Race and intelligence (explanations)

There is substantial debate about the influence of various environmental factors on IQ test score differences between races and ethnic groups in a given country, and whether or not genetics may also play a role.

Measuring intelligence
Several different ways of measuring intelligence have been proposed. IQ tests are the most common and what is usually used in research. The American Psychological Association (APA) states in its 1997 report Intelligence: Knowns and Unknowns that "the dominant psychometric approach, which has not only inspired the most research and attracted the most attention (up to this time) but is by far the most widely used in practical settings...The psychometric approach is the oldest and best established, but others also have much to contribute. We should be open to the possibility that our understanding of intelligence in the future will be rather different from what it is today."'

Christine E. Daley, Anthony J. Onwuegbuzie, Howard Gardner and other psychologists have challenged the classicist model of a single index for "intelligence" on which hereditarian assumptions of racial disparity are based. They advocate a theory of multiple intelligences. They write that through research on multiple intelligences they may reveal a more nuanced perspective in to the strengths and weaknesses of students and in to the ways that people view the intelligences of people from other ethnic groups. Cultural differences may lead children to develop different strengths in different areas of intelligence. Leroy G. Baruth and M. Lee Manning write "Knowing that a relationship exists between cultures and education is a prerequisite to effective teaching, but continuing to teach with styles and strategies appropriate only for middle-class Anglo learners fails to meet the needs of culturally diverse children and adolescents." .

Ketty M. Sarouphim writes in Discovering Multiple Intelligences through a Performance-Based Assessment: Consistency with Independent Ratings that the use of standardized tests to assess the intelligence of culturally diverse groups has been much criticized. Some researchers have attributed the problem of underrepresentation of minority students in programs for the gifted to the wide use of such tests in which narrow definitions of giftedness are adopted. Sarouphim writes that the field of intelligence assessment seems to be witnessing a paradigm shift, as evidenced by recent definitions of giftedness, the emergence of nontraditional theories of intelligence , and the rise of alternative assessment methods, namely performance-based assessments.

Facial recognition ability has shown differences by race. In general, other-race faces are less accurately recognized than same race faces but classified faster by race. In the US Blacks' performance is significantly better than that of whites', and blacks are better at recognizing faces of whites than whites are at recognizing blacks. Richard Ferraro writes that facial recognition is an example of a neuropsychological measure that can be used to assess cognitive abilities that are salient within African-American culture. One possibility is that expertise in perceiving faces of particular races is associated with increased ability to extract information about the spatial relationships between different features.

Test bias
While the existence of average IQ test score differences has been a matter of accepted fact for decades, during the 1960s and 1970s a great deal of controversy existed among scholars over the question of whether these score differences reflected real differences in cognitive ability. Some claim that there is no evidence for test bias since IQ tests are equally good predictors of IQ-related factors (such as school performance) for U.S. 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. Still, a 2007 study at Case Western Reserve University found that cultural differences in the provision of information account for racial differences in IQ. The study also found that test problems, similar to some problems found on conventional IQ tests, were only solvable on the basis of specific previous knowledge. Such specific knowledge based questions showed evidence of test bias since the performance on non-specific knowledge based questions did not always correlate with the performance on the knowledge based question.

On a test (Black Intelligence Test of Cultural Homogeneity) oriented toward the language, attitudes, and life-styles of Afro-Americans, white students perform more poorly than blacks do on tests oriented toward white middle-class values, indicating that there are important dissimilarities in the cultural backgrounds of blacks and whites. Some argue that these findings indicate that test bias plays a role in producing the gaps in intelligence test scores. The Chitling Intelligence Test is another example of a culturally biased test that tends to favor African Americans, although it should be noted that this test, despite its name, is more concerned with knowledge than intelligence. These criticisms may not apply to "culture free" tests of intelligence. However, due to their cultural backgrounds some test takers do not have the familiarity with the language and culture of the psychological and educational tests that is implicitly assumed in the assessment procedure, even on "culture free" tests.

Environmental explanations
Regarding the IQ gaps in the U.S., numerous explanations beside genetics have been proposed. Joel Wiesen lists more than a hundred. It has been suggested by John Ogbu and others that African-American culture disfavors academic achievement and fosters an environment that is damaging to IQ. Likewise, it is argued that the persistence of negative racial stereotypes reinforces this effect. Ogbu writes that the condition of being a "caste-like minority" affects motivation and achievement, depressing IQ. Although cultural differences may play a role in creating the gaps, much of the present gap found in IQ tests scores is likely the result of a combination of socioeconomic factors and health factors, such a breastfeeding. A 2006 study found that strongest and most robust predictors of intelligence were family income, parental education and breast feeding, with these three variables explaining 7.5% of the variation in intelligence at age 14. The impact of racial stereotypes has also been shown to play a key role. Making race salient in testing stations depresses the performance of minority students who belong to racial groups that have been historically stereotyped as less intelligent. (See:Race and intelligence (media portrayal)) Recent developments in intervention methods to counteract the impact of negative stereotypes have proved promising.

Arguing that IQ tests are often wrongly described as measuring "innate" rather than developed ability, write that this "labeling bias" causes people to inappropriately attribute the Black-White gap to "innate" differences. They argue that non-cultural environmental factors cause gaps measured by the tests, rather than innate difference based on genetics, and that to use these tests as a measure of innate difference is misleading and improper.

