Environmentalists like admixture analysis too (until they don’t)

See previous post about quotes from the medical genetics and physical anthropology literature on admixture analysis and the causal interpretation.

There’s quite a few older admixture studies that examined relationships between racial ancestry and intelligence. Most of these used quite crude methods such as interviewer judgement. Some used a better method, namely objectively measured skin tone/color/reflectance. A small few used something approaching modern technology, namely blood groups. Curiously, you only hear about the ones with the smallest samples probably because these found no relationships, and are thus perceived to provide evidence (sometimes seen as conclusive) for non-genetic models. Thus, we can find environmentalists defending this kind of analysis, now that they like the results.

Templeton provides a particularly good quote, my bolding below.

There is a way of testing if differences in phenotypic means between two populations have a genetic basis. The test was developed by Mendel and requires that the populations be crossed and that the hybrids and their descendants be raised in a ‘‘common garden” (i.e., a common environment). Despite the extreme interest in the genetic basis of between population differences in intelligence, only a handful of studies have even attempted to use this standard research design of genetics. These few studies (Green, 1972; Loehlin, Vandenberg, & Osborne, 1973; Scarr, Pakstis, Katz, & Barker, 1977) have several common features. First, they take advantage of the strong tendency of humans to interbreed when brought into physical proximity. For example, in the Americas, geographically differentiated human populations of European and sub-Saharan African origin were brought together and began to hybridize. However, most matings still occurred within populations. Given this assortative mating, the genetic impact of hybridization is extremely sensitive to the cultural environment. In North America, the hybrids were culturally classified as blacks, and hence most subsequent matings involving the hybrids were into the population of African origin. Therefore, a broad range of variation in degree of European and African ancestry can be found among North American individuals who are all culturally classified as being members of the same “race”, in this case blacks (a “common garden” cultural classification). In Latin America, different cultures have different ways of classifying hybrids, but in general a number of alternative categories are available and social class is a more powerful determinant of mating than is physical appearance (e.g., skin color). As a consequence, individuals in Latin America can be culturally classified into a single social entity that genetically represents a broad range of variation in amount of European and African ancestry. Thus, these studies use a “common garden” design in a cultural sense that nevertheless includes hybrid individuals and their descendants. Second, these studies quantify the degree of European and African ancestry in a population of individuals that is culturally classified as being a single “race.” Because the original geographically disparate populations do show genetic differences due to isolation by distance, the degree of European and African ancestry of a specific individual can be estimated using blood group and molecular genetic markers. Finally, the shared premise of these studies is that if a trait that differentiates European and sub-Saharan Africans has a genetic basis, it should show variation in the hybrid population that correlates with the degree of African ancestry. This is indeed the case for many morphological traits, such as skin color (Scarr et al., 1977). However, there is no significant correlation with the degree of African ancestry for any cognitive test result, either within the cultural environment of being “black” (Loehlin et al., 1973; Scarr et al.,1977) or in the cultural environment of being “white” (Green, 1972). Hence, even though these populations differ in their average test scores, there is no evidence for any genetic differentiation among these populations at genetic loci that influence these IQ test scores.

So if such admixture patterns were to be found,, Templeton would have to agree this constitutes evidence for “genetic differentiation among these populations at genetic loci that influence these IQ test scores”.

Nisbett spends an entire appendix trying to argue for a 0% genetic contribution to US black-white gap. After ignoring most of the evidence on the topic (even in his included categories), he specifically advocates using admixture studies in his discussion (again my bolding):

Racial Ancestry and IQ
All of the research reported above is most consistent with the proposition that the genetic contribution to the black/white dif-ference is nil, but the evidence is not terribly probative one way or the other because it is indirect. The only direct evidence on the question of genetics concerns the racial ancestry of a given individual. The genes in the U.S. “black” population are about zo percent European (Parra et al., 1998; Parra, Kittles, and Shriver, 1004). Some blacks have completely African ancestry, many have at least some European ancestry, and some—about to percent—have mostly European ancestry. Does it make a difference how African versus European a black person is? A hereditarian model demands that blacks with more European genes have higher IQs. Herrnstein and Murray (1994) and Rushton and Jensen (2005), as it happens, scarcely deal with this direct evidence.

[discussion of various studies]

So what do we have in the way of studies that examine the effects of racial ancestry—by far the most direct way to assess the contribution of genes versus the environment to the black/white IQ gap? We have one flawed adoption study with results consistent with the hypothesis that the gap is substantially genetic in origin, and we have two less-flawed adoption studies, one of which indicates slightly superior African genes and one of which suggests no genetic difference. We have downs of studies looking at racial ancestry as indicated by skin color and “negroidness” of features that provide scant support for the genetic theory. In addition, three different studies of Europeanness of blood groups, using two different designs, indicate no support for the genetic theory. One study of illegitimate children in Germany demonstrates no superiority for children of white fathers as compared to children of black fathers. One study shows that exceptionally bright “black children have no more European ancestry than the best-available estimate for the population as a whole. And one study indicates that A is more advantageous for a mixed-race child to be raised by a family having a white mother than by a family having a black mother. All of these racial ancestry studies are subject to alternative interpretations Most of these alternatives boil down to the possibility that there was self-selection for IQ in black-white unions. If whites who mated with blacks had much lower IQs than whites in general, their European genes would convey little IQ advantage. Similarly, if blacks who mated with whites had much higher IQs than blacks in general, their African genes would not have been a drawback. Yet the extent to which white genes contributing to mixed-race unions would have to be inferior to white genes in general, or black genes would have to be superior to black genes in general, would have to be very extreme to result in no IQ difference at all between children of purely African heritage and those of partially European origin. Moreover, self-selection by IQ was probably not very great during the slave era, when most black-white unions probably took place. It is unlikely, for example, that the white males who mated with black females had on average a lower IQ than other white males. Indeed, if such unions mostly involved white male slave-owners and black female slaves, which seems likely to be the case (Parra et al., 1998), and if economic status was slightly positively related to IQ (as it is now), thew whites probably had IQs slightly above average. The black female partners were nor likely chosen on the bask of IQ, as opposed to comeliness. Similarly, it scarcely seems likely that either black or white soldiers in World War II were selecting their German mates on the basis of IQ. Several studies, moreover, are immune to the self-selection hypothesis. In particular, the study involving black and white children raised in an institutional setting, and the study involving black children adopted into either black or white middle-class homes, could not be explained by self-selection for IQ in mating. In short, though one would never know it by reading Herrnstein and Murray’s book (1994) or Rushton and Jensen’s article (zoos), the great mass of evidence on racial ancestry—the only direct evidence we have—points toward no contribution at all of genetics to the black/white gap.


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