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The cluster structure of the genetic data is dependent on the initial hypotheses of the researcher and the populations sampled.
When one samples continental groups, the clusters become continental; if one had chosen other sampling patterns, the clusters would be different.
Kaplan 2011 therefore concludes that, while differences in particular allele frequencies can be used to identify populations that loosely correspond to the racial categories common in Western social discourse, the differences are of no more biological significance than the differences found between any human populations (e.g., the Spanish and Portuguese).
Earl B.
Hunt agrees that racial categories are defined by social conventions, though he points out that they also correlate with clusters of both genetic traits and cultural traits.
Hunt explains that, due to this, racial IQ differences are caused by these variables that correlate with race, and race itself is rarely a causal variable.
Researchers who study racial disparities in test scores are studying the relationship between the scores and the many race-related factors which could potentially affect performance.
These factors include health, wealth, biological differences, and education.
[44] Group differences The study of human intelligence is one of the most controversial topics in psychology.
It remains unclear whether group differences in intelligence test scores are caused by heritable factors or by "other correlated demographic variables such as socioeconomic status, education level, and motivation.
"[45] Hunt and Carlson outlined four contemporary positions on differences in IQ based on race or ethnicity.
The first is that these reflect real differences in average group intelligence, which is caused by a combination of environmental factors and heritable differences in brain function.
A second position is that differences in average cognitive ability between races are caused entirely by social and/or environmental factors.
A third position holds that differences in average cognitive ability between races do not exist, and that the differences in average test scores are the result of inappropriate use of the tests themselves.
Finally, a fourth position is that either or both of the concepts of race and general intelligence are poorly constructed and therefore any comparisons between races are meaningless.
[40] United States test scores In the United States, individuals identifying themselves as East Asian tend to have higher average IQ scores than do Caucasians, who, in turn, have higher average IQs than African Americans.
Nevertheless, greater variation in IQ scores exists within each ethnic group than between them.
[46] Rushton & Jensen (2005) wrote that, in the United States, self-identified blacks and whites have been the subjects of the greatest number of studies.
They stated that the black-white IQ difference is about 15 to 18 points or 1 to 1.1 standard deviations (SDs), which implies that between 11 and 16 percent of the black population have an IQ above 100 (the general population median).
According to Arthur Jensen and J. Philippe Rushton the black-white IQ difference is largest on those components of IQ tests that are claimed best to represent the general intelligence factor g.[47] The 1996 APA report "Intelligence: Knowns and Unknowns" and the 1994 editorial statement "Mainstream Science on Intelligence" gave more or less similar estimates.
[48][49] Roth et al.
(2001), in a review of the results of a total of 6,246,729 participants on other tests of cognitive ability or aptitude, found a difference in mean IQ scores between blacks and whites of 1.1 SD.
Consistent results were found for college and university application tests such as the Scholastic Aptitude Test (N = 2.4 million) and Graduate Record Examination (N = 2.3 million), as well as for tests of job applicants in corporate sections (N = 0.5 million) and in the military (N = 0.4 million).
[50] East Asians have tended to score relatively higher on visuospatial subtests with lower scores in verbal subtests while Ashkenazi Jews score higher in verbal subtests with lower scores in visuospatial subtests.
The few Amerindian populations who have been systematically tested, including Arctic Natives, tend to score worse on average than white populations but better on average than black populations.
[50] The racial groups studied in the United States and Europe are not necessarily representative samples for populations in other parts of the world.
Cultural differences may also factor in IQ test performance and outcomes.
Therefore, results in the United States and Europe do not necessarily correlate to results in other populations.
[51] Global variation of IQ scores A number of studies have compared average IQ scores between the world's nations, finding patterns of difference between continental populations similar to those associated with race.
Richard Lynn and Tatu Vanhanen have argued that populations in the third world, particularly populations in Africa, tend to have limited intelligence because of their genetic composition and that, consequently, education cannot be effective in creating social and economic development in third world countries.
Lynn and Vanhanen's studies have been severely criticized for relying on low quality data and for choosing sources in ways that seem to be biased severely towards underestimating the average IQ potential of developing nations, particularly in Africa.
[55] Nonetheless there is a general consensus that the average IQ in developing countries is lower than in developed countries, but subsequent research has favored environmental explanations for this fact, such as lack of basic infrastructure related to health and education.
In the 2002 book IQ and the Wealth of Nations, and IQ and Global Inequality in 2006, Richard Lynn and Tatu Vanhanen created estimates of average IQs for 113 nations.
They estimated IQs of 79 other nations based on neighboring nations or via other means.
They saw a consistent correlation between national development and national IQ averages.