Increases in IQ scores over time


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. This means, given the same test, the mean black American performance today could be higher than the mean white American performance in 1920, though the gains causing this appear to have occurred predominantly in the lower half of the IQ distribution. If changes in environment can cause changes in IQ over time, they argue, then contemporary differences between groups could also be due to an unknown environmental factor. On the supposition that the effect started earlier for whites, because their social and economical conditions began to improve earlier than did those of blacks, they anticipate that the IQ gap among races might change in the future or is even now changing. 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", and other researchers have found reason to concur. wrote that "the gains cannot be explained solely by increases at the level of the latent variables (common factors), which IQ tests purport to measure". An analysis by reported that the IQ increases associated with the Flynn effect did not produce changes in g, which Rushton compares to the finding by  that IQ increases associated with adoption likewise do not increase g. disagrees with Rushton's analysis.

have proposed a solution which rests on genotype-environment correlation, hypothesizing that small initial differences in environment cause feedback effects which magnify into large IQ differences. and others find this hypothesis unsupported by the available evidence. respond to these criticisms. Such differences would need to develop before age 3, when the black-white IQ gap can be first detected.

The Flynn effect consists of 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.

Comparing the Flynn effect (IQ differences within races over time) to contemporary IQ differences between races is contested; for example, one report concludes "the nature of the Flynn effect is qualitatively different from the nature of black-white differences in the United States," and that "the implications of the Flynn effect for black-white differences appear small" However, this refers to "measurement invariance", is not a statement about the role of genetics in the B-W gap, and is a relatively minor statement that not mentioned in the abstract..

A recent theory hypothesizes that fluid cognition (gF') may be separable from general intelligence, and that gF' may be very susceptible to environmental factors, in particular early childhood stress. Some IQ tests, especially those used with children, are poor measures of gF', which means that the effect of the environment on intelligence regarding racial differences, the Flynn effect, early childhood intervention, and life outcomes may have been underestimated in many studies. The article has received numerous peer commentaries for and against.

A recent, newly available, large, and nationally representative data set find only very small (0.06 SD between whites and blacks) racial differences on measures for mental function for children aged eight to twelve months. These differences disappear when controlling for a limited set of factors such as differences in SES. "These findings pose a substantial challenge to the simplest, most direct, and most often articulated genetic stories regarding racial differences in mental function." "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." In their 2006 study, Black Americans reduce the racial IQ gap: Evidence from standardization samples, William T. Dickens and James R. Flynn write that blacks have gained 5 or 6 IQ points on non-Hispanic whites between 1972 and 2002. Gains have been fairly uniform across the entire range of black cognitive ability.

Racism and discrimination
Reserchers such, as Jack Demaine find racial categorizations problematic in educational settings. Racial categorizations, Jack Demaine writes, may have adverse impacts on the education of minorities. Similarly, Alastair Bonnett, Bruce Carrington state:

"The collection of ethnic and racial statistics has become common in a growing number of institutional settings. Yet contemporary approaches to race and ethnicity suggest that the very process of compelling people to assign themselves to one of a small number of racial or ethnic 'boxes' is, at best, essentialist and, at worst, racist."

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 (Aronson, Wilson, & Akert, 2005). Stereotype threat has been documented by the social psychologists Claude Steele, Joshua Aronson, Irwin Katz, and Steven Spencer, who have conducted several studies on this topic.

"'When capable black college students fail to perform as well as their white counterparts, the explanation often has less to do with preparation or ability than with the threat of stereotypes about their capacity to succeed.' - Claude M. Steele, The Atlantic Monthly, August 1999 Thin Ice: Stereotype Threat and Black College Students"

Steele and Aronson write that making race salient when taking a test of cognitive ability negatively affected high-ability African American students. Steele writes that the stigma of being African American is still relevant, as it has an effect on the educational outcomes of African Americans. Stereotypes such as: Asian-Americans excelling in mathematics or African-Americans always testing poorly can be extremely harmful. Stereotype threats can seriously alter academic achievement and motivation.

In a paper prepared for an APA convention, Steele writes: "Thus the predicament of 'stereotype vulnerability': The group members then know that anything about them or anything they do that fits the stereotype can be taken as confirming it as self-characteristic, in the eyes of others, and perhaps even in their own eyes. This vulnerability amounts to a jeopardy of double devaluation: once for whatever bad thing the stereotype-fitting behavior or feature would say about anyone, and again for its confirmation of the bad things alleged in the stereotype."

Steele and Aronson are not first to test stereotype threat. During the 1960’s Irwin Katz, psychologist, suggested that stereotype threat could also influence performance on IQ tests. Katz found that Blacks were able to score better an IQ subtest if the test was presented as a test of eye-hand coordination. Blacks also scored higher on an IQ test when they believe the test will be compared to that of other blacks. Katz concluded that his subjects were thoroughly aware of the judgment of intellectual inferiority held by many white Americans. With little expectation of overruling this judgment, their motivation was low, and so were their scores. Paul Sackett, a psychologist agrees that stereotype threat is a real phenomenon and that it is is a potentially important contributor to the racial achievement gap. He cautions however, that these findings may be widely misinterpreted to mean that eliminating stereotype threat eliminates the entire Black-White performance gap, and encourages researchers to continue their study of this and other phenomena.

Since stereotype threat appears to be one key contributing factors to the gaps in test scores, researchers Geoffrey L. Cohen, Julio Garcia, Nancy Apfel, and Allison Master proposed intervention methods to address the problem in 2006. The intervention, a brief in-class writing assignment, significantly improved the grades of African American students and reduced the racial achievement gap by 40%. These results suggest that the racial achievement gap, a major social concern in the United States, could be ameliorated by the use of timely and targeted social-psychological interventions.

