Science Fair Project Encyclopedia
Race and intelligence
Many studies of cognitive ability have shown that some racial groups differ in measured intelligence. These results have sparked public debates concerning not only the reliability of the studies and the motives of their authors, but the validity and fairness of intelligence tests in general and extent to which measured intelligence is determined by biological and social factors.
Racial distinctions are most often based on skin color, facial features, ancestry, genetics, and national origin. Some scientists argue that common racial classifications are not meaningful, often on the basis of research claiming that more genetic variation exists within such races than between them. See the article Race for further discussion.
Intelligence is most commonly measured by IQ tests. In turn, IQ tests principally measure the psychometric variable g. Some question the validity of all IQ testing or claim that there are aspects of intelligence not reflected in IQ tests. See the articles Intelligence (trait), IQ, and g theory for further discussion.
The debates described in the following article assumes that IQ tests measure some interesting aspect of intelligence and that some interesting information may be gained by studying racial group differences. For a critique of these assumptions, please see the previously mentioned articles.
IQ gap among races
The modern controversy surrounding intelligence and race focuses on the results of IQ studies conducted during the second half of the 20th century mainly in the United States and some other industrialized nations. IQ studies outside these nations are few and small. It is uncertain what the average IQ or subgroup IQ tests scores would be with more complete studies in the developing world. IQ test scores in the developing world may be affected by factors less important in the developed world such as nutritional deficiencies. Most of the following article refers to studies attempting to explain race differences in IQ test scores in the US and do not refer to the world as a whole.
IQ gap in the US
In almost every testing situation where tests were administered and evaluated correctly, a difference of approximately one standard deviation was observed in the US between the mean IQ score of Blacks and Whites. In the United States, the mean IQ score among Blacks is approximately 85 and the mean IQ score among Whites is approximately 100; the mean IQ score of Hispanics is usually reported to be between the mean Black and White scores (Herrnstein and Murray report a mean "Latino" IQ of 89 in The Bell Curve). The mean score for people of East Asian and Jewish descent is usually higher than the mean score of Whites, but the extent of that difference is not precisely known (Herrnstein and Murray report mean IQ scores for East Asians and Jewish Americans of 106 and 113, respectively, in The Bell Curve). However, several studies place the median IQ of Ashkenazi Jews (who make up the overwhelming majority of American Jews) at approximately one standard deviation above the mean for other Whites, with the primary Jewish advantage in verbal reasoning and the East Asian advantage primarily in spatial reasoning. In a normal distribution, only about 16% of the population is at least one standard deviation above the mean. Thus, there is substantial overlap in the distribution of IQ scores among individuals of each race.
Similar gaps are seen in other tests of cognitive ability or aptitude, including university admission exams such as the SAT and GRE as well as employment tests for corporate settings and the military (Roth et al. 2001). Likewise, the gap is reflected by gaps in the academic, economic, and social factors correlated with IQ (Gordon 1997; Gottfredson 1997). The practical importance of intelligence makes the source and meaning of the IQ gap a pressing social concern.
Is the gap closing?
Richard Nisbett and others have argued that the Black-White gap on various ability tests has narrowed from the 1970s to the 1990s (Grissmer, 1994; Grissmer, Flanagan, & Williamson, 1998; Grissmer, Williamson, Kirby, & Berends, 1998; Hedges & Nowell, 1998; Nisbett, 1995, 1998, 2005). These tests include the Equality of Educational Opportunity (EEO) survey, the National Longitudinal Study, the High School and Beyond survey, the National Education Longitudinal Study, and the National Assessment of Educational Progress program (NAEP).
The Journal of Blacks in Higher Education found that although the Black-White gap on the SAT declined from 1976-1988, it has been increasing since 1988. On the other hand, some studies find that the gap has been decreasing for the most of the 20th century and that this continued during the nineties. .
Jensen (1998 pp. 375-376, 407-408, 494-495) has argued that the Black-White differences in g have not narrowed. In support of this claim, he presents evidence that gains in scholastic achievement do not equal gains in g. Jensen also argues that Black-White differences in g seen in measures of reaction time have not narrowed.
