other? Evo and Proud, July 28. https://evoandproud.blogspot.com/2019/07/why-that-stereotype-and-not-other.html

 

Frost, P. (2021). Commentary on Fuerst et al: Do Human Populations Differ in Their Mental Characteristics? Mankind Quarterly 62(2). http://doi.org/10.46469/mq.2021.62.2.9   

 

Fuerst, J., E.O.W. Kirkegaard and D. Piffer. (2021). More research needed: There is a robust causal vs. confounding problem for intelligence-associated polygenic scores in context to admixed American populations. Mankind Quarterly 62(1): 151-185. https://www.researchgate.net/profile/John-Fuerst/publication/354767141_More_Research_Needed_There_is_a_Robust_Causal_vs_Confounding_Problem_for_Intelligence-associated_Polygenic_Scores_in_Context_to_Admixed_American_Populations/links/614bc1dfa595d06017e4c017/More-Research-Needed-There-is-a-Robust-Causal-vs-Confounding-Problem-for-Intelligence-associated-Polygenic-Scores-in-Context-to-Admixed-American-Populations.pdf

 

Lasker, J., B.J. Pesta, J.G.R. Fuerst, and E.O.W. Kirkegaard. (2019). Global Ancestry and Cognitive Ability. Psych 1(1):431-459. https://doi.org/10.3390/psych1010034  

 

Oesterdiekhoff, G.W. (2012). Was pre-modern man a child? The quintessence of the psychometric and developmental approaches. Intelligence 40, 470–478. https://doi.org/10.1016/j.intell.2012.05.005

 

Piffer, D. (2019). Evidence for Recent Polygenic Selection on Educational Attainment and Intelligence Inferred from Gwas Hits: A Replication of Previous Findings Using Recent Data. Psych 1(1):55-75. https://doi.org/10.3390/psych1010005

 

Piffer, D., and R. Lynn. (2022). In Italy, North-South Differences in Student Performance Are Mirrored by Differences in Polygenic Scores for Educational Attainment. Mankind Quarterly 62(4), Article 2. https://doi.org/10.46469/mq.2022.62.4.2   

 

Rindermann, H. (2018). Cognitive Capitalism. Human Capital and the Wellbeing of Nations, 1st ed.; Cambridge University Press.

John Fuerst's latest paper

 

The Igbo used their location on the Niger Delta to become traders between the coast and the interior. In the 18th century, they were already described as high achievers with a strong commercial sense. Cognitive ability seems to be higher in populations that specialize in trade, probably because the cognitive demands are likewise higher.

(Map of Niger - Wikicommons)

 

 

 

I was asked to comment on a recent study by John Fuerst, Emil Kirkegaard, and Davide Piffer. This study is well worth reading. It provides the strongest evidence to date for differences in mean cognitive ability between human populations, specifically by showing how cognitive ability correlates with alleles for educational attainment and with degree of European / African / Amerindian ancestry in European Americans, African Americans, and Hispanic Americans. Also worth reading is a related paper by John Fuerst, Meng Hu, and Gregory Connor.

 

The following is the abstract of my commentary:

 

Human populations may differ genetically not only in their anatomy but also in their mental characteristics. Our species is not too young for such differentiation. In fact, human genetic evolution has proceeded faster over the past 10,000 years than over the previous million. With the rise of farming, and social complexity, humans were no longer adapting solely to a limited range of natural environments. They were adapting to an ever-widening range of cultural environments, each of which imposed its demands on mind and body.

 

Thus, mental characteristics do not have the same adaptive value in all environments, and differences in adaptive value will lead, over time, to genetic differences. Are the latter large enough to explain IQ differences between human populations? That question has led to studies of people who are ancestrally diverse but raised in the same environment, such as transracial adoptees. Unfortunately, the environment can never be fully equalized. We should measure genetic differences directly, and a promising step in that direction has come with research to identify alleles associated with educational attainment. There is no need to identify all of them, just a large enough sample. These “witnesses” can then be questioned to determine the strength and direction of natural selection, and its consequences.

