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Genetics of race and intelligence
Main article:
Race and genetics
The decoding of the
human genome has enabled scientists to search for sections of the genome that may contribute to cognitive abilities.
Geneticist,
Alan R. Templeton argues that the question about the possible genetic effects on the test score gap is muddled by the general focus on "race" rather than on populations defined by gene frequency or by geographical proximity, and by the general insistence on phrasing the question in terms of heritability.
[111] Templeton points out that racial groups neither represent
sub-species nor distinct
evolutionary lineages, and that therefore there is no basis for making claims about the general intelligence of races.
[111] From this point of view the search for possible genetic influences on the black-white test score gap is a priori flawed, because there is no genetic material shared by all Africans or by all Europeans.
Mackintosh (2011) points out that by using genetic cluster analysis to correlate gene frequencies with continental populations it could possibly be the case that African populations had a higher frequency of certain genetic variants that contribute to an average lower intelligence. Such a hypothetical situation could hold without all Africans carrying the same genes or belonging to a single Evolutionary lineage. According to Mackintosh, a biological basis for the gap thus cannot be ruled out on a priori grounds.
Intelligence is a
polygenic trait. This means that intelligence is under the influence of several genes, possibly several thousand. The effect of most individual genetic variants on intelligence is thought to be very small, well below 1% of the variance in
g. Current studies using
quantitative trait loci have yielded little success in the search for genes influencing intelligence.
Robert Plomin is confident that QTLs responsible for the variation in IQ scores exist, but due to their small effect sizes, more powerful tools of analysis will be required to detect them.
[112] Others assert that no useful answers can be reasonably expected from such research before an understanding of the relation between DNA and human phenotypes emerges.
[93] Several candidate genes have been proposed to have a relationship with intelligence.
[113][114] However, a review of candidate genes for intelligence published in
Deary, Johnson & Houlihan (2009) failed to find evidence of an association between these genes and general intelligence, stating "there is still almost no replicated evidence concerning the individual genes, which have variants that contribute to intelligence differences".
[115]
A 2005 literature review article by Sternberg, Grigorenko and Kidd stated that no gene has been shown to be linked to intelligence, "so attempts to provide a compelling genetic link of race to intelligence are not feasible at this time".
[116] Hunt (2010), p. 447 and
Mackintosh (2011), p. 344 concurred, both scholars noting that while several environmental factors have been shown to influence the IQ gap, the evidence for a genetic influence has been circumstantial, and according to Mackintosh negligible. Mackintosh however suggests that it may never become possible to account satisfyingly for the relative contributions of genetic and environmental factors. The 2012 review by the
Nisbett et al. (2012) concluded that "Almost no genetic polymorphisms have been discovered that are consistently associated with variation in IQ in the normal range". Hunt and several other researchers however maintain that genetic causes cannot be ruled out and that new evidence may yet show a genetic contribution to the gap. Hunt concurs with Rushton and Jensen who considered the 100% environmental hypothesis to be impossible. Nonetheless, Nisbett and colleagues (2012) consider the entire IQ gap to be explained by the environmental factors that have thus far been demonstrated to influence it, and Mackintosh does not find this view to be unreasonable.
[24]