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Nature and nurture
The cause(s) of group average IQ test score differences are not known but several hypotheses have been proposed. Many scholars have offered descriptions of the variety of hypotheses that have been proposed. These descriptions usually distinguish between those hypotheses which invoke a contribution of genetic factors and those which solely invoke environmental (i.e., non-genetic) factors. Some descriptions of the positions are themselves controversial. In a review published in 2007, psychologists Earl Hunt and Jerry Carlson listed four positions.[1] The first position, attributed to Jensen and Rushton, is that group differences in IQ reflect differences in intelligence that are "due in substantial part to genetically determined differences in brain structure and/or function"[2] The second position, attributed to Ogbu and Sowell, is that the differences in intelligence test scores are due to social factors. The third view, attributed to Sternberg and colleagues, is that the use of IQ scores to argue for differences in intelligence is an inappropriate use of tests in different groups. The fourth position, attributed to Fish and others, is that there is no such thing as race: "a term motivated by social concerns and not a scientific concept".[3]
Prominent proponents of the genetic hypothesis are predominately psychologists or experts in intelligence testing, such as Charles Spearman, Hans Eysenck, Arthur Jensen, J. Philippe Rushton, Linda Gottfredson, Richard Herrnstein, and David C. Rowe. A few prominent proponents have come from other fields, such as political scientist Charles Murray, evolutionary psychologist Satoshi Kanazawa, and philosopher Neven Sesardic. Not all proponents share the same views, nor have all of their views been constant throughout their careers. The genetic hypothesis has also received support from scholars whose primary research is in other fields, such as William Shockley and James D. Watson. Prominent critics come from many backgrounds, and hold a diversity of views. They include psychologists and experts in intelligence testing such as James Flynn (prior to his work on IQ Flynn was a political scientist), Ulric Neisser, Robert Sternberg, and a number of less well known psychometricians who criticize the underlying IQ test data. Other critics include economists Thomas Sowell and Roland G. Fryer Jr, anthropologists Ashley Montagu and Leonard Lieberman, sociologists John Ogbu and Claude S. Fischer, paleontologist Stephen J. Gould, and biologists Richard C. Lewontin and Jared Diamond.
The genetic hypothesis has been characterized as inherently racist. For example, Turkheimer argues that proponents of a genetic contribution, whom he calls "racialists", deserve "vigorous disapprobation" and do not deserve "respect".[4] Turkheimer writes that he disagrees with his colleagues, such as Flynn and Ceci, who "agree that the question is a legitimate matter for scientific inquiry, to be settled by cool-headed evaluation of the empirical evidence".[5] Flynn does not accept Turkherimer's arguments.[6] Rushton and Jensen argue that the policy issues should be regarded as separate from the academic inquiry.[7]
What is heritability?
The consensus among intelligence researchers is that IQ differences between individuals of the same racial-ethnic group reflect real, functionally and socially significant, and substantially heritable differences in intelligence.[8][9][10][11][12][13][14][15][16]
The nature versus nurture debates concern the relative importance of an individual's inherited qualities ("nature") versus personal experiences and cultural environment ("nurture") in determining or causing individual differences in physical and behavioral traits. Often the heritability of a trait is used to estimate the relative contribution of genetic and environmental factors to the variation seen in a phenotype within a population, sometimes this is presented as a percentage and sometimes as a simple proportion. For example, a heritability of 50% (or 0.5) means that half of the variation between individuals in a group is associated with genetic differences between individuals and half of the variation is associated with non-genetic differences. Heritability cannot distinguish genetic and environmental contributions to the phenotype of a particular person, and it does not measure the direct effect of any genetic or environmental factor on a trait or phenotype. Instead, it captures the effects of the genetic and environmental factors that actually vary within a population.[17] Thus, a heritability of 100% does not mean that environmental factors are irrelevant for development, and a heritability of 0% does not mean that genetic factors are irrelevant for development. Reported heritability of IQ from twin studies and other study designs consistently fall in the range of 50% to 80%, with the estimated heritability in young (preschool) children in the lower range and adults in the higher range.