Physiological responses to racism
Stereotype threat can result in physiological responses that can be measured objectively. For example, a study by Blascovich J, Spencer SJ, Quinn D and Steele C. reported that African Americans under stereotype threat exhibited larger increases in arterial blood pressure during an academic test, and performed more poorly on difficult test items. Some researchers feel this may explain the higher death rates from hypertension related disorders among African Americans.

A study by Toni Schmader and Michael Johns reported that stereotype threat can effectively reduce working memory capacity, another factor in poor test performance.

Stereotype threat may undermine intellectual performance by triggering a disruptive mental load. Jean-Claude Croizet, Gérard Després, Marie-Eve Gauzins, Pascal Huguet, Jacques-Philippe Leyens and Alain Méot reported increased heart rates for test subjects operating under stereotype threat.

Caste-like minorities


John Ogbu writes that caste-like minorities are not the same as other racial minorities. Caste-like minorities are incorporated into a country involuntarily and permanently. These include Blacks, American Indians, Mexicans, Native Hawaiians, Puerto Ricans and others. Membership is a low caste acquired at birth and retained permanently. Caste members are regarded by the white majority as inferior and not desirable as neighbors or workmates. Often, they lack political power and are economically subordinate. They face a job ceiling, and are not hired on the basis of training and skills like other minorities. Caste-like groups also reject the ideology and beliefs of the dominant group culture. They believe their problems are due to the "system" and racism than their own inadequacies. They may develop a "collective institutional discrimination perspective". This leads them into channel efforts into collective struggle.

Like Blacks and Hispanics in the U.S., minorities in non-US societies show achievement gaps (such as the Māori 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 U.S. 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).

Quality of education
Some researches 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 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. 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. 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 Whites on both cancellation tasks which assess visual scanning. (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.

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. Scores 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.

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. 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. Recently, a number of scholars have examined the issue of disproportionate representation of minority students in special education programs

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. 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.

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."

Socio-economic factors




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, in the American Psychological Association report argue that economics cannot be the whole explanation. According to Richard J. Herrnstein and Charles Murray, authors of The Bell Curve, to the moderate extent that IQ and income are related, it appears that IQ determines income, and not the other way around. There are, however, many other potential Socio-economic factors factors beside income.

Researchers have reported 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)

Work by on average Black-Hispanic-White differences in IQ, education, and income casts doubt on conventional explanations of Black-White differences: "Hispanic children start with cognitive and noncognitive deficits similar to those of black children. They also grow up in similarly disadvantaged environments and are likely to attend schools of similar quality. Hispanics complete much less schooling than blacks. Nevertheless, the ability growth by years of schooling is much higher for Hispanics than for blacks. By the time they reach adulthood, Hispanics have significantly higher test scores than do blacks. Conditional on test scores, there is no evidence of an important Hispanic-white wage gap. Our analysis of the Hispanic data illuminates the traditional study of black-white differences and casts doubt on many conventional explanations of these differences because they do not apply to Hispanics, who also suffer from many of the same disadvantages. The failure of the Hispanic-white gap to widen with schooling or age casts doubt on the claim that poor schools and bad neighborhoods are the reasons for the slow growth rate of black test scores."

A recent 1996 study using multiple socio-economic factors have accounted for 80% of the gap, and suggest that any remaining gap is statistically insignificant.

Health


In the developing world there are are many factors can greatly decrease IQ scores. Examples include nutrition deficiencies in iodine and iron; certain diseases like malaria; unregulated toxic industrial substances like lead and mercury; and poor health care for pregnant women and infants. Also in the developed world there are many biological factors that can affect IQ. Increased rates of low birth weight babies and lower rates of breastfeeding in Blacks as compared to Whites are some factors of many that have been proposed to affect the IQ gap.

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. has proposed a "neurotoxity hypothesis" where pre- and post-natal exposure to heavy metal poisons differentially impacts Blacks. Black children have much higher lead levels than white children. Drug abuse during pregnancy (e.g., alcohol and phenobarbital) can negatively affect IQ.

Infant mortality may be an indicator of environmental conditions that are sublethal but damaging to health. The rate of infant mortality in the U.S. Black population is twice that of the White population, which in turn is twice the rate of infant mortality among Asians. The rates of low birth weight (LBW), defined as less than 5.5 pounds, are correlated with infant death. LBW is different than premature birth; LBW can occur in full-term babies. LBW babies are at risk for many developmental, behavioral and cognitive abnormalities, including mental retardation. LBW (and premature birth) affect Blacks at twice the overall rate for the U.S. population. Mother's age is the strongest predictor of LBW, where teenagers are especially susceptible. Most of the Black-White differences in LBW are not account for by other environmental variables such as socioeconomic status, poverty status, mother's age, and education; but differential prenatal care explains some of the gap. Thus, the cause of the Black-White gap in LBW is a mystery. Environmental intervention has strong but short-lasting effects on IQ among LBW babies. Studies of LBW Black and White babies matched for birth weight and gestational age still find a one standard deviation IQ gap.