 

Also promising is the study of IQ and ancestry in ethnically mixed groups. This research instrument is not without problems. Large continental populations often have high-achieving minorities who may contribute disproportionately to the founding of new groups or to admixture with old ones. In addition, natural selection can alter the distribution of alleles within a new group, even after a few generations.

 

References

 

Frost, P. (2021). Commentary on Fuerst et al: Do Human Populations Differ in Their Mental Characteristics? Mankind Quarterly 62(2): 366-380.

http://doi.org/10.46469/mq.2021.62.2.9

https://www.academia.edu/73260728/Commentary_on_Fuerst_et_al_Do_Human_Populations_Differ_in_Their_Mental_Characteristics

 

Fuerst, J., Hu, M. & Connor, G. (2021). Genetic ancestry and general cognitive ability in a sample of American youths. Mankind Quarterly 62(1): 186-216.

https://www.researchgate.net/profile/John-Fuerst/publication/354766492_Genetic_Ancestry_and_General_Cognitive_Ability_in_a_Sample_of_American_Youths/links/614bbacaa595d06017e4bdbc/Genetic-Ancestry-and-General-Cognitive-Ability-in-a-Sample-of-American-Youths.pdf

 

Fuerst, J., Kirkegaard, E.O.W. and Piffer, D. (2021). More research needed: There is a robust causal vs. confounding problem for intelligence-associated polygenic scores in context to admixed American populations. Mankind Quarterly 62(1): 151-185.

https://www.researchgate.net/profile/John-Fuerst/publication/354767141_More_Research_Needed_There_is_a_Robust_Causal_vs_Confounding_Problem_for_Intelligence-associated_Polygenic_Scores_in_Context_to_Admixed_American_Populations/links/614bc1dfa595d06017e4c017/More-Research-Needed-There-is-a-Robust-Causal-vs-Confounding-Problem-for-Intelligence-associated-Polygenic-Scores-in-Context-to-Admixed-American-Populations.pdf  

Differences in the genetic architecture of cognition?

Tableau III, Piet Mondrian (1872-1944). The polygenic score can provide a measure of innate cognitive ability in various human populations. However, it is less valid for African Americans, apparently because of differences in the genetic architecture of cognition.



When IQ is measured in European Americans and African Americans, the two groups differ on average by about 15 points. Is the difference genetic? Or is it due to different environments?

After years of debate, we are coming close to an answer. The weight of evidence is shifting, especially because of two unrelated developments: 

- We can now easily measure ethnic ancestry by means of genetic data. Previously, we had to rely on self-report or indirect measures like skin color.

- We can now measure the genetic component of cognitive ability: the polygenic score. This score is a summation of alleles associated with high educational attainment. Initially a crude measure, it is getting better and better as we identify more and more of these alleles.

Both research tools were used in a recent study. Lasker et al. (2019) applied them to the Philadelphia Neurodevelopmental Cohort, a sample of 9421 individuals from the Philadelphia area who received medical care from the Children's Hospital of Philadelphia network. They ranged in age from 8 to 21 with a mean of 14.2. They were 51.7% female, 55.8% European American, 32.9% African American, and 11.4% other. All of them were genotyped and given a series of cognitive tests.

This dataset had advantages over those of previous studies:

- All participants came from the same geographic area. 

- Heritabilities of cognitive ability were already estimated by another research team, specifically 0.61 for the African American participants and 0.72 for the European American participants.

- Skin, hair, and eye color could be estimated from the genetic data to control for the effects of "colorism" (discrimination favoring lighter-skinned over darker-skinned African Americans)

- Polygenic scores could be calculated from the genetic data

The main disadvantage was the participants’ young age. Before adulthood the brain is plastic and still developing, so the heritability of cognitive ability is lower. 