A meta-analysis by Roth et al. (2001) found a mean Black-White score difference of 1.1 standard deviations (6,246,729 samples; ranging from 0.38 to 1.46 depending on the g loading of the test). As to whether the IQ gap is narrowing, they speculated that any reduction was "either small, potentially a function of sampling error ... or nonexistent for highly g loaded" tests (Roth et al. 2001).
Gottfredson (2005) agreed that the Black-White gap observed in the National Assessment of Educational Progress test has narrowed from 1.07 to 0.89 standard deviations. However, she then argues that reduction stopped by the mid-1980s and is compatible with stable group differences in g.
A large (21,260 children) and probably the most recent (1998) study found that Black-White gap for young children in reading and math scores was much smaller than in earlier studies and that all of the remaining difference could be explained by a few environmental factors.  One possible explanation is that the Flynn effect started earlier for Whites but has now stopped while continuing for Blacks. Reading and math scores are correlated with, but not substitutable for, IQ, so these findings alone may not indicate convergence in the IQ gap. Still, the correlation of IQ with grades is highest in elementary school (0.6 to 0.7; Jensen 1998), so convergence in scores may, in fact, indicate that the IQ gap is narrowing.
IQ gaps in other nations
Attempted compilations of average IQ by race generally place East Asians at the top, followed by Whites, other Asians, Arabs and Blacks. See IQ and the Wealth of Nations for an attempted compilation of average IQ for different nations and a discussion of associated problems.
The IQ scores vary greatly between different nations for the same group. Blacks in Africa score much lower than Blacks in the US. The black-white gap is much smaller in the UK than in the US . . Another example is Jews who score much lower in developing nations and Koreans who score much lower in Japan.
There are many examples of IQ score differences between close neighbours in the same nation, for example between French vs. Flemish speakers in Belgium, Slovaks vs. Gypsies in Slovakia, Irish & Scottish vs. English in Great Britain, and white speakers of Afrikaans vs. white speakers of English in South Africa. The difference between the white neighbours Protestants and Catholics in Northern Ireland is a large as the differences between whites and blacks in the US .
In 1991, Richard Lynn tested 1,468 9-year old children consisting of Blacks from South Africa, East Asians from Hong Kong and Japan, and Whites from Britain and Ireland. The content of the tests involved flipping a switch after one or more lights came on. Lynn found that the decision times (the time taken to make a decision about what to do) had a low correlation with IQ data on Raven Progressive Matrices tests also administered during the same study, although movement times (the time taken to execute the decision) did not. He found that the Asians had the fastest decision times, followed by the Whites, and then by the Blacks. He also determined that the Black children had movement times that were substantially faster than those of Whites and Asians on certain tests.  
Group differences in average IQ tend to mirror group differences in brain size. Numerous historical and modern studies, using skull and head measurements, weighing of brains at autopsy, and more recently, magnetic resonance imaging report racial differences. These studies are usually accompanied by a great deal of controversy.
In his 1839 Crania Americana, anthropologist Samuel George Morton reported that the mean cranial capacity of the skulls of Whites was 87 in³, while that of Blacks was 78 in³. Based on the measurement of 144 skulls of Native Americans, he reported an a figure of 82in³.
In his controversial 1995 work Race, Evolution, and Behavior, J. Philippe Rushton reported an average endocranial volume of 1,415 cm³ for "Orientals [sic]", 1,362 for Whites, and 1,268 for Blacks. When adjusted for average body size, the differences become more pronounced.
Modern studies using MRI imaging have revealed similar results and have shown that brain size correlates with IQ by a factor of roughly .35 to .40. In 1991, Willerman et al. used data from 40 White American university students and reported a correlation coefficient of .35. Other studies done on samples of Caucasians show similar results, with Andreasen et al (1993) determining a correlation of .38, while Raz et al (1993) obtained a figure of .43 and Wickett et al (1994) obtained a figure of .40. The correlation between brain size and IQ seems to hold for comparisons betweeen and within families (Gignac et al. 2003; Jensen 1994; Jensen & Johnson 1994). However, one study found no within family correlation (Schoenemann et al. 2000). A study on twins (Thompson et al., 2001) showed that frontal gray matter volume was correlated with g and highly heritable. A related study has reported that the correlation between brain size (reported to have a heritability of 0.85) and g is 0.4, and that correlation is mediated entirely by genetic factors (Posthuma et al 2002).
