The costs of outbreeding: a few more points

Although the house mouse (Mus musculus) is a single species, fertility is reduced in crosses between geographic populations (Wikipedia - Bolid74)



It's widely believed that mating between two species never produces fertile offspring. Untrue. Interspecific mating sometimes produces fertile offspring, and the genomes of closely related species often show gene flow from one to the other. It would be more accurate to say that such crossings are usually compromised in one way or another—incompatibilities arise during development, which lead to miscarriage, sterile offspring, or less viable offspring. 

There is a similar misunderstanding about mating within a species. It isn't true that mating within a species is uniformly possible. Even below the species level, genetic incompatibilities can arise between different geographic populations. Keep in mind the arbitrariness of all these terms: “geographic population,” “race,” “subspecies,” and “species.” These are points on a continuum of increasing reproductive isolation and genetic incompatibility. 

Various problems have been observed in crossings between geographic populations of the same species (Presgraves 2010; Turner et al. 2011). In the case of crossings between two subspecies of the house mouse (M. m. domesticus and M. m. musculus), the problems have no single cause. There is not only reduced fertility but also reduced immune function, as shown by higher loads of intestinal parasites in hybrids. Even the reduction in fertility has multiple causes: "The type and severity of fertility defects observed depends on the geographic origin of the strains and also varies among individuals within regions. This variability suggests that multiple genetic incompatibilities contribute to hybrid male sterility" (Turner et al. 2011). Multiple loci seem to be responsible, and incompatibilities that cause sterility likely act in combination with each other and with other incompatibilities (Turner et al. 2011).

In general, as genetic distance increases, so does the risk of faulty interactions between different gene loci:

The Dobzhansky-Muller model proposes that populations diverging independently in allopatry will accumulate differences through drift or selection, but will maintain coadaptation between divergent loci within each single population. If secondary contact and hybridization occurs between these populations, these divergent loci from the previously allopatric populations may interact deleteriously in hybrids leading to lowered hybrid fitness (Leppälä and Savolainen 2011)

Icelandic and Danish studies on outbreeding

The above model is consistent with two recent studies on humans, one in Iceland and the other in Denmark. The Icelandic study used a database of couples born between 1800 and 1965. In the authors' own words, "the advantage of using the Icelandic data set lies in this population being small and one of the most socioeconomically and culturally homogeneous societies in the world, with little variation in family size, use of contraceptives, and marriage practices" (Helgason et al. 2008). The authors found that fertility rose and then fell with increasing genetic distance between husband and wife. Specifically, the relationship was:

[...] an n-shaped curve from the relatively low reproductive success of couples related at the level of second cousins or closer, to the maximum for couples related at the level of third and fourth cousins, after which there is a steady decrease in reproductive success with diminishing kinship between spouses. A similar picture emerges when the number of grandchildren per couple is examined (Helgason et al. 2008)

The Icelandic study was criticized in a tweet by Jayman:

Doubt it. The Icelandic study is likely confounded in various ways so I don't think genetic relatedness is the thing directly affecting fertility.

He was retweeted by hbdchick, but neither of them identified the "various" confounds. Nor did I find this criticism in the 140 papers that cited the study on Google Scholar. So we're not in the realm of common knowledge.

The original paper did identify a possible confound. From 1800 to 1965, there was a decrease in mean fertility and an increase in marriages with more distantly related individuals. The relationship between fertility and kinship may thus be an historical confound:

These results are based on couples born during a period of almost 200 years, in the course of which there was a marked decline both in the mean fertility and in kinship between couples. (Helgason et al. 2008)

To eliminate the confound, the authors broke the data down by 25-year intervals:

Nonetheless, the same general relationship between kinship and reproductive outcome was observed within each 25-year subinterval (fig. S2). We evaluated the correlation between the standardized variables of kinship and reproductive outcome for all couples and for each time interval separately (Table 2), adjusting for the impact of geographical differences in the kinship and fertility of couples within Iceland (10). Each test revealed as significant association with kinship, with correlation coefficients of 0.063 (P = 1.5 × 10-129) for the number of children, 0.045 (P = 3.6 × 10-66) for the number of children who reproduced, and 0.042 (P = 7.6 × 10 -58) for the number of grandchildren. (Helgason et al. 2008)

There might be other confounds. That's why the Danish study is important. It went through a number of socioeconomic factors: "education, family income, urbanicity, mother's age at first birth, and six variables representing proximity to kin [maternal radius, i.e., distance between mother's and child's birthplace; paternal radius; and presence of each grandparent in the child's birth parish or neighboring parish" (Laboriau and Amorim 2008). Controlling for those factors did not change the findings.