They found the highest national IQs among Western and East Asian developed nations and the lowest national IQs in the world's least developed nations among the indigenous peoples in the regions of Sub-Saharan Africa, Southeast Asia and Latin America.
[56] In a meta-analysis of studies of IQ estimates in Sub-Saharan Africa, Wicherts, Dolan & van der Maas (2009, p. 10) concluded that Lynn and Vanhanen had relied on unsystematic methodology by failing to publish their criteria for including or excluding studies.
They found that Lynn and Vanhanen's exclusion of studies had depressed their IQ estimate for sub-Saharan Africa, and that including studies excluded in "IQ and Global Inequality" resulted in average IQ of 82 for sub-Saharan Africa, lower than the average in Western countries, but higher than Lynn and Vanhanen's estimate of 67.
Wicherts at al.
conclude that this difference is likely due to sub-Saharan Africa having limited access to modern advances in education, nutrition and health care.
A 2010 systematic review by the same research team, along with Jerry S. Carlson, found that compared to American norms, the average IQ of sub-Saharan Africans was about 80.
The same review concluded that the Flynn effect had not yet taken hold in sub-Saharan Africa.
A 2007 meta-analysis by Rindermann found many of the same groupings and correlations found by Lynn and Vanhanen, with the lowest scores in sub-Saharan Africa, and a correlation of .60 between cognitive skill and GDP per capita.
Hunt (2010, pp.
437–439) considers Rindermann's analysis to be much more reliable than Lynn and Vanhanen's.
By measuring the relationship between educational data and social wellbeing over time, this study also performed a causal analysis, finding that when nations invest in education this leads to increased well-being later on.
Kamin (2006) has also criticized Lynn and Vanhanen's work on the IQs of sub-Saharan Africans.
Wicherts, Borsboom & Dolan (2010) argue that studies reporting support for evolutionary theories of intelligence based on national IQ data suffer from multiple fatal methodological flaws.
For example, they state that such studies "...assume that the Flynn Effect is either nonexistent or invariant with respect to different regions of the world, that there have been no migrations and climatic changes over the course of evolution, and that there have been no trends over the last century in indicators of reproductive strategies (e.g., declines in fertility and infant mortality)."
They also showed that a strong degree of confounding exists between national IQs and current national development status.
Similarly, Pesta & Poznanski (2014) showed that the average temperature of a given U.S. state is strongly associated with that state's average IQ and other well-being variables, despite the fact that evolution has not had enough time to operate on non-Native American residents of the United States.
They also noted that this association persisted even after controlling for race, and concluded that "Evolution is therefore not necessary for temperature and IQ/well-being to co-vary meaningfully across geographic space."
Flynn effect and the closing gap For the past century raw scores on IQ tests have been rising; this score increase is known as the "Flynn effect", named after James R. Flynn.
In the United States, the increase was continuous and approximately linear from the earliest years of testing to about 1998 when the gains stopped and some tests even showed decreasing test scores.
For example, in the United States the average scores of blacks on some IQ tests in 1995 were the same as the scores of whites in 1945.
[63] As one pair of academics phrased it, "the typical African American today probably has a slightly higher IQ than the grandparents of today's average white American.
"[64] Flynn has argued that given that these changes take place between one generation and the next it is highly unlikely that genetic factors could account for the increasing scores, which must then be caused by environmental factors.
The Flynn Effect has often been used as an argument that the racial gap in IQ test scores must be environmental too, but this is not generally agreed – others have asserted that the two may have entirely different causes.
A meta-analysis by Te Nijenhuis and van der Flier (2013) concluded that the Flynn effect and group differences in intelligence were likely to have different causes.
They stated that the Flynn effect is caused primarily by environmental factors and that it's unlikely these same environmental factors play an important role in explaining group differences in IQ.
[65] The importance of the Flynn effect in the debate over the causes for the IQ gap lies in demonstrating that environmental factors may cause changes in test scores on the scale of 1 SD.
This had previously been doubted.
A separate phenomenon from the Flynn effect has been the discovery that the IQ gap has been gradually closing over the last decades of the 20th century, as black test-takers increased their average scores relative to white test-takers.
For instance, Vincent reported in 1991 that the black-white IQ gap was decreasing among children, but that it was remaining constant among adults.
Similarly, a 2006 study by Dickens and Flynn estimated that the difference between mean scores of blacks and whites closed by about 5 or 6 IQ points between 1972 and 2002, a reduction of about one-third.
In the same period, the educational achievement disparity also diminished.
[68] However, this was challenged by Rushton & Jensen who claim the difference remains stable.
In a 2006 study, Murray agreed with Dickens and Flynn that there has been a narrowing of the difference; "Dickens' and Flynn's estimate of 3–6 IQ points from a base of about 16–18 points is a useful, though provisional, starting point".