Critics have also questioned the interpretation of heritability. Lewontin suggests that some genotypes are more influenced by environments than others, leading to the possibility that populations that have similar genetic variance in the same environment can have different heritabilities because of their different genotypes.[18] David Layzer (1974) contends that the development of a trait can be influenced by genetic differences qualitatively and that heritability estimates cannot measure such qualitative differences, as such it is possible that even with a heritability of close to 100% it is possibly for phenotypic variance to be due largely to environment.[19]
Genetic hypothesis
The primary theoretical argument in favor of a genetic contribution to the Black-White (B-W) difference in average IQ was made popular by Arthur Jensen and later by James Flynn and others. The argument states that the constraints on an environmental explanation for group differences are so strong as to make it unlikely that the B-W gap is due entirely to environemental factors and thus that genetic factors must also be involved. According to Jensen[21] and Flynn[22], the very high within-group heritability of IQ (within both white and black populations) presents a problem for environmental explanations of group differences in IQ. They consider two general classes of environmental factors: common environmental factors and X-factors. Common environmental factors vary within and between populations. X-factors vary between populations, but do not vary substantially within populations. They first consider common environmental factors. To account for a 1 SD B-W IQ gap only in terms of common environmental factors would require very large environmental differences. For example, if the within-group heritability of IQ is 80%, then a B-W IQ difference of 2.24 SD in common environmental factors is required. For a heritability of 40%, a difference of 1.29 SD is required.
| Within-Group Heritability | 0% | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
| Required environmental difference | 1 | 1.05 | 1.12 | 1.19 | 1.29 | 1.41 | 1.58 | 1.82 | 2.24 | 3.16 | impossible |
Jensen and Flynn agree that it is an empirical question whether common environmental factors that influence IQ differ between whites and blacks to such an extent, and both agree that most commonly suggested environmental factors do not. Jensen believes that empirical evidence supports the view that the B-W IQ gap is caused by both common environmental factors and genetic factors. Flynn disagrees and believes that empirical evidence supports the view that the B-W IQ gap is caused by yet unrecognized environmental factors. Socioeconomic factors are commonly cited as plausible common environmental factors to explain the B-W IQ gap. Several lines of evidence are used to argue against this. First, IQ differences are larger between racial groups than between income groups.[24] Second, the children of whites from the lowest income bracket have higher IQs than the children of blacks from the highest income bracket.[25][26] Third, ethnicity is the single most powerful demographic predictor of academic achievement.[27] Finally, transracial adoption studies control for all aspects of the home environment that differ between blacks and whites (such as parenting, income, nutrition, neighborhood).
| Source | % of Variance | Average IQ Difference |
|---|---|---|
| Between races (within social classes) | 14 | 12 |
| Between social classes (within races) | 8 | 6 |
| Interaction of race and social class | 8 | |
| Between families (within race and social class) | 26 | 9 |
| Within families (siblings) | 39 | 11 |
| Measurement error | 5 | 4 |
| Total | 100 | 17 |
The alternative to common environmental factors is to hypothesize that X-factors account for the B-W IQ gap. A frequently-cited example from Lewontin describes the effect of a hypothetical X-factor. Imagine that the height of "ordinary genetically varied corn" is 100% heritable when grown in a uniform environment. Further imagine that two populations of corn are grown: one in a normal nutrient environment and the other in a deficient nutrient environment. Consequently, the average height of the corn grown in the deficient nutrient environment is less than the average height of the corn grown in the normal environment. In such a scenario, the within-group heritability of height is 100% in both populations, but the substantial difference between group are due entirely to environmental factors. The quality of the nutrient is an "X-factor" in the language of Jensen and Flynn. With respect to the B-W IQ gap, Jensen suggests that effects associated with racism (both overt and institutionalized racism) might be X-factors. Flynn believes that attributing the B-W gap to the effects of racism is incorrect, because the most plausible ways in which discrimination could affect IQ are themselves common environmental factors. These may include psychological effects such as stereotype threat; biological