A study of LBW babies indicates that breastfeeding can significantly improve their IQ scores tested at 8 years old. After controlling for possible confounding factors, an improvement of 8.3 IQ points was reported in the breastfed group as compared to the formula fed group. 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. Several recent studies shows that the intake of certain micronutrients, like those present in breast milk or fish oil, affects IQ scores even in developed nations. have shown larger head size at birth and higher IQ scores at 4 years of age when mothers took fish oil supplements during pregnancy and lactation. believes that dietary supplementation is a promising avenue of research for raising Black children's levels of g. has proposed a nutritional hypothesis for the Flynn effect.

Exposure to violence in childhood has been associated with lower school grades and lower IQ in children of all races. A group of largely African American urban first-grade children and their caregivers were evaluated using self-report, interview, and standardized tests, including IQ tests. The study reported that exposure to violence and trauma-related distress in young children were associated with substantial decrements in IQ and reading achievement. Exposure to Violence or Trauma lead to a 7.5-point (SD, 0.5) decrement in IQ and a 9.8-point (SD, 0.66) decrement in reading achievement. Violence may have a negative impact on IQ, or IQ may be protective against violence. The causal mechanism and direction of causation is unknown. Neighborhood risk has been related to lower school grades for African-American adolescents in another study from 2006.

Culture
Many anthropologists have argued that intelligence is a cultural category; some cultures emphasize speed and competition more than others, for example. Speculations about innate differences in intelligence between ethnic groups have occurred throughout history. Aristotle in the 4th century B.C. and Cicero in the 1st. century B.C. disparaged the intelligence of the northern Europeans of the time, as did the Moors in Iberia in the 11th century.

Ogbu elaborates on this idea suggesting that African American popular culture serves to disengage students from academic achievement by proving the wrong kind of role models.

"'What amazed me is that these kids who come from homes of doctors and lawyers are not thinking like their parents; they don't know how their parents made it,' Professor Ogbu said in an interview. 'They are looking at rappers in ghettos as their role models, they are looking at entertainers. The parents work two jobs, three jobs, to give their children everything, but they are not guiding their children.'"

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's been a personal challenge for Dylan Pritchett, a Lafayette High School senior in Williamsburg who will head to Old Dominion University in August. Friends accused him of not 'acting black' when he signed up for AP courses."

Speculations about innate differences in intelligence between ethnic groups have occurred throughout history. Aristotle in the 4th century B.C. and Cicero in the 1rst. century B.C. disparaged the intelligence of the northern Europeans of the time, as did the Moors in Iberia in the 11th century.

It has been suggested that Black culture disfavors academic achievement and fosters an environment that is damaging to IQ. Likewise, it is argued that a persistence of racism reinforces this negative effect. John Ogbu has developed a hypothesis that the condition of being a "caste-like minority" affects motivation and achievement, depressing IQ. However, Arthur Jensen has criticized these arguments on the grounds that they cannot explain the higher scores of East Indians and East Asians. 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 East Asian minorities are complemented – and sometimes challenged – by the observation that East 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". East Asian and Jewish populations have suffered past discrimination and persecution which some argue is evidence against the importance of discrimination for IQ differences. While the severe discrimination against Jews and East Asians have today diminished, many argue that discrimination continue against blacks and that this is impacting the IQ scores of Blacks.

Pidgin language barriers
Sandra Lee McKay author of Sociolinguistics and Language Teaching writes that language may present a barrier for students who speak pidgin and creole languages. Unlike other languages such as Spanish and Chinese, pidgin and creole languages such as African American Vernacular English (AAVE) are not commonly recognized in classroom settings. As a result of this, students are not taught the Standard American English (SAE) used on tests as a second language in the same way as students who speak Spanish or Chinese. Students who speak AAVE face challenges similar to those learning English as a Second Language. (ESL)

John Russel Rickford author of Unequal partnership: Sociolinguistics and the African American speech community rebutts misconceptions about the cognitive limitations of the use of AAVE notes the unfair disadvantages IQ tests pose for its speakers. Geneva Smitherman writes that "80 to 90 percent of American blacks” speak AAVE “at least some of the time". Anne H. Charity, Hollis S. Scarborough, Darion M. Griffin found in a 2004 study that higher familiarity with SE was associated with better reading achievement for urban African American students in kindergarten through second grade. The improvement in reading ability was independent of other cognitive measures suggesting that speakers of AAVE face barriers in education. The preponderance of code-switching indicates that AAVE and SAE are met with different reactions or discernments. AAVE is often perceived by members of mainstream American society as indicative of low intelligence or limited education.

Positive language effects
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 and Japanese, develop spatial reasoning better than the alphabetic writing systems prevalent in Europe and America, though there are no studies that support this hypothesis. The same reasoning has been used to explain why students from some Asian countries (e.g., Singapore) 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. However, even though few native-born Asian Americans learn to read and write Chinese characters, their performance is above-average on IQ tests.

A direct comparative test between Greek and Chinese students showed no difference in IQ or g, contradicting earlier studies which do not take the finer architecture of mental processing into account. The Chinese did outperform the Greeks in visuo/spatial ability, but this difference was smaller at earlier ages, grew during the first years of schooling and decreased later. The authors suggest that this pattern can be explained as follows: the Chinese students train their visuo/spatial ability during their early school years, as they have to learn many characters of the Chinese writing system. Later in life, the Greek students adopt compensating strategies to deal with visuo/spatial information, and therefore the difference decreases in this realm.

Role-model effects
Thomas S. Dee, in his studyTeachers, Race, and Student Achievement in a Randomized Experiment found that the race of the teacher has impacts on student achievement. An own-race teacher significantly increased the math and reading achievement of both black and white students. Using single-equation regression models Mark O. Evans has also found evidence of effects for African-American students. These findings may confirm the suggestion for the aggressive recruitment of minority teachers are based on hypothesized role-model effects for minority students.