But he argued that this has stalled and that there has been no further narrowing for people born after the late 1970s.
A subsequent study by Murray, based on the Woodcock–Johnson Tests of Cognitive Abilities, estimated that the black-white IQ difference decreased by about one-half of one standard deviation from those born in the 1920s to those born in the second half of the 1960s and early 1970s.
Recent reviews by Flynn and Dickens (2006), Mackintosh (2011), and Nisbett et al.
2012 accept the gradual closing of the gap as a fact.
In his review of the historical trends, Hunt (2010, p. 411) states: "There is some variety in the results, but not a great deal.
The African American means are about 1 standard deviation unit (15 points on the IQ scale) below the white means, and the Hispanic means fall in between."
Some studies reviewed by Hunt (2010, p. 418) found that rise in the average achievement of African Americans was caused by a reduction in the number of African American students in the lowest range of scores without a corresponding increase in the number of students in the highest ranges.
A 2012 review of the literature found that the IQ gap had diminished by 0.33 standard deviations since first reported.
A 2013 analysis of the National Assessment of Educational Progress found that from 1971 to 2008, the size of the black–white IQ gap in the United States decreased from 16.33 to 9.94 IQ points.
It has also concluded however that, while IQ means are continuing to rise in all ethnic groups, this growth is occurring more slowly among 17-year-old students than among younger students and the black-white IQ gap is no longer narrowing.
As of 2008, a study published in 2013 by Heiner Rindermann, Stefan Pinchelmann, and James Thompson have estimated the IQ means of 17-year-old black, white, and Hispanic students to range respectively from 90.45–94.15, 102.29–104.57 and 92.30–95.90 points.
They explain that the gap may persist due to the crack epidemic, the degradation of African-American family structure, the rise of fraud in the educational system (especially with respect to No Child Left Behind), the decrease in unskilled real wages and employment among African-Americans due to globalization and minimum wage increases, differences in parental practices (such as breastfeeding or reading to children), and "environmental conditions shaped by [African-Americans] themselves."
To resolve this, they ultimately recommend the reestablishment of "meritoric principles" and "blindly graded objective central exams," as opposed to "ethnically based policies," in education.
[74] Environmental influences on group differences in IQ The following environmental factors are some of those suggested as explaining a portion of the differences in average IQ between races.
These factors are not mutually exclusive with one another, and some may, in fact, contribute directly to others.
Furthermore, the relationship between genetics and environmental factors may be complicated.
For example, the differences in socioeconomic environment for a child may be due to differences in genetic IQ for the parents, and the differences in average brain size between races could be the result of nutritional factors.
All recent reviews agree that some environmental factors that are unequally distributed between racial groups have been shown to affect intelligence in ways that could contribute to the test score gap.
However, currently, the question is whether these factors can account for the entire gap between white and black test scores, or only part of it.
One group of scholars, including Richard E. Nisbett, James R. Flynn, Joshua Aronson, Diane Halpern, William Dickens, Eric Turkheimer (2012) have argued that the environmental factors so far demonstrated are sufficient to account for the entire gap.
Nicholas Mackintosh (2011) considers this a reasonable argument, but argues that probably it is impossible to ever know for sure; another group including Earl B.
Hunt (2010), Arthur Jensen, J. Philippe Rushton and Richard Lynn have argued that this is impossible.
Jensen and Rushton consider that it may account for as little as 20% of the gap.
Meanwhile, while Hunt considers this a vast overstatement, he nonetheless considers it likely that some portion of the gap will eventually be shown to be caused by genetic factors.
Health and nutrition at least 10 µg/dL.
Black and Hispanic children have much higher levels than white children.
A 10 µg/dL increase in blood lead at 24 months is associated with a 5.8-point decline in IQ.
[76] Although the Geometric Mean Blood Lead Levels (GM BLL) are declining, a CDC report (2002) states that: "However, the GM BLL for non-Hispanic black children remains higher than that for Mexican-American and non-Hispanic white children, indicating that differences in risk for exposure still persist.
"[77] Percentage of children aged 1–5 with blood lead levels10 µg/dL.
Black and Hispanic children have much higher levels than white children.
A 10 µg/dL increase in blood lead at 24 months is associated with a 5.8-point decline in IQ.Although the Geometric Mean Blood Lead Levels (GM BLL) are declining, a CDC report (2002) states that: "However, the GM BLL for non-Hispanic black children remains higher than that for Mexican-American and non-Hispanic white children, indicating that differences in risk for exposure still persist."
Environmental factors including childhood lead exposure,[76] low rates of breast feeding,[78] and poor nutrition[79][80] can significantly affect cognitive development and functioning.