effects such as poor nutrition, health care and living close to toxic environments; and educational effects such as a lack of good schools. Instead, Flynn and his colleague William Dickens have developed more complicated models to explain the black-white gap in terms of environmental factors. One initial motivation of the Dickens-Flynn theory was Flynn's observation that IQ test scores have been rising over time in countries around the world – termed the Flynn effect. Flynn and others believe an explanation for the Flynn effect may elucidate the cause of the B-W gap. Jensen and others disagree. Structural equation models have been used to test for the existence of factor X effects (those which are affect individuals of a particular racial ethnic group uniquely) that are acting on IQ. These studies failed to find evidence for such effects.[29][30] Also, peers have little effect on developed intelligence.[31]
Rushton and Jensen describe 10 categories of research evidence from around the world to contrast "a hereditarian model" (50% genetic-50% cultural) and a culture-only model (0% genetic-100% cultural). In their article "Thirty Years of Research on Race Differences in Cognitive Ability", published in the APA journal Psychology, Public Policy and Law, Rushton and Jensen say that the totality of the evidence supports the a hereditarian model of Black-White group differences.[32][33] In addition to arguing that common envirnomental factors are not strong enough to account for the Black-White gap, and arguing that no X-factors have yet been identified, Rushton and Jensen argue that a number of lines of evidence are contradictory to many culture-only hypotheses. According to Rushton and Jensen, the Black-White differences in IQ are strongest among the children of parents with the highest socioeconomic levels, and thus who have the greatest social advantages. Jensen argues that this pattern extends to siblings, such that for example, when black and white sibling pairs are found in which one member of each pair had an IQ of 120, the other sibling had and IQ of 113 for whites whereas the other sibling has an IQ of 99 for blacks. According to Jensen this is an anomaly for the culture-only theory but is explained by genetic theory through regression to the mean.[34] [35][36] Rushton and Jensen argue that culture-only explanations such as stereotype threat, caste-like minorities and race stigma do not explain the low IQ of black Africans south of the Sahara or in the Caribbean, where blacks are in the majority. They also argue that the high IQs of East Asians and Jews discredit claims that discrimination alone explains the lower IQs of Black. They also cite the example of the Inuit, who live above the Arctic Circle and have higher average IQs than do either American or Jamaican Blacks even though the socioeconomic conditions of the Inuit are extremely poor, provide another counter example.[37][38][39] They also argue that several properties of Black-White IQ differences are most consistent with a genetic cause, such as the finding that the size of the Black-White IQ gap on individual tests is largest on tests with the least ostensive cultural content, which are also the tests that are the strongest measures of the g factor and the tests with the highest measured within-group heritability. Rushton and Jensen conclude that "Mean Black-White-East Asian differences [are] not fully explained by any model [examined], not by a 100% culture-only model, by a 100% genetic-only model, or, with precision, even by the 50% genetic-50% environmental model." They suggest that the hereditarian model of "50% genes-50% environment may need to be revised, perhaps to 80% genes-20% environment" and that the culture-only model fails as stated, but hypothesizing a "large Gene X Environment interaction might make it viable."[40][41]
| Race | Location | N. Samples | N. Countries | IQ | Range |
|---|---|---|---|---|---|
| Bushmen | S. W. Africa | 3 | 1 | 54 | 48–62 |
| Aborigines | Australia | 17 | 1 | 62 | 53–74 |
| Aborigines | New Guinea | 5 | 1 | 63 | 50–60 |
| Sub-Saharan Africans | Africa | 57 | 17 | 67 | 59–89 |
| Sub-Saharan Africans | Caribbean | 14 | 6 | 71 | 60–80 |
| Sub-Saharan Africans | United States | 29 | 1 | 85 | 77–93 |
| Sub-Saharan Africans | Netherlands | 7 | 1 | 85 | 83–88 |
| Sub-Saharan Africans | Britain | 18 | 1 | 86 | 73–94 |
| S. Asians & N. Africans | South Asia | 37 | 17 | 84 | 77–96 |
| S. Asians & N. Africans | Britain | 16 | 1 | 92 | 83–96 |
| S. Asians & N. Africans | Europe | 18 | 3 | 85 | 75–94 |
| S. Asians & N. Africans | Africa | 6 | 2 | 86 | 77–91 |
| S. Asians & N. Africans | Fiji, etc. | 3 | 3 | 85 | 82–89 |
| Pacific Islanders | Pacific Islands | 14 | 9 | 85 | 80–89 |
| Pacific Islanders | New Zealand | 12 | 1 | 90 | 81–96 |
| Southeast Asians | South E. Asia | 11 | 6 | 87 | 85–93 |
| Southeast Asians | United States | 7 | 3 | 93 | 87–96 |
| Native Americans | North America | 19 | 2 | 86 | 69–94 |
| Native Americans | Latin America | 10 | 5 | 86 | 79–92 |