Cranial vault size and shape have changed greatly during the last 150 years in the US. These changes must occur by early childhood because of the early development of the vault. The explanation for these changes may be related to the Flynn effect. 
It has been suggested that IQ tests may be biased against minorities, and that this accounts for part or all of the IQ gap. Some claim that there is no evidence for test bias. IQ tests are equally good predictors of IQ-related factors (such as school performance) for Blacks and Whites. The performance differences persist in tests and testing situations in which care has been taken to eliminate bias. It has also been suggested that IQ tests are formulated in such a way as to disadvantage minorities. Controlled studies have shown that test construction does not substantially contribute to the IQ gap.
The lack of test bias is widely accepted in the research community. From the American Psychological Association's summary of their 1996 task force report, "Intelligence: Knowns and Unknowns": "The differential between the mean intelligence test scores of Blacks and Whites does not result from any obvious biases in test construction and administration, nor does it simply reflect differences in socio-economic status." From The Wall Street Journal: Mainstream Science on Intelligence (PDF): "Intelligence tests are not culturally biased against American Blacks or other native-born, English-speaking people in the U.S. Rather, IQ scores predict equally accurately for all such Americans, regardless of race or social class."
Since the U.S. Supreme Court outlawed employee selection, including testing, which is "fair in form, but discriminatory in operation" (Griggs vs. Duke Power Co., 1971; see this page on disparate impact), American companies have had a strong incentive to construct valid tests which do not produce an IQ gap between ethnic groups, called "selection bias" in employment. Despite this incentive, these efforts have generally failed. For example, in one case regarding a police selection test in Nassau County, NY, a scandal ensued when tests which showed no "selection bias" (Black-White score gap) were found to have been denuded of their ability to measure intelligence (Gottfredson, 2003, pp. 24-26 PDF).
One environmental source of the IQ gap which has been suggested is poor motivation among low scorers. This hypothesis is seemingly discredited by findings promoted by the researcher Arthur Jensen (1998) using elementary cognitive tasks to measure intelligence. For example, one such test asks the subject to lift a finger from a depressed button to strike a light when it flashes. When more than one light is offered as a target the task involves a decision of which to hit (i.e. the one which is lit). These tests measure both reaction time (from when the bulb illuminates to when the subject lifts their finger) and movement time (from when the subject lifts their finger to when the subject reaches the bulb). While movement time measurements show no difference (or an advantage to Blacks), reaction time measurements negatively correlate with IQ scores and show the same performance gaps between those two races. Jensen argues that it is difficult to imagine that people could be motivated during one part of each segment of the test but not motivated during the other. The correlation between IQ and reaction time is low (from .20 to .40). A review by Deary (2000) that combined several studies with six measures produced a multiple correlation of reaction time to IQ of .67. This correlation is within the range of correlations between different kinds of IQ tests.
IQ is correlated with economic factors. Blacks and Hispanics suffer poorer economic conditions than Whites. It has been suggested that the effects of poverty are responsible for some or all of the IQ gap. However, some argue that economics cannot be the whole explanation. First, the gaps are slightly smaller but still persist for individuals from the same socioeconomic backgrounds. (For counter-argument see figure) Second, except for extreme environments, some argue that factors associated with poverty account for little of the variance in IQ scores. (Some studies claim to prove that the socioeconomic environment completely "overrides" the race factor, at least temporarily, in adopted Black children (e.g. Capron and Duyme, 1989). Third, some scientists believe that IQ determines income in developed nations, and not the other way around (Murray, 1998). (Even if true, this is not evidence that IQ is genetic since there are many other potential environmental factors beside income)
Other researchers have come across what they see as additional reasons for the IQ gap. The paper Poverty and Brain Development in Early Childhood holds that there is a large amount of neural damage in many American Black and Hispanic children due to inadequate nutrition, substance abuse of the children's parents, a high incidence of maternal depression, exposure to environmental toxins, psychological trauma, and the neural effects of physical abuse.