IQ results

IQ scores were 100 for European Americans, 98 for self-described biracial Americans, and 85 for African Americans. The three groups were respectively 99% European, 80% European, and 19% European.

African Americans only

European admixture significantly correlated with IQ among the African American participants. The correlation remained significant after controlling for either skin color or socioeconomic status. Interestingly, skin and hair color didn't significantly correlate with IQ independently of European admixture, but eye color did. Brown eyes correlated positively with IQ. No explanation was offered by the authors. Did they get the same finding with European Americans?

Biracial Americans only - smarter than expected

As with African Americans, skin color didn't seem to influence intelligence independently of European admixture. On the other hand, "biracial status had a significant effect independently of European ancestry." In other words, racially mixed individuals who identified equally as African American and European American, and not just as African American, tended to be more intelligent than what their degree of European admixture would predict. 

The term "biracial" as a badge of identity is recent and seems to be most popular among middle-class people: 

Interestingly, many of the respondents here who identify as biracial are middle class, educated in private schools, and raised in predominantly white neighborhoods with mostly white social networks (Rockquemore and Brunsma 2008, p. xxii)

It may be, then, that self-identified "biracial" people have parents who are, on average, of higher quality than other people of the same racial background.

African Americans, Biracial Americans, and European Americans combined

When all three groups were combined, the most important factor was European admixture. Next came socioeconomic status, which correlated with cognitive ability independently of European admixture. Finally, self-identification as a European American had an effect over and above that of European admixture. The last factor suggests that European American culture has a positive influence on cognitive ability.


Polygenic score results

The polygenic scores ran into a problem that others have noted: the genetic architecture of cognition seems to be different in African Americans. This is a problem because researchers have used only Europeans or European Americans to identify genetic variants that are associated with high educational attainment. Those variants did correlate with cognitive ability in the African American sample, but to a much lower degree than in the European American sample. Their validity as a measure of cognitive ability was only 20% of what it was in the European American sample.

The authors used a subset of the same variants to create a polygenic score that would be less sensitive to linkage disequilibrium decay and thus more valid across different human populations. This polygenic score had good validity in both the African- and European-American samples (r = 0.112 and r = 0.227 respectively).

The authors then tried to create an even better polygenic score by excluding variants that are rare in African Americans. There was no effect on the results for either African Americans or European Americans. But what about the reverse? Perhaps cognitive ability is improved by some variants that are rare in European Americans but common in African Americans.


Conclusion

This is an excellent study, on a par with the Minnesota Transracial Adoption Study (Frost 2019). The main problem is the participants’ young age. Had adults been used, there would have been less noise in the data, and the results would have been better.

Another problem is the apparently different genetic architecture of cognition in people of sub-Saharan African origin. Piffer (2019) has noted that polygenic scores underestimate African American IQ. He disagrees, however, with the "different genetic architecture" hypothesis, pointing out that no divergence exists between mean population IQ and the polygenic scores he has calculated for various sub-Saharan African groups (Esan, Gambians, Luhya, Mende, Yoruba). None of them, however, are Igbo, and the Igbo are really the one group that stands out from other West Africans on measures of intellectual and educational attainment (Frost 2015a, 2015b). They also contributed to the gene pool of African Americans: "Many of the enslaved Igbo people in the United States were concentrated in Virginia's lower Tidewater region and at some points in the 18th century they constituted over 30% of the enslaved black population" (Wikipedia 2019).

While the polygenic score is a good measure of raw cognitive ability in most humans, we need to develop a modified version for people whose ancestry comes primarily from sub-Saharan Africa.