| Arctic Peoples | North America | 15 | 2 | 91 | 78–96 |
| Europeans | Europe | 71 | 25 | 99 | 87–105 |
| Europeans | Outside Europe | 23 | 12 | 99 | 93–103 |
| East Asians | East Asia | 60 | 7 | 105 | 100–120 |
| East Asians | United States | 26 | 1 | 101 | 96–109 |
| East Asians | Elsewhere | 9 | 5 | 102 | 95–110 |
Other evidence, such as the Minnesota Transracial Adoption Study, certain racial admixture studies, behavior genetic modeling of group differences, "life-history" traits, and evolutionary explanations have also been proposed to indicate a genetic contribution to the IQ gaps and explain how these arose.[42]. The Minnesota Transracial Adoption Study examined adoptees with 2 black, 1 black and 1 white, or 2 white biological parents adopted into upper-middle class white families in the northern U.S. state of Minnesota. By age 17, the average IQ difference between the adopted children with 2 black biological parents and the adopted children with 2 white biological parents was 18 points[43], which was about the size of the mean black-white IQ gap in the US population.[44] Rushton and Jensen argue this is not surprising because home environment has no lasting effect on developed IQ in large-scale studies performed on whites.[45] The Minnesota Transracial Adoption Study was intended to test of genetic effects. In this study, the difference in average IQ between adoptees with 2 black biological parents and those with 1 black biological parent was 10 points, although they share the same social identity in U.S. culture.[46] Moreover, a dozen of the mixed-race adoptees were raised under the mistaken impression that they had 2 black parents, but the average IQ of this subgroup did not differ from the other mixed race children.[47] A criticism of this view is that race was confounded with age of adoption.[48] Others argue that it is unlikely that preadoption experiences account for the observed differences.[49][50][51] In meta-analyses, neither age of adoption nor abusive or neglectful environments affect the developed IQs of adoptees.[52]
| Children's background | Age 7 IQ | Age 17 IQ |
|---|---|---|
| Adopted, with two White biological parents | 111.5 | 101.5 |
| Adopted, with one White and one Black biological parent | 105.4 | 93.2 |
| Adopted, with two Black biological parents | 91.4 | 83.7 |
Rushton is noted for the theory presented in his book Race, Evolution and Behavior, which argues that racial differences (Asian-White-Black) on a variety of traits, including intelligence and brain size, have a common evolutionary history related to the effect of local environmental differences on natural selection. Jensen argues that the genetic component of within-group difference in IQ is likely associated with a large number of genes. Because allelic variants of many genes occur in different frequencies in different populations, he argues, some of the genes associated with IQ are likely to occur at different frequencies in different populations. For this reason alone, according to Jensen, we can know that even if genetic factors do not contribute to the average differences between populations, different populations are unlikely to be genetically uniform with respect to genes related to IQ. Jensen further argues that it is not implausible that the genetic differences between groups add up to cause some of the observed differences in average IQ between groups.
According to Nathan Brody, chronometric measures of intelligence provide the best available evidence for a genetic contribution to the Black-White differences in intelligence. These are simple tasks that anyone can perform in seconds. Individual differences in performance on these tasks are correlated with IQ, and this correlation is mediated by genetic effects.[54][55] Black-White differences are found in chronometric measures of simple and choice reaction times.[56] According to Jensen, this implies that the poorer performance of black research subjects on these tasks are caused by genetic differences that affect intelligence.[57][58]
Environmental impacts
Critics of the significantly genetic view, such as Robert Sternberg, argue that these studies are either flawed and thus inconclusive, or else that they support a primarily environment (<20% genetic) hypothesis.[59] For example, Dolan and Hamaker (2001) argue that the statistical methods linking the Black-White gap to g are insufficient. They reanalyzed the data from several earlier studies and concluded that Spearman's hypothesis is not an "empirically established fact" (i.e., that Black-White IQ differences may be due to differences in common factors other than g) due to insufficient power in the data to choose between alternative models. "This leaves the validity of Spearman's hypothesis, considered a central justification for the genetic explanation, an unresolved question." However, they did confirm that the Black-White IQ gap is not due to measurement artifacts, and is instead due to some measured factor that varies both within and between groups.