Davenport's study on outbreeding in Jamaica

Jayman also criticized Davenport's study, apparently in reference to the finding that between 5 and 30% of the "brown" children did worse than predicted.

As for mixed race children, you have to keep in mind that people don't enter those relationships randomly - i.e., also confounded

Jayman is arguing that the parents of the brown children were atypical, apparently in the sense of being below-average. In its early days, Jamaica did have many poor whites, but most of them left because of competition from slave labor. European ancestry entered the Jamaican population largely from landowners and businessmen, particularly people of Scottish descent:

Jamaica has more people using the Campbell surnames than the population of Scotland itself, and it also has the highest percentage of Scottish surnames outside of Scotland. Scottish surnames account to about 60% of the surnames in the Jamaican phone books. The first Jamaican inhabitants from Scotland were exiled "rebels". Later, they would be followed by ambitious businessmen who spent time between their great country estates in Scotland and the island. As a result, many of the slave owning plantations on the island were owned by Scottish men, and thus a large number of mixed-race Jamaicans can claim Scottish ancestry. (Wikipedia 2020).

Today, "brown" Jamaicans are generally middle class, and they were even more so during the time of Davenport's study. It is indeed strange to intimate that his biracial subjects were the offspring of below-average whites and blacks.


Conclusion

Davenport's study probably corresponds to the highest degree of outbreeding possible. Even in that case, adverse mental effects were observable only in a minority of offspring. The most widespread effect is probably lower fertility and perhaps a higher risk of testicular cancer. Joffe (2009) suggests that an increase in outbreeding might explain a century-long decrease in semen quality and a corresponding increase in testicular cancer:

One implication of the proposed pathogenesis is that as D&D [duplications and deletions of genetic material during meiosis] accumulate, mating between individuals who are genetically unalike would be associated with lower reproductive success as pairing at the start of meiosis would be more likely to be impaired. On the face of it, this contradicts the earlier observation that hybrid vigour would tend to increase fertility in unrelated individuals. Yet these two ideas may be compatible: both extremes—genetic similarity (inbreeding) and genetic distance (D&D accumulation)—could decrease fertility, so that an intermediate degree of relatedness would be associated with the highest degree of fertility. This could explain the evidence from Iceland that the greatest reproductive success, measured as the number of grandchildren, was observed in couples who were third or fourth cousins (Helgason et al., 2008). Comparable findings have been reported from elsewhere, for example Denmark (Labouriau and Amorim, 2008).

I realize this may be a sensitive subject. If you feel offended, please read something else.


References

Davenport, C.B. and M. Steggerda. (1928). Race Crossing in Jamaica. Washington: Carnegie Institution, Publication no. 395.
http://www.velesova-sloboda.info/archiv/pdf/davenport-race-crossing-in-jamaica.pdf  

Helgason, A., S. Pálsson, D.F. Guðbjartsson, þ. Kristjánsson, K. Stefánsson. (2008). An association between the kinship and fertility of human couples. Science 319(5864): 813-816.
http://facelab.org/debruine/Teaching/EvPsych/files/Helgason_2008.pdf   

Joffe, M. (2009). What has happened to human fertility? Human Reproduction 25(2): 295-307.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1003.7270&rep=rep1&type=pdf 

Labouriau, R., and A. Amorim. (2008). Comment on "An Association Between the Kinship and Fertility of Human Couples." Science 322(5908): 1634
http://science.sciencemag.org/content/322/5908/1634.2.full   

Leppälä, J. and O. Savolainen. (2011). Nuclear-cytoplasmic interactions reduce male fertility in hybrids of Arabidopsis lyrata subspecies. Evolution 65(10): 2959-2972
https://onlinelibrary.wiley.com/doi/full/10.1111/j.1558-5646.2011.01361.x

Presgraves, D.C. (2010). The molecular evolutionary basis of species formation. Nature Reviews Genetics 11:175-180.
https://www.nature.com/articles/nrg2718  

Turner, L.M., D.J. Schwahn, and B. Harr. (2011). Reduced male fertility is common but highly variable in form and severity in a natural house mouse hybrid zone. Evolution 66(2): 443-458.
https://onlinelibrary.wiley.com/doi/full/10.1111/j.1558-5646.2011.01445.x  

Wikipedia (2020). Jamaica
https://en.wikipedia.org/wiki/Jamaica

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