References

Frost, P. (2015a). The Jews of West Africa? Evo and Proud, July 4
http://evoandproud.blogspot.ca/2015/07/the-jews-of-west-africa.html 

Frost, P. (2015b). No, blacks aren't all alike. Who said they were? Evo and Proud. October 10.
http://evoandproud.blogspot.com/2015/10/no-blacks-arent-all-alike-who-said-they.html 

Frost, P. (2019). IQ of biracial children and adults. Evo and Proud. March 10.
http://evoandproud.blogspot.com/2019/03/iq-of-biracial-children-and-adults.html

Lasker, J., B.J. Pesta, J.G.R. Fuerst, and E.O.W. Kirkegaard. (2019). Global ancestry and cognitive ability. Psych 1(1)
https://www.mdpi.com/2624-8611/1/1/34 

Piffer, D. (2019). Evidence for Recent Polygenic Selection on Educational Attainment and Intelligence Inferred from Gwas Hits: A Replication of Previous Findings Using Recent Data. Psych 1(1): 55-75
https://www.mdpi.com/2624-8611/1/1/5  

Rockquemore, K.A. and D.L. Brunsma. (2008). Beyond Black. Biracial Identity in America. Lanham: Rowman & Littlefield Publishers.
https://books.google.ca/books?id=Nf0eAAAAQBAJ&printsec=frontcover&hl=fr&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

Wikipedia. (2019). Igbo Americans. 
https://en.wikipedia.org/wiki/Igbo_Americans

IQ of biracial children and adults

First snow in Minnesota (c. 1895), Robert Koehler. Biracial children have IQ scores halfway between those of white children and black children, even when they are conceived by white single mothers and adopted into middle-class white families in their first year of life.

You may have heard about the Minnesota Transracial Adoption Study. It was a longitudinal study of black, biracial, and white children adopted into white middle-class Minnesotan families, as well as the biological children of the same families (Levin, 1994; Lynn, 1994; Scarr and Weinberg, 1976; Weinberg, Scarr, and Waldman, 1992). IQ was measured when the adopted children were on average 7 years old and the biological children on average 10 years old. They were tested again ten years later. Between the two tests, all four groups declined in mean IQ. On both tests, however, the differences among the four groups remained unchanged, particularly the 15-point gap between black and white adoptees. 

The biracial children remained halfway between the black and white adoptees. Could this be due to the parental environment being likewise half and half? Well, no. All of them were raised by white parents, and they were adopted at an early age: 19 months on average for the white adoptees, 9 months for the biracial adoptees, and 32 months for the black adoptees. The last figure is emphasized by Scarr and Weinberg (1976) as a reason for the IQ gap between the black and white adoptees. 

Fine, but what about the IQ gap between the biracial and white adoptees? Almost all of the biracial children were adopted at a young age and born to white single mothers who had completed high school. From conception to adulthood they developed in a "white" environment. If anything, the white adoptees should have encountered more developmental problems because they were adopted at an older age.

Could color prejudice be a reason? Perhaps the biracial children were unconsciously treated worse than the white children. By the same reasoning, they may have been treated better than the black children. We can test the second half of this hypothesis. Twelve of the biracial children were wrongly thought by their adoptive parents to have two black parents. Nonetheless, they scored on average at the same level as the biracial children correctly classified by their adoptive parents (Scarr and Weinberg 1976).

The Eyferth study

Another study found no difference in IQ between white and biracial children. This was a study of children fathered by American soldiers in Germany and then raised by German mothers (Eyferth 1961). It found no significant difference in IQ between children with white fathers and children with black fathers. Both groups had a mean IQ of about 97.

These findings were criticized by Rushton and Jensen (2005) on three grounds:

1. The children were still young when tested. One third were between 5 and 10 years old and two thirds between 10 and 13. Since IQ is strongly influenced by family environment before puberty, a much larger sample would be needed to find a significant difference between the two groups.

2. Between 20 and 25% of the “black” fathers were actually North African.

3. At the time of the study, the US Army screened out low IQ applicants with its preinduction Army General Classification Test. The rejection rate was about 30% for African Americans and 3% for European Americans. African American soldiers are thus a biased sample of the African American population.