Researchers have found that many American Blacks and Hispanics are not given sufficient opportunity to learn language and thinking skills during the first three years of life, possibly due to economic status. The first three years are especially critical years for neural development of the brain, and previous studies have shown that when human children were deprived of most or all language skills at an early age, they never developed the ability to master language at a later age; if they only mastered a small amount of language and thinking skills at a young age, then they could only make small improvements in later years. A recent study has shown that many American Blacks and Hispanics are raised in homes where their parents speak relatively few sentences, and the sentences usually show only simple grammar. As a result, their children never hear millions of words during the time when their brains are developing linguistic skills. Without this linguistic input during their developing years, many are observed to quickly fall behind, and they can never catch up. Children in poorer welfare families, which includes a higher percentage of many minority populations, apparently hear up to 30 million fewer words by age three than children in higher income, usually White, families. (Source: The Early Catastrophe: The 30 Million Word Gap by Age 3)
The recent paper Socioeconomic status modifies heritability of IQ in young children finds that the role of the environment is more important in poorer families. "The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse."
Many anthropologists have argued that intelligence is a cultural category; some cultures emphasize speed and competition more than others, for example. During WWI African-Americans from the north tested higher than those from the south. This could be because African-Americans in the north had received more formal education (see Race: Science and Politics, written by Ruth Benedict in 1940). Thousands of ethnographic studies indicate that innate capacities for cultural evolution are equal among all human populations. The American Anthropological Association has endorsed a statement deriding all studies of race and intelligence .
It has been suggested that Black culture disfavors academic achievement and fosters an environment that is damaging to IQ (Boykin, 1994). Likewise, it is argued that a persistence of racism reinforces this negative effect. John Ogbu (1978, 1994) has developed a hypothesis that the condition of being a "caste-like minority" affects motivation and achievement, depressing IQ. Even proponents of the view that the IQ gap is caused partly by genetic differences, such as Arthur Jensen, recognize that non-genetic factors are likely involved. Indeed, one author has compiled a list of over one hundred possible causes of the Black-White IQ gap .
Cultural explanations for the IQ deficit among Blacks and Hispanics compared to Whites and Asian minorities are complemented – and sometimes challenged – by the observation that Asian minorities score well on IQ tests and on average enjoy greater economic success than other minorities. Along these lines, East Asians are sometimes referred to as "model minorities". Likewise, Jewish populations have suffered past discrimination and persecution, but do not exhibit an IQ deficit. However, Jews and East Asians are today less discriminated than Blacks.
Black mothers are known to breastfeed infants less and for a shorter time than White mothers. Studies have shown IQ gains lasting into adulthood with increased duration of breastfeeding.
Some argue that the higher IQ test scores in East Asian nations are in part attributed to some IQ tests' inherent bias towards testing spatial reasoning. They argue that logographic writing systems like those used by Chinese or Japanese develop spatial reasoning better than the alphabetic writing systems prevalent in Europe and America. The same reasoning has been used to explain why students from Asia-Pacific countries (eg Singapore, South Korea) tend to score better than average in tests of mathematics. Some argue that the East Asian advantage can also be explained by more rigorous education programs.
The Flynn effect
The Flynn effect is large documented worldwide increases in IQ scores for at least several decades. Attempted explanations have included improved nutrition, a trend towards smaller families, better education, greater environmental complexity, and heterosis.
Some think that there are no genetic differences and that the Flynn effect will eliminate differences in IQ test scores in the future. They argue that the Flynn effect started and will end sooner for the more affluent parts of society and that Blacks will or have started to close the gap.
However, comparing the Flynn effect (IQ differences within races over time) to contemporary IQ differences between races is contested; for example, one report concludes "that the nature of the Flynn effect is qualitatively different from the nature of B-W differences in the United States" (Wicherts et al., 2004). Others note that this is not mentioned in the abstract, refers to "measurement invariance", and is not a statement about the role of genetics in the B-W gap.