In Sabrina Zirkel's longitudinal study of young adolescents students who reported having at least one race- and gender-matched role model performed better academically up to 24 months later, reported more achievement-oriented goals, enjoyed achievement-relevant activities to a greater degree, thought more about their futures, and looked up to adults rather than peers more often than did students without a race- and gender-matched role model. These effects held only for race- and gender-matched role models—not for non-matched role models.

The effort gap
Researchers Stephan Thernstrom and John Ogbu have suggested that black students perform poorly in part due to simple lack of effort. Stephan Thernstrom studied different kinds of schools and concluded that, while many environmental factors play a role in the achievement gap, a strong commitment to education was an essential element for academic success. Ogbu wrote that the black students were quite open in telling the researchers that, in general, their white classmates studied more, worked harder and cared more about getting good grades. "'In spite of the fact that the students knew and asserted that one had to work hard to succeed in Shaker schools, black students did not generally work hard. In fact, most appeared to be characterized by the low-effort syndrome ... (They) were not highly engaged in their schoolwork and homework.' --John Ogbu"

Freeman A. Hrabowski III, president of the University of Maryland, Baltimore County, said that many black boys grow up with few male role models and in high-crime neighborhoods, where being smart in school is not considered cool. “You can’t just ignore the needs of a group and say all children are the same,” he said.

Arguments against environmental explanations
Although, researchers such as Jensen, who argue that genetics play a significant role in do acknowledge that environmental factors are important. They have raised a number of questions about various environmental explanations since if any one environmental explanation or any combination of environmental explanations accounted for the entire gap in test scores, the question of a genetic contribution would no longer be relevant. According to, "the total between-populations variance accounted for by empirically demonstrable environmental factors does not exceed 20 to 30 percent." Jensen argues that attributing between population difference to factors that have not been shown to affect differences within populations is "useless from the standpoint of scientific explanation". Further, writes that many environment-IQ correlations which have been reported, though real and significant, can be disqualified because these studies completely confound the genetic and environmental causes of IQ variance, and in those cases where genetic and environmental causes have been examined, nearly all of the correlations have been found to actually have a genetic basis.

According to Arthur Jensen, we know hardly anything about the magnitude and psychometric properties of test score gaps among "caste-like" minorities in Indian and Japan, but a Black-White gap can be found in places where blacks have never been slaves, where they have never been a minority population, and where there has never been a color line. In response to Sowell, Jensen writes that the poor performance of recent immigrants is usually attributed to deficiencies in English language skills, but that those groups from Europe and Asia perform as well as the majority population within a single generation.

One environmental source of the IQ gap which has been suggested is poor motivation among low scorers. This hypothesis has been disputed by the researcher Arthur Jensen (1998). 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, reaction time measurements negatively correlate with IQ scores and show the same performance gaps between 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, although no correlation between movement speed and intelligence is claimed. The correlation between IQ and reaction time is low (from .20 to .40).

The effects of test construction on minority groups, such as the use of standard English, were examined by the 1996 APA report, which wrote that "controlled studies have shown, however, that none of them contributes substantially to the Black/White differential under discussion here (Jensen, 1980; Reynolds 82 Brown, 1984; for a different view see Helms, 1992). Moreover, efforts to devise reliable and valid tests that would minimize disadvantages of this kind have been unsuccessful." However the language gap still presents a barrier to education through adverse impacts in classroom settings where the need to teach English as a second language to students who come to school speaking AAVE is not commonly recognized.

According, the Coleman report says that "Negligible, and in some cases, negative correlations were found between IQ and variables such as pupil expenditure, teachers' salaries, teachers' qualifications, student/teacher ratios, and the availability of other school professionals". According to Lisa Sanbonmatsu and colleagues, moving families from public housing to neighborhoods with lower poverty rates produces no significant effects on children's test scores.

Media portrayal


Race and intelligence are sometimes portrayed as related in media. People of various races have been portrayed as more or less intelligent in media such as films, books, and newspapers. Likewise, reporting on research into race and intelligence has been criticized: either for giving scientific theories of race too much credit, or for rejecting the theories of some researchers in the name of racial harmony.

Critics of contemporary media have highlighted portrayals of minorities as less intelligent (or in the case of Asians, on occasion more intelligent ) in films and movies. Entman and Rojeki assert that media images of Blacks may have profound effects on the perceptions by both Blacks and Whites about black intellectual potential.

Even so-called positive images of Black people can lead to stereotypes about intelligence. In Darwin's Athletes: how sport has damaged Black America and preserved the myth of race John Hoberman writes that the prominence of African-American athletes encourages a de-emphasis on academic achievement in black communities. Film director Spike Lee explains that these images have negative impacts "In my neighborhood, we looked up to athletes, guys who got the ladies, and intelligent people," said Lee. "[Now] If you're intelligent, you're called a white guy or girl."

Blacks are not the only ethnic group in the US to be stereotyped as stupid. Shortly after the large waves of immigration in the 19th century and number of immigrant groups such as the Irish were stereotyped as being more closely related to apes or dogs and therefore intellectually inferior. This changed after the definition of "white" was expanded to include the Irish. Unlike other racial stereotypes of intelligence, the 19th century pseudo-scientific ideas about Irish stupidity and inferiority are not supported by most hereditarian proponents of modern race research. Other stereotypes, of Blacks, Jews and Asians endure to this day as do the findings of Hereditarian researchers about these groups.