Nisbett (2005) argues that many studies find results that do not support the genetic hypothesis. They include studies on IQ and skin color that reported that the average correlation between skin color and IQ is .1 (the average correlation between IQ and judged “Negroidness” of features is even lower); IQ and self-reported European ancestry; IQ and blood groups showing degree of European Ancestry; IQ among children in post WWII Germany born to black and white American soldiers; and IQ among mixed-race children born to either a black or a white mother. He argues that these are direct tests of the genetic hypothesis and of more value than indirect variables, such as skull size and reaction time. He argues that "There is not a shred of evidence in this literature, which draws on studies having a total of five very different designs, that the gap has a genetic basis." He argues further that many intervention and adoption studies also find results that do not support the genetic hypothesis. He also argues "that the Black-White IQ gap has lessened considerably in recent decades."[60] Hunt and Carlson[61] argue that Nisbett's interpretations are far too strong in light of problems with these studies that have been recognized for decades.[62] Gottfredson writes that the studies Nisbett cites "actually lack the ability to rule out any hypothesis at all, genetic or not"[63].
Dickens (2005) states that "Although the direct evidence on the role of environment is not definitive, it mostly suggests that genetic differences are not necessary to explain racial differences. Advocates of the hereditarian position have therefore turned to indirect evidence...The indirect evidence on the role of genes in explaining the black-white gap does not tell us how much of the gap genes explain and may be of no value at all in deciding whether genes do play a role. Because the direct evidence on ancestry, adoption, and cross-fostering is most consistent with little or no role for genes, it is unlikely that the black-white gap has a large genetic component."[64]
Fryer and Levitt (2006), with data from "the first large, nationally representative sample" of its kind, report finding only a very small racial difference when measuring mental function for children aged eight to twelve months, and that even these differences disappear when including a "limited set of controls". "On tests of intelligence, Blacks systematically score worse than Whites, whereas Asians frequently outperform Whites. Some have argued that genetic differences across races account for the gap. Using a newly available nationally representative data set that includes a test of mental function for children aged eight to twelve months, we find only minor racial differences in test outcomes (0.06 standard deviation units in the raw data) between Blacks and Whites that disappear with the inclusion of a limited set of controls. The only statistically significant racial difference is that Asian children score slightly worse than those of other races." They argue that their report poses "a substantial challenge to the simplest, most direct, and most often articulated genetic stories regarding racial differences in mental function." They conclude that "to the extent that there are any genetically-driven racial differences in intelligence, these gaps must either emerge after the age of one, or operate along dimensions not captured by this early test of mental cognition."[65]
The Flynn effect
The secular, international increase in test scores, commonly called the Flynn effect, is seen by Flynn and others as reason to expect the eventual convergence of average black and white IQ scores. Flynn argues that the average IQ scores in several countries have increased about 3 points per decade during the 20th century, which he and others attribute predominantly to environmental causes.[66] This means, given the same test, the mean performance of Black Americans today could be higher than the mean for White Americans in 1920, though the gains causing this appear to have occurred predominantly in the lower half of the IQ distribution.[67] If an unknown environmental factor can cause changes in IQ over time, they argue, then contemporary differences between groups could also be due to an unknown environmental factor. An added complication to this hypothesis is the question of whether the secular IQ gains can be predominantly a real change in cognitive ability. Flynn's face-value answer to this question is "No",[68] and some other researchers have found reason to concur. In terms of the mixture of ability factors that IQ tests were designed to measure, such as g and verbal and mathematical ability, changes in IQ scores over time are different than either within-group individual differences and between group differences.[69][70] For example, there has been little increase over time in performance on either the forward digit-span or reverse digit-span subtests, and tests of school achievement have been less affected than tests of abstract reasoning.[71] Other recent studies have found that g has improved substantially.[72][73] Cranial vault size has increased and the shape changed during the last 150 years in the US; these changes must occur by early childhood because of the early development of the vault.[74]
Health
Numerous explanations beside genetics have been proposed to account for the IQ gaps in the U.S. [75] High rates of low birth-weight babies, lower rates of breastfeeding, and exposure to toxins are some factors. The Flynn effect is often cited as evidence that average IQ scores have changed greatly and rapidly, for reasons poorly understood, thus the IQ gap between races could change in the future or is changing, especially if the Flynn effect started earlier for Whites.