Part of the gap may well be genetic; there is no a priori reason to believe that every ethnic group or race has precisely the same distribution of genes that affect intelligence; a small amount of random variation early in human evolution may have later crystallized into differences seen today. Also there might have been smaller evolutionary pressure towards greater intelligence in some environments.
The genetic hypothesis is often ignored or disregarded in primary research on group differences. It has been well-studied by researchers doing meta-analyses that combine multiple sources of primary materials.
Jensen has concluded that the IQ gap is at least partly genetic. Jensen and colleagues reach this conclusion based on an evaluation of some of the following evidence. While accepting that some fraction of the supporting evidence may be false, they argue that the partly-genetic hypothesis is favored over the culture-only hypothesis by a preponderance of the evidence (Rushton and Jensen 2005; Gottfredson 2005). In this view, average intelligence differences among races are like average skin color differences: a product of different allelic frequencies within each population. Others are critical of Jensen's methods and evaluation (Sternberg 2005; Suzuki & Aronson 2005; Nisbett 2005).
The results of most (indirect) analyses used to test the genetic hypothesis do not logically contradict an environmental explanation of the lower IQ of Blacks. That is, a plausible (but some argue ad hoc) environmental explanation for the lower mean IQ in Blacks can be offered in most cases. However, many argue that the higher average IQ of East Asians than Whites is anomalous for an environment-only theory of IQ differences (Rushton & Jensen, 2005).
The heritability of intelligence within groups is high. It is widely recognized that within group heritability does not in itself indicate that between group differences are genetic in origin, although it is likely a necessary condition. Different kinds of evidence are needed to address the question of between group heritability. As Herrnstein and Murray explain in The Bell Curve:
- As we discussed in Chapter 4, scholars accept that IQ is substantially heritable, somewhere between 40 and 80 percent, meaning that much of the observed variation in IQ is genetic. And yet this tells us nothing for sure about the origin of the differences between races in measured intelligence. This point is so basic, and so commonly misunderstood, that it deserves emphasis: That a trait is genetically transmitted in individuals does not mean that group differences in that trait are also genetic in origin. Anyone who doubts this assertion may take two handfuls of genetically identical seed corn and plant one handful in Iowa, the other in the Mojave Desert, and let nature (i.e., the environment) take its course. The seeds will grow in Iowa, not in the Mojave, and the result will have nothing to do with genetic differences. (Herrnstein & Murray, The Bell Curve, 1994, pg. 298)
In most studies, measured heritabilities for intelligence are the same for Blacks as for Whites. A 1975 review by Loehlin et al. found some evidence suggesting lower heritability in Blacks than Whites (e.g., Scarr-Salapatek 1971), but a larger body of evidence suggested equal heritabilities for both races. An analysis of the Georgia Twin Study by Osborne (1980) found equal heritabilities for both Blacks and Whites.
Two kinds of environmental effects can be distinguished: shared and nonshared effects (see nature versus nurture). Twin and adoption studies, used to measure heritability, can also be used to quantify the two types of environmental effects (Plomin, DeFries, & Loehlin, 1977). Shared environmental effects are due to factors experienced in common by all children raised in the same family but that differ between families. Examples of shared environmental effects include socio-economic factors, family cultural practices, and parental influences on children. Nonshared effects are unique for each child, and thus differ within families. Examples include chance events such as accidents, illness, and childhood friends. Anything that happens to one sibling and not to the other contributes to nonshared effects. McGue et al. (1993) found that the nonshared environmental effects on IQ remain approximately constant throughout life, shared environmental effects remain approximately constant until 20-years of age but then drop to zero in adulthood, and genetic factors increase throughout development but especially after 20-years of age. Plomin et al. (2001) corroborates these results. Environmental factors usually proposed to explain the Black-White gap are shared effects (e.g. social class, religion, cultural practices, father absence, and parenting styles). Jensen (1997) argues that because these effects account for little variance within a race, they are unlikely to account for the differences between races in developed nations. Others studies do support that shared environmental factors in developed nations can affect IQ  .