Other aspects of the media portrayal of race and intelligence include recent books asserting or disputing a genetic cause for group differences (such as The Bell Curve by Hernstein and Murray, and The Mismeasure of Man by Gould) and surveys or consensus statements made by groups of scientists (the APA statement on Race and Intelligence, the Snyderman and Rothman survey, and the Mainstream Research on Intelligence manifesto by Gottfredson.)

Genetic explanation
Arthur Jensen and others argue that the Black-White IQ gap is significantly genetic. That is, they argue that the same mix of genetic and environment factors that cause IQ differences among individuals or between families of the same race also causes the differences seen between races. In this view, the genetic contribution to 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.

The results of most (indirect) analyses used to test the genetic hypothesis do not logically contradict a primarily 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. Arthur Jensen and others have concluded that the US IQ gap is partially genetic. Rushton and Jensen say that while plausible environmental explanation for the lower mean IQ in Blacks in the U.S. can be offered in many cases, these explanations are less capable of explaining the higher average IQ of East Asians than Whites.

To support their theory, they often cite several arguments and observations:


 * 1) Black–White–East Asian differences in IQ, reaction time, and brain size are observed worldwide in a range of cultures and environments. In the United States, significant Black-White IQ differences are observable at every age above 3 years, within every occupation or socioeconomic level tested, in every region of the country, and at every time since the invention of ability tests.
 * 2) Jensen and others have argued that the magnitude of race differences on different IQ subtests correlate with the extent to which those subtests measures g, which also correlates with measures of the subtests heritability. From these and other findings, they argue that race differences have a partly biological basis.
 * 3) The rising heritability of IQ with age (within all races; studies have reported on average in the developed world heritability starts at 20% in infants, rises to 40% in middle childhood, and peaks at 80% in adulthood); and studies showing the virtual disappearance (~0.0) by adulthood of shared environmental effects on IQ (for example, family income, education, and home environment), with adopted siblings partaking in the studies no more similar in IQ than with strangers From these studies, they argue that most suggested environmental explanations for IQ difference between groups do not have a strong enough effect on IQ to fully account for group differences.
 * 4) Studies of US comparisons of both parents to children and siblings to each other finding regression to differing means for different races (85 for Blacks and 100 for Whites) across the entire range of IQs, despite the fact that siblings are matched for shared environment and genetic heritage, with regression unaffected by family socioeconomic status and generation examined
 * 5) Evidence against test construction and cultural bias: the internal consistency of item difficulty for all groups, the equivalent validity of tests in predicting academic and occupational outcomes for all groups, and the persistence of the IQ gap on relatively culture-free tests.

believe that the best explanation is that 50%-80% of the group differences in average US IQ is genetic.

Other evidence, such as transracial adoption, 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. Critics of this 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. For example, argue that the statistical methods linking the Black-White gap to g are insufficient.

According to Linda Gottfredson, a researcher at the University of Delaware IQ differences among individuals of the same race reflect (1) real, (2) functionally/socially significant, and (3) substantially genetic differences in the general intelligence factor (, p. 311). Also, again according to Dr Gottfredson, average IQ differences among races reflect (1) real and (2) significant differences in the same g factor (, p. 311). However, it is a matter of debate whether IQ differences among races in a given country are primarily environmental, primarily genetic or simply an artifact of an inaccurate use of social racial identification as a proxy for genetics.

A recent review summarizing the arguments for a genetic explanation can be found here. A critique of genetic explanations can be found here. See also the table below for counter-arguments.

"Heritability"


In developed nations, IQ has a high "heritability". Heritability is often confused with genetics, the concepts are related but distinct.

Herrnstein and Murray explain a limitation of within-group heritability 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 and Murray 1994, p. 298.)"

There are a number of points to consider when interpreting heritability:
 * High heritability of a trait within a given group has no necessary implications for the source of a difference between groups.
 * A high heritability does not mean that the environment has no impact on the development of a trait, or that learning is not involved. Vocabulary size, for example, is very substantially heritable (and highly correlated with general intelligence) although every word in an individual's vocabulary is learned. In a society in which plenty of words are available in everyone's environment, especially for individuals who are motivated to seek them out, the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions.
 * A common error is to assume that because something is heritable it is necessarily unchangeable. This is wrong. Heritability does not imply immutability. As previously noted, heritable traits can depend on learning, and they may be subject to other environmental effects as well. The value of heritability can change if the distribution of environments (or genes) in the population is substantially altered. For example, an impoverished or suppressive environment could fail to support the development of a trait, and hence restrict individual variation. This could affect estimates of heritability. Another example is Phenylketonuria which previously caused mental retardation for everyone who had this genetic disorder. Today, this can be prevented if following a modified diet.
 * On the other hand, there can be effective environmental changes that do not change heritability at all. If the environment relevant to a given trait improves in a way that affects all members of the population equally, the mean value of the trait will rise without any change in its heritability (because the differences among individuals in the population will stay the same). This has evidently happened for height: the heritability of stature is high, but average heights continue to increase.

According to, the complement of the maxim that "heritability within groups does not imply... heritability between groups" is also logically true: that environmentalability (the contribution of environment to phenotypic differences) within groups does not imply environmentability between groups.

