Yale Universitesinden Robert J. Sternberg, Elena L. Grigorenko ve Kenneth K. Kidd tarafından kaleme alınan bu yayında sıkça tartışılan zeka ve ırk kavramlarının bilimsel kökenlerine açıklık getirilmiştir. Yazarlar ırk’ın tamamen sosyal bir tanımlama olduğunu ve bilimsel /genetik karşılığının olmadığı sonucuna varmışlardır. Zeka’yı belirleyen bir DNA bağlantısının da henüz kurulmamış olduğu düşünülürse (ki muhtemelen bu özellik tek gene bağlı değil ancak multifaktöriyeldir) zeka ile ırk arasında bir bağlantı kurmanın bilimsel açıdan “feasible” olmayacağı sonucuna varılmıştır. İşte yazıda göze çarpan değerlendirmeler:

-To study the interrelationships among intelligence race and genetics, we need to know what intelligence is. We do not know. Hence, any conclusions about its relationships to other constructs will be, at best, tentative.

-…it is not clear that tests of IQ measure the same construct among all people…

-Modern Homo sapiens evolved in Africa about 200,000 years ago and then spread throughout the rest of the world and simultaneously diversified starting approximately 50,000 to 100,000 years ago. During that spreading out, modern humans supplanted now-archaic humanlike populations identifiable as having spread outside of Africa, such as Neanderthals.

-…those humans of whom we are descendants all lived in Africa (e.g., Tishkoff et al., 1996; Tishkoff & Kidd, 2004; Walter et al., 2000). They first appeared roughly 200,000 years ago.

-As people migrated, they adapted so as better to fit their environments. Much of that adaptation was cultural—different clothing, different foods, for example—but some of may be very different from those in the parent population. As the new population grows over a few generations, magnitude of the sampling error per generation decreases, and the new population will continue to have very different frequencies from the parent population.

– gene flow or genetic exchange, by which interbreeding among certain groups of individuals potentially results in those populations becoming increasingly similar to each other. Two populations that start off quite different genetically, if they mate, can produce offspring that represent the genes present in both of the original populations.

-natural selection, by which organisms with gene patterns that are adaptive to a given environment become more prevalent over time. For example, organisms that can adapt to changing climatic patternsare at an advantage over those that adapt only with great difficulty.

-When multiple forms of a DNA sequence, either a coding sequence or a noncoding sequence, are present, the sequence is referred to as polymorphic and the forms as alleles at the polymorphism. Among populations of various kinds, allele frequency differences at polymorphisms are the rule because of the chance effects known as random genetic drift.

-In other words, as a result of both natural and social events, only some genotypes are transmitted through generations; the others are lost.

-Where does race fit into the genetic pattern? Actually, it fits nowhere. Race is a socially constructed concept, not a biological one. It derives from people’s desire to classify. People seem to be natural classifiers. Perhaps this tendency reflects, in part, what Gardner (1999a, 1999b) has referred to as “naturalistic intelligence.” Or perhaps it merely reflects a need to discern order in or even to impose it on nature. Any set of observations can be categorized in multiple ways. People impose categorization and classification schemes that make sense to them and that, in some cases, favor their particular goals.

-How mixtures are labeled is a function of social status. In the United States, Blacks generally have lower social status than Whites, so supposed admixtures of blood determine degrees of “blackness.” Possessing any blackness makes one Black to some degree. So one can be light. Black, medium-skinned, or dark Black, but one is still Black. Even if individuals of mixed parentage inherited none of the physical features of blackness, they would still be classified as Black, although they might pass for White (Fish, 2002). In areas where Blacks are of higher social status, degrees of whiteness may all be seen as departures from true blackness.

-For example, being born with two eyes is 100% under genetic control (except in the exceedingly rare case of severe dismorphologies, with which we do not deal here). Regardless of the environment into which a human being is born, he or she will have two eyes. But it is not meaningful to speak of the heritability of having two eyes, because there are no individual differences. Heritability is not 1; it is meaningless (because there is a zero in the denominator of the ratio) and cannot be sensibly calculated. Consider a second complementary example, occupational status. It is associated with a statistically significant heritability coefficient (Plomin, DeFries, & McClearn, 1990), but certainly it is not under direct genetic control. Clearly, there is no gene or set of genes for occupational status. How could it be heritable, then? Heredity can affect certain factors that in turn lead people to occupations of higher or lower status. Thus, if factors such as intelligence, personality, and interpersonal attractiveness are under some degree of genetic control, they may lead in turn to
differences in occupational status. The effects of genes are at best indirect (Block, 1995). Other attributes, such as divorce, may run in families (i.e., show familiality), but again they are not under direct genetic control; in fact, the reason for such familiality may be that these attributes are culturally “inherited.”

When one speaks of heritability, one needs to remember that genes always operate within environment contexts. All genetic effects occur within a reaction range such that, inevitably, environment will have differential effects on the same genetic structure. The reaction range is the range of phenotypes (observable effects of genes) that a given genotype (latent structure of genes) for any particular attribute can produce, given the interaction of environment with that genotype. For example, genotype sets a reaction range for the possible heights a person can attain, but childhood nutrition, diseases, and many other factors affect the adult height realized. Moreover, if different genotypes react differently to environmental variation, heritability will show differences depending on the mean and variance
in relevant environments (Lewontin, 1974). Thus, the statistic is not a fixed value. There are no pure genetic effects on behavior, as would be shown dramatically if a child were raised in a small closet with no stimulation. Genes express themselves through covariation and interaction with the environment, as discussed further later.

New evidence indicating that many uninterrupted or rarely interrupted chunks of DNA (referred as haplotypes) appear to be common across different populations socially classified as belonging to different races (Wilson et al., 2001). To appreciate the significance of this finding, consider the example of population variability in mutations in the phenylalanine hydroxylase (PAH) gene, the gene whose disrupted protein results in the manifestation of PKU (as described earlier). It has been established that multiple mutations in this gene result in PKU. The mutations differ in terms of their specific location within the gene, and the frequencies of individual mutations vary across populations. However, each of these mutations appears to arise on one of a limited number of haplotypes and continues to be associated with that haplotype. Most common haplotypes are seen in all populations, and the greatest number of haplotypes are seen in African populations (J. R. Kidd et al., 2000).

Race is a social construction, not a biological construct, and studies currently indicating alleged genetic bases of racial differences in intelligence fail to make their point even for these social-defined groups. In general, we need to be careful, in psychological research, to distinguish our folk conceptions of constructs from the constructs themselves.

genedispersalmap

Advertisements