Intelligence as measured by g, a general factor of cognitive ability, and its various biological correlates (e.g., the volume of gray matter in the frontal cortex) are claimed to be partly genetically determined (see g theory). g has the highest measured heritability of any cognitive ability factor. The degree to which Black and White cognitive test scores differ is linearly correlated with the test's degree of g-loading, with a correlation of 0.6 (Jensen, 1998); a phenomena called Spearman's hypothesis. This study combined scores on 149 psychometric tests obtained from 15 independent samples totaling 43,892 Blacks and 243,009 Whites (Jensen, 1998). Dolan and Hamaker (2001) have reanalyzed the data from several previous studies (Jensen & Reynolds, 1982; Naglieri & Jensen, 1987) that used the statistical method invented by Jensen (the method of correlated vectors) with a more recent and improved method (multigroup confirmatory factor analysis). Their results statistically were consistent with a "weak version" of Spearman's hypothesis: that Black-White group differences were predominantly on the g factor. However, their analysis of the data set failed to "establish Spearman's hypothesis as an empirically established fact". However, they also speculate that "it is possible that the analysis of all available data sets ... will demonstrate that a model incorporating the weak version of Spearman's hypothesis provides the best description of the data." . This leaves the validity of Spearman's hypothesis, considered a central justification for the genetic explanation, an unresolved question.
Minority-specific effects on intelligence arising from cultural background differences between the races would be expected to affect the correlations between the measures of environmental background variables and outcome measures. Rowe et al. (1994) compared cross-sectional correlation matrices using both independent variables (e.g., home environment, peer characteristics) and developmental outcomes (e.g., achievement, delinquency). Rowe et al. (1995) compared correlations between academic achievement and family environment. They found that the covariance matrix of each group were equal. That is, they failed to find evidence for distortions in the correlations between the background variables and the outcome measures that would suggest a minority-specific developmental factor.
Similarly, Carretta (1995), Owen (1992), and Rushton et al. (2000, 2002, 2003) found nearly identical statistical structure on psychometric variables in each group. The factor structure of cognitive ability is nearly identical for Blacks and for Whites; there were no race-specific factors.
Using structural equation modeling Rowe and Cleveland (1996) estimated the genetic architecture for Black and White siblings. They found that the best-fitting model for the source of differences between and within races was the same: both genetic and environmental factors. Jensen (1998b, p. 465) reanalyzed a subset of this data. This analysis found that the Black-White IQ difference was best explained by a model of both genetic and environmental factors, and that the genetic-only and the environmental-only models were inadequate.
Nichols (1972) using differential heritabilities among Blacks and Whites and later Rushton (1989) using inbreeding depression calculated in Japan found that the Black-White gap is least on IQ subtests most affected by the environment, and greatest on subtests that are least affected by the environment. It is difficult to attribute the relationship between inbreeding depression from Japan with the Black-White IQ gap in the U.S. to an environmental (not-genetic) cause.
Other evidence that is interpreted as indirect support for the hypothesis that the Black-White-Asian IQ gap is party due to genetic differences, rather than culture alone, includes:
- World-wide Black-White-Asian differences in IQ, reaction time, and brain size.
- Correlations between an IQ subtest's g-loading, heritability, and the magnitude of the Black-White-Asian score gap for that subtest.
- Rising heritability of IQ with age, and decreasing shared-family effects (e.g., socioeconomic factors) on IQ after adolescence.
- Studies suggesting that IQ heritability and gene-environment interactions within races are the same for Blacks and Whites.
- The finding that when Black and White children are matched for IQ, their siblings tend to have IQs that regress towards different means (85 for Blacks and 100 for Whites). For example, among Black and White children matched with an IQ of 120, the siblings of the Black children have an average IQ of 100 whereas the siblings of the White children have an average IQ of 110. This is a stronger test of the party-genetic hypothesis than regression from parents to offspring because siblings share a similar environment (Jensen, 1973).