High levels of lead at an early age may affect intelligence; studies indicate that black and hispanic children have measurably higher levels than white children. A 10 µg/dL increase in blood lead at 24 months of age is associated with a 5.8-point lower IQ later in life. [76] In 1976 77.8% of all children had at least this much lead in their blood.[77]
Exposure to lead is frequently attributed to housing conditions including lead based paint, which is no longer used but has accumulated in older buildings; people of lower economic means are more frequently exposed to lead from housing.[78]
Stereotype threat
Stereotype threat is the fear that one's behavior will confirm an existing stereotype of a group with which one identifies; this fear may in turn lead to an impairment of performance.[79] Testing situations that highlight the fact that intelligence is being measured tend to lower the scores of individuals from racial-ethnic groups that already score lower on average. Stereotype threat conditions cause larger than expected IQ differences among groups but do not fully explain the gaps found in non-threatening test conditions.
Quality of education
Some researchers have written that studies that find test performance gaps between races even after adjusting for education level, such as the analysis found in The Bell Curve, fail to adjust for the quality of education. Not all high school graduates or college graduates have received the same quality of education. A 2006 study reported that years of education is an inadequate measure of the educational experience among multicultural elders, and that adjusting for quality of education greatly reduced the overall effect of racial differences on the tests.[80] A 2004 study reported that quality of education and cultural experience influence how older African Americans approach neuropsychological tasks and concluded that adjustment for these variables may improve specificity of neuropsychological measures.[81] Yet another study reported that, although significant differences were observed between the ethnic groups when matched for years of education, equating for literacy level eliminated all performance differences between African Americans and European Americans on both cancellation tasks which assess visual scanning [82] (like reaction time tests, cancellation task tests are sometimes regarded as "culture free" tests of intelligence). Eric A. Hanushek and Steven G. Rivkin wrote in their 2006 book that unequal distributions of inexperienced teachers and of racial concentrations in schools can explain all of the increased achievement gap between grades 3 and 8.[83]
A 2004 study in South Africa found highly significant effects for both level and quality of education within the black African first language groups taking the Wechsler IQ tests. The scores of black African first language groups with advantaged education were comparable with the US standardization, whereas scores for black African first language participants with disadvantaged education were significantly lower than this. The study cautioned that faulty conclusions may be drawn about the effects of ethnicity and the potential for neuropsychological misdiagnosis.[84]
Racial discrimination in education
Roslyn Arlin Mickelson writes that racial discrimination in education arises from actions of institutions or individual state actors, their attitudes and ideologies, or processes that systematically treat students from different racial/ethnic groups disparately or inequitably.[85] Despite advancement in education reform efforts, to this day African American students continue to experience inequities within the educational system. Hala Elhoweris , Kagendo Mutua, Negmeldin Alsheikh and Pauline Holloway conducted a study of the effect of students' ethnicity on teachers' educational decision making. The results of this study indicated that the student's ethnicity did make a difference in the teachers' referral decisions for gifted and talented educational programs.[86]Recently, a number of scholars have examined the issue of disproportionate representation of minority students in special education programs [87][88]
Teachers' perceptions of a students cultural background may effect school achievement. African American students with African American cultural backgrounds, for example, have been found to benefit from culturally responsive teaching.[89] In a 2003 study researchers found that teachers perceived students with African American culture-related movement styles as lower in achievement, higher in aggression, and more likely to need special education services than students with standard movement styles irrespective of race or other academic indicators. [90]