"Factor X"
Jensen (1973) argued that because the heritability of IQ is so high and the correlation between environment and IQ is so low, a 15 point IQ gap between Blacks and Whites could be explained by environmental factors alone only if the environmental gap between Blacks and Whites was implausibly immense.

Lewontin (1976) argued that Jensen's conclusion does not follow. He offered the example of two genetically indistinguishable populations of plants, one grown under optimal conditions and another grown under poor conditions. Under those conditions, the heritability of a trait such as height may be 100% within groups but the differences between groups would be entirely environmental.

Jensen writes that to avoid concluding that between group differences are partly-genetic in origin, one must believe either that: (1) similar environmental factors have greater effect between groups than within groups or (2) there are environmental factors that have effects between groups but not within groups. Jensen calls the second alternative Factor X. If group differences were caused by racism (discrimination based on race), then racism would be a "Factor X." Jensen labels this an ad hoc hypothesis that violates Occam's razor.

Dickens and Flynn argue that neither Lewontin nor Jensen is correct. Lewontin is mistaken, he argues, because he posits an implausible Factor X model, and they argue that Jensen's model also have implausible implications, such as that the Flynn effect is significantly due to genetic factors. Instead, they propose an alternative model. "Heritability" include not only a direct genetic effect on IQ, but also the effect on environmental factors caused by those with higher IQ seeking out a more stimulating environent. The direct genetic effect can be quite small despite a high "heritability" measure and large IQ differences due to feedback loops between prior IQ and seeking out more stimulating environments. Also, there is no need to have a large Factor X causing permanent effects in IQ. Many small and transient environmental effects that persistently lean in one direction can substitute for a single persistent environmental cause. Averaged together, the total impact can be large, even if each individual effect is small. David Rowe and others have both complimented and criticized the published account of the Flynn-Dickens model, which Flynn and Dickens defend.

Shared and nonshared environmental effects

 * see also IQ: genetics vs environment

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. explain this view: "The cause of individual differences within groups has no necessary implication for the cause of the average difference between groups. A high heritability within one group does not mean that the average difference between it and another group is due to genetic differences, even if the heritability is high in both groups. However, within-groups evidence does imply the plausibility of the between-groups differences being due to the same factors, genetic or environmental. If variations in level of education or nutrition or genes reliably predict individual variation within Black and within White groups, then it would be reasonable to consider these variables to explain the differences between Blacks and Whites. Of course, independent evidence would then be needed to establish any relationship."

According to, there is some evidence suggesting lower heritability in Blacks than Whites (e.g., ), but a larger body of evidence suggested equal heritabilities for both races. An analysis of the Georgia Twin Study by reported 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. Shared environmental effects are due to factors experienced in common by all children raised in the same family but that differ among 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 among 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.

reported (among a population of people studied in the U.S.) that the nonshared environmental effects on IQ remain approximately constant throughout life. Shared environmental effects in their study remained approximately constant (40% to 30%) from 4 to 20 years of age but then drop to zero in adulthood. Genetic factors increase throughout development (from 40% to 50%) but especially after 20 years of age (from 50% to 80%). 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). argues that because these effects account for little variance within a race, they are unlikely to account for the differences among races in developed nations.

However, others studies do support that shared environmental factors in developed nations can affect IQ, including IQ gains lasting into adulthood (Capron and Duyme, 1989). However, many such studies measure IQ in children (those shared effects that have disappeared in studies don't disappear until adulthood) or, some critics claim, do not have the controls needed to differentiate genetic and environmental effects. Others argue that some IQ gains disappear exactly because the interventions cease, continuing interventions like Head Start have showed that the IQ gains then remain.

In a re-analysis of adoption data from, reported that the IQ gains that result from being adopted into high socioeconomic-status homes do not produce gains in g, but only in non-g factors. Jensen also reported that the g factor scores of the adopted children reflected the socioeconomic level of their biological parents, not their adopted parents. This is consistent with Jensen's theory that g is the predominant genetic component of IQ scores; see Spearman's hypothesis below from the relationship between g and racial difference in IQ.

Only shared environmental effects captured in heritability studies disappear in adulthood; more extreme environmental deprivation may likely have a lasting impact on IQ in adults. Heritability only tells us what is the contribution of genes to variation in a trait, not what it could be. Thus, heritability measures in the U.S. population cannot be extrapolated to populations in developing nations.

Biased older studies?
Stoolmiller (1999) found that the range restriction of family environments that goes with adoption, that adopting families tend to be more similar on for example SES than the general population, means that role of the shared family environment have been understimated in previous studies. Corrections for range correction applied to adoption studies indicate that SE could account for as much as 50% of the variance in IQ. However, the effect of restriction of range on IQ for adoption studies was examined by Matt McGue and colleagues, who write that "restriction in range in parent disinhibitory psychopathology and family SES had no effect on adoptive-sibling correlations [in] IQ".

According to Eric Turkheimer and colleages (2003), not using an adoption study, the heritability of IQ among young children is lower for poor families, meaning that for poor children environmental influences play a greater role than genetics, which has a contribution close to zero. They suggest that the role of shared environmental factors may have been underestimated in older studies which often only studied affluent middle class families.

Maternal (foetal) environment
A meta-analysis, by Devlin and colleages in Nature (1997), of 212 previous studies evaluated an alternative model for environmental influence and found that it fits the data better than the 'family-environments' model commonly used. The shared maternal (foetal) environment effects, often assumed to be negligible, account for 20% of covariance between twins and 5% between siblings, and the effects of genes are correspondingly reduced, with two measures of heritability being less than 50%. They argue that the shared maternal environment may explain the striking correlation between the IQs of twins, especially those of adult twins that were reared apart.

Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of hertiability is not substantially different than previous reports and that their conclusions regarding prenatal effects stands in contradiction to many previous reports. They write that: "Chipuer et al. and Loehlin conclude that the postnatal rather than the prenatal environment is most important. The Devlin et al. (1997a) conclusion that the prenatal environment contributes to twin IQ similarity is especially remarkable given the existence of an extensive empirical literature on prenatal effects. Price (1950), in a comprehensive review published over 50 years ago, argued that almost all MZ twin prenatal effects produced differences rather than similarities. As of 1950 the literature on the topic was so large that the entire bibliography was not published. It was finally published in 1978 with an additional 260 references. At that time Price reiterated his earlier conclusion (Price, 1978). Research subsequent to the 1978 review largely reinforces Price’s hypothesis (Bryan, 1993; Macdonald et al., 1993; Hall and Lopez-Rangel, 1996; see also Martin et al., 1997, box 2; Machin, 1996)."

The Dickens and Flynn model
Dickens and Flynn (2001) argue that the arguments regarding the disappearance of the shared family environment should apply equally well to groups separated in time. This is contradicted by the Flynn effect. Changes here have happened to quickly to be explained by genetics. This paradox can be explained by observing that the measure "heritability" includes both a direct effect of the genotype on IQ and also indirect effects where the genotype changes the environment, in turn affecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but feedback loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decay over time unless the stimulus continues (the model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects). The Flynn effect can be explained by a generally more stimulating environment for all people. The authors suggest that programs aiming to increase IQ would be most likely to produce long-term IQ gains if they taught children how to replicate outside the program the kinds of cognitively demanding experiences that produce IQ gains while they are in the program and motivate them to persist in that replication long after they have left the program. Regarding the arguments that high IQ people can seek out stimulating environments to increase their IQ, has argued that while it's possible to substantially increase IQ through cultural stimulation, the IQ gains are "hollow" with respect to g, and that g itself appears to be a wholly biological variable, and not something that has proven amenable to cultural or psychological manipulation. However, Dickens and Flynn, in the above and later papers, have discussed Jensen's model and rejected it, for example arguing that it implies that the Flynn effect is substantially due to genetic factors.

Spearman's hypothesis
IQ tests contain one or more sets of test questions of different varieties. Individually administered tests often are composed of subtests that have different homogeneous item contents. The mean Black-White difference varies considerably across tests with different contents. For example, the BW gap is larger on tests that require the recall of a series of digits in reverse order than on tests that require the recall of a series of digits in forward order. Across a large number of test, the standardized mean Black-White gap varies from near zero to over one standard deviation. According to, "this variation between tests in the size of the standardized mean W-B difference is not explainable in terms of test bias or in terms of differences in types of item content or other formal or superficial characteristics of the tests."

The English psychologist Charles Spearman, in his 1904 book, General Intelligence - Objectively Determined and Measured, described his two-factor theory of intelligence, using statistics. The theory is still used today by researches such as Jensen. It states that individual differences in the general intelligence factor, g, and its various biological correlates (e.g., the volume of gray matter in the frontal cortex) are partly caused by genetic differences between individuals. g has the highest measured heritability of any cognitive ability factor. Jensen formulated a hypothesis now referred to as Spearman's hypothesis which states that the degree of difference between black and white cognitive test scores will be correlated with the degree to which the test measures g (called the test's g-loading). Spearman's hypothesis has a strong form, which says that all test-score differences can be traced to g, and a weak form, which claims that some but not all differences are due to g.

Jensen reported that black-white cognitive test score differences and test g-loadings correlate with a correlation coefficient of 0.6, and concluded that the weak form of Spearman's hypothesis was thus confirmed. Jensen's study combined scores on 149 psychometric tests obtained from 15 independent samples totaling 43,892 Blacks and 243,009 Whites.

The magnitude of a test's g-loading is a positively correlated with a number of other variables:
 * heritability of the test score
 * magnitude of inbreeding depression of the test score
 * magnitude of heterosis
 * head size
 * neurophysiological variables, such as average evoked potential (AEP) habituation and complexity and glucose metabolic rate measured by PET scan
 * average reaction time in elementary cognitive tests
 * the size of the White-Black difference in test scores

Paleontologist Stephen Jay Gould rejected that idea that IQ is measures "general intelligence"; (or g). Gould's writings about g are disputed by many scholars, including statistician David J. Bartholomew and Arthur Jensen. According to "Virtually all present day researchers in psychometrics now accept as a well established fact that individual differences in all complex mental tests are positively correlated, and that a hierarchical factor model, consisting of a number of group factors dominated by g at the apex (or the highest level of generality), is the best representation of the correlational structure of mental abilities."

have reanalyzed the data from several previous studies that used the statistical method invented by Jensen (the method of correlated vectors) with a more recent and improved method (multigroup confirmatory factor analysis). "On the basis of the present, as well as other results (Dolan, 2000), we are convinced that the Spearman correlation cannot be used to demonstrate the importance of g in b-w differences with any confidence." and "It is possible that the analysis of all available data sets (perhaps using an appropriate meta-analytic procedure) will demonstrate that a model incorporating the weak version of Spearman's hypothesis provides the best description of the data. However, until this work is undertaken, we cannot accept Spearman's hypothesis as an "empirically established fact"

Similarly, in a group paper