Another factor is that the capacity for intelligence seems to be more malleable in children than in adults. We see this with the Minnesota Transracial Adoption Study. In the enriched learning environment of middle-class Minnesota families, all of the children showed impressive IQ scores at 7 years of age. By 17 years of age, however, this benefit had largely washed out:

-------------------- Age 7 Age 17

Black children ----- 97 ----- 89

Biracial children - 109 ----- 99

White children --- 112 ---- 106

Does intelligence really decline with age because of wear and tear on the brain? Perhaps we’re programmed to be most intelligent in childhood. That’s when we have to familiarize ourselves with the world. The capacity for intelligence may then be gradually deactivated as we get older because it’s less necessary.

This deactivation may follow different trajectories in different human groups. In early Homo sapiens, it may have begun not long after puberty. As ancestral humans made the transition to farming, sedentary living, and increasingly complex societies, this learning capacity became more necessary in adulthood, with the result that natural selection favored those individuals who retained it at older ages. This gene-culture coevolution would have gone farther in some populations than in others.

The Fuerst et al study

A recent study led by John Fuerst has confirmed the intermediate IQ of biracial individuals, this time in adults. The research team used the General Social Survey, which includes not only ethnic, sociological, and demographic data but also a measure of intelligence (WordSum):

The relationship between biracial status, color, and crystallized intelligence was examined in a nationally representative sample of adult Black and White Americans. First, it was found that self-identifying biracial individuals, who were found to be intermediate in color and in self-reported ancestry, had intermediate levels of crystallized intelligence relative to self-identifying White (mostly European ancestry) and Black (mostly sub-Saharan African ancestry) Americans. The results were transformed to an IQ scale: White (M = 100.00, N = 7569), primarily White-biracial (M = 96.07, N = 43, primarily Black-biracial (M = 94.14 N = 50), and Black (M = 89.81, N = 1381).

The same study also found a significant negative correlation among African Americans between facial color and WordSum scores. The correlation was low (r = -0.102), but it would be difficult to get a higher correlation because of the measures used. Self-reported skin color correlates imperfectly with actual skin color, which in turn correlates imperfectly with European admixture. Wordsum likewise correlates imperfectly with IQ (r = 0.71). On a final note, the correlation between facial color and WordSum scores was not explained by region of residence, interviewer’s race, parental socioeconomic status, or individual educational attainment.

References

Eyferth, K. (1961). Leistungen verscheidener Gruppen von Besatzungskindern in Hamburg-Wechsler Intelligenztest für Kinder (HAWIK). Archiv für die gesamte Psychologie 113: 222-241.

Fuerst, J.G.R., R. Lynn, and E.O.W. Kirkegaard. (2019). The Effect of Biracial Status and Color on Crystallized Intelligence in the U.S.-Born African-European American Population. Psych 1(1): 44-54. https://dx.doi.org/10.3390/Psychology1010004

Levin, M. (1994). Comment on the Minnesota transracial adoption study. Intelligence 19: 13-20.

http://www.sciencedirect.com/science/article/pii/0160289694900493   

Lynn, R. (1994). Some reinterpretations of the Minnesota Transracial Adoption Study. Intelligence 19: 21-27.

http://www.sciencedirect.com/science/article/pii/0160289694900507   

Rushton, P. and A.R. Jensen. (2005). Thirty years of research on race differences in cognitive ability. Psychology, Public Policy, and Law 11: 235-294.

https://isgp-studies.com/miscellaneous/race-and-iq/2005-APA-Rushton-Jensen-Thirty-Years-of-Research-on-Race-Differences-in-Cognitive-Ability.pdf

Scarr, S., and Weinberg, R.A. (1976). IQ test performance of Black children adopted by White families. American Psychologist 31: 726-739.

http://www.kjplanet.com/amp-31-10-726.pdf   

Weinberg, R.A., Scarr, S., and Waldman, I.D. (1992). The Minnesota Transracial Adoption Study: A follow-up of IQ test performance at adolescence. Intelligence 16: 117-135.

http://www.sciencedirect.com/science/article/pii/016028969290028P