- American Blacks have a lower average IQ than Hispanic and Native American groups, which are more socio-economically deprived. For example, the Inuit, who live in the Arctic, have higher average IQs than North American Blacks (Berry 1966; MacArthur 1968) despite being extremely poor (Vernon 1965, 1979).
- Average Black-White-Asian differences in IQ (both positive and negative) remain following transracial adoption.
- The three-way difference in SAT scores persists even after controlling for income, which seems to counter arguments that the gap is due to socioeconomic conditions.  In addition, many researchers have argued that intelligence is largely responsible for determining socioeconomic status, rather than the other way around.
- Ashkenazi Jews have often been persecuted and discriminated against, but they still display the highest average IQ of any ethnic group, as well as SAT scores higher than those of non-Jewish Caucasians. This seems to counter arguments that depressed IQ scores of African Americans are due to discrimination or prejudice.
- The three-way difference in average IQ can be measured in very young children, before significant cultural forces can take effect. For example, a 1 standard deviation gap is observed in Black and White 3-year olds matched for gender, birth order, and maternal education (Peoples, Fagan, & Drotar, 1995). Lynn (1996) found that by age 6 the average IQ of East Asian children is 107, 103 for White children and 89 for Black children.
- Three-way differences in reaction times have been demonstrated, and it is difficult to explain differences in reaction time through lack of motivation or cultural differences on the part of the subjects.
Critics note that there are arguments against all of the above. For example:
- The only nationwide IQ tests have been done in a few developed countries. See the criticism against IQ and the Wealth of Nations. Reaction time and brain size has only been studied in small samples in some developed countries with almost no studies in the developing world.
- That Blacks are less socio-economically deprived than Hispanics or Native Americans is controversial.
- Environmental factors certainly can affect very young children, for example nutrition by the mother during pregnancy and breastfeeding.
- Persecution of and discrimination against Jews was strongest in the past, while Blacks are still being discriminated against in various ways today.
- Regression towards the mean only shows that mean IQ scores are different which is not a new finding. That is not evidence that the cause of this difference is genetic.
- Differences in reaction time or brain volume may be caused by environmental factors. As noted, there have been large changes in cranial vault size and shape during the last century in the US for both Black and Whites, far beyond what can be explained genetically.
- Adjustments for socioeconomic conditions almost completely eliminate differences in IQ scores between black and white children. The remaining difference is statistically insignificant. .
- Several adoption studies finds no IQ difference. (www.apa.org/journals/features/bul1312301.pdf)
- IQ have very low positive to low negative correlation with Whiteness of skin, degree of European blood groups, or self-reported degree of European ancestry among Blacks.
- g-loading and the method of correlated vectors, the statistical method used in many older studies, has been criticized heavily in recent research as discussed previously.
- Many older studies have only studied middle class families but SES has recently been shown to be relatively more important in poorer families.
- One estimate is that the average IQ in the U.S. was below 75 before the Flynn effect started and it seems likely that the effect started earlier and may end sooner for Whites.
- The gap in the US may be narrowing. One large recent study found much smaller differences than earlier studies in math and reading skills in young children and found that all of the remaining differences could be explained by a few environmental factors. 
The two most widely-known works concerning race and intelligence are The Mismeasure of Man by Stephen Jay Gould, originally published in 1981, and The Bell Curve by Richard Herrnstein and Charles Murray, published in 1994. Media controversy surrounding The Bell Curve motivated Gould to revise and expand The Mismeasure of Man to respond to arguments from The Bell Curve, publishing the book's second edition in 1996. Many current researchers think that both books are outdated due to new research.
A recent paper in the Psychological Review, "Heritability Estimates Versus Large Environmental Effects: The IQ Paradox Resolved" by William T. Dickens of The Brookings Institution and James R. Flynn presents a mechanism by which environmental effects on IQ may be magnified by feedback effects. This work may provide a resolution of the contradiction between the viewpoint of The Bell Curve's authors and the 'nurture' effects observed by others. A latter paper responded to objections .