Ellis Cose writes that low expectations may have a negative impact on the achievement of minorities. He writes that black people did not need to read The Bell Curve to be aware of the low expectations held for them by the majority culture. He recalls examples of low expectations from his teachers in school who regarded his use of AAVE as "laziness" and teachers who did not feel it was important to purchase new text books because they did not expect the students to be able to read anything complex. He contrasts these low expectations with the high expectations philosophy of Xavier University where, using the ideas Whimbey articulated in his book Intelligence can be Taught teachers created a program called SOAR. SOAR raised the performance of black students and lead Xavier to become the university that sends the greatest number of black students to medical school in the United States. The SOAR program produced gains equivalent to 120 points on an SAT test. Cose writes that "..we must treat people, whatever their color, as if they have unlimited intellectual capacity."[91]
Caste-like minorities
The book Inequality by Design: Cracking the Bell Curve Myth claims that it is not lower average intelligence that leads to the lower status of ethnic minorities, it is instead their lower status that leads to their lower average intelligence test scores. The following table from the same book compares social status or caste position to test scores and school success in nations around the world.[92]
| Status or Caste Position | Test Scores, School Success | |||
|---|---|---|---|---|
| Country | High | Low | High | Low |
| United States[93] | Whites | Blacks | Whites | Blacks |
| Asians | Latinos | Whites | Latinos | |
| Whites | American Indians[94] | Whites | American Indians | |
| Great Britain[95] | English | Irish, Scottish | English | Irish, Scottish |
| Northern Ireland[96] | Protestants | Catholics | Protestants | Catholics |
| Australia[97] | Whites | Aborigines | Whites | Aborigines |
| New Zealand[98] | Whites | Maoris | Whites | Maoris |
| South Africa[99] | English | Afrikaners | English | Afrikaners |
| Belgium[100] | Flemish | French | Flemish | French |
| Israel[101] | Jews | Arabs | Jews | Arabs |
| Western Jews | Eastern Jews | Western Jews | Eastern Jews | |
| India[102] | Nontribals | Tribal people | Nontribals | Tribal people |
| Brahmin | Dalit | Brahmin | Dalit | |
| High caste | Low caste | High caste | Low caste | |
| Czechoslovakia[103] | Slovaks | Gypsies | Slovaks | Gypsies |
|
Notes:
| ||||
Viewpoints of notable scientists and researchers
Researchers who believe that there is no significant genetic contribution to race differences in intelligence include Flynn (1980), Brody (1992), Neisser et al. (1996), Nisbett (1998), Mackintosh (1998), Jencks and Phillips (1998), and Fish (2002). Some scientists who emphasize cultural explanations do not necessarily exclude a small genetic influence. Reynolds (2000) suggests up to 20% genetic influence be included in the cultural explanation. Researchers who believe that there are significant genetic contributions to race differences in intelligence include McGurk (1953), Garrett (1961), Shuey (1966), Shockley (1968), Eysenck (1971), Baker (1974), Loehlin et al. (1975), Vernon (1979), Lynn (1991a), Waldman et al. (1994), Scarr (1995), Levin (1997), Jensen (1998b), Rushton (2000), and Gottfredson (2005b).[106] Coming advances in genetics and genomics are expected to soon provide the ability to test hypotheses about group differences more rigorously than has as yet been possible.[107]
Interpretations
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Given the observed differences in IQ scores between certain groups, a great deal of debate revolves around the significance of these observations. Various interpretations of test data lead to a multitude of conflicting conclusions as to which specific explanations the data support.
Jared Diamond's Guns, Germs and Steel instead argues that historical differences in economic and technological development for different geographic areas can be explained by differences in geography (which affects factors like population density and spread of new technology) and differences in available crops and domesticatable animals. Richard Nisbett argues in his 2004 The Geography of Thought that some of these regional differences shaped lasting cultural traits, such as the collectivism required by East Asian rice irrigation, compared with the individualism of ancient Greek herding, maritime mercantilism, and money crops wine and olive oil [108] However, it has been suggested that these environmental differences may operate in part by selecting for higher levels of IQ.[109]
J. Philippe Rushton, a professor of psychology at the University of Western Ontario and the current head of the Pioneer fund, has written a controversial book called Race, Evolution, and Behavior: A Life History Perspective. Rushton claims in the book that race is a valid biological concept and that racial differences frequently arrange in a continuum of Mongoloids (Orientals, East Asians) at one extreme, Negroids (blacks, Africans) at the opposite extreme, and Caucasoids (whites, Europeans) in the middle. It has been heavily criticized.[110]
Differing rates of economic growth have also been attributed to numerous factors other than racial IQ gaps such as local availability of resources, climate, and sociopolitical factors. See for example the Global Competitiveness Report, the Ease of Doing Business Index, and the Index of Economic Freedom or works by Kenneth Pomeranz[111], Eric Jones[112], Joel Mokyr[113], and Douglass C. North[114].