Some cite research that they believe indicates that discriminated or lower-status minorities do tend to have lower IQ, some without apparent genetic differences. Like Blacks and Hispanics in the US, minorities in some societies show achievement gaps (such as the Maori in New Zealand, aboriginals in Australia, scheduled castes ("untouchables") in India, non-European Jews in Israel, and the Burakumin in Japan). The most prominent finding cited is that Northern Irish Catholics used to score about 15 points lower than Protestants. Similarly, Irish, Italian and Polish immigrants in the USA are reported to have all scored about 80 in the beginning of the 19th century, but now tend to reach 100. The same is true of persons from rural versus urban areas in general (see e.g. this article by conservative columnist and economist Thomas Sowell and this page on European and Greek IQ. More arguments of the kind are to be found here).
In 1974, an English biologist named John Randal Baker presented a lengthy argument in favor of innate racial differences in intelligence in a volume entitled "Race" (Oxford University Press, 1974), in which he claimed there had been a systematic, politically motivated suppression of classical biological anthropology outside Germany since the 1930s, and went on to attempt to demonstrate a relation among five historical civilizations (Sumerian, Egyptian, Indus valley, Helladic-Minoan and Sinic) and the supposed biological dispositions of their creators.
Comparative economic analyses are sometimes presented as evidence that environmental explanations of the IQ gap are incomplete. IQ and the Wealth of Nations, a book by Richard Lynn and Tatu Vanhanen, is the most recent prominent example of an economic analysis of the IQ and race issue. The book, which was not peer-reviewed, is sharply criticized in the peer-reviewed paper The Impact of National IQ on Income and Growth .
Opinions of scholars
A survey performed in the 1980s of a broad sample of 1,020 scholars in specialties that would give them reason to be knowledgeable about IQ asked, "Which of the following best characterizes your opinion of the heritability of the Black-White difference in I.Q.?" (emphasis original).  The responses were divided into five categories:
- The difference is entirely due to environmental variation: 15%.
- The difference is entirely due to genetic variation: 1% (8 respondents).
- The difference is a product of both genetic and environmental variation: 45%.
- The data are insufficient to support any reasonable opinion: 24%.
- No response: 14%.
No single response was endorsed by more than half of those surveyed. In general, this could be considered as evidence that no consensus opinion exists on the cause of the Black-White IQ gap. However, the survey is old and it is unclear what specialties are represented.
The source of and meaning of the IQ gap is not known. Many theories have been proposed, but none are generally accepted. Most of the theories are supported by only indirect evidence. The cause may be environmental. Many attribute the difference primarily to cultural factors that disadvantage caste-like minorities. Many researchers in the field of intelligence suggest that the difference is partially genetic and partially environmental. Other observers insist that the differences may be entirely environmental. The cause of the IQ gap may be identical to the cause of IQ differences between all individuals, or it may represent a race-specific effect. This is an active area of research.
Biological differences in brain volume and reaction time, which show low to moderate correlations with IQ, are not by themselves evidence for genetic differences. Even if the IQ gap does indicate differences in intelligence, this may be due to environmental differences in factors such as nutrition during pregnancy or early childhood which may produce biological differences with no genetic component. In general, simple correlations cannot decide the role of genetics. Advanced statistical methods are instead used with hotly debated results.
Because the cause of the IQ gap is ultimately an empirical question, it should be possible to resolve this question in the future. Irrefutable direct evidence is currently lacking and may continue to be so until intelligence is mapped to specific genes.
Most research has been done in the US and a few other developed nations. That research cannot directly be generalized to the world as a whole. Blacks in the US are not random sample from Africa and environmental conditions differ between nations. IQ tests done in developing countries are likely affected by impoverished environmental conditions that are common in the developing world, such as nutritional deficiencies (e.g., iodine deficiency is known to affect intelligence) and the impact of diseases (e.g., HIV, anemia or chronic parasites that may affect IQ test scores).
Finally, genetic engineering may soon be able to directly change the genetic determinants of intelligence. This may make genetic intelligence and other genetic characteristics a matter of voluntary parental (or enforced governmental) decision. This could theoretically, in a single generation, dramatically increase human intelligence and make the current concept and discussion of race and possible associated characteristics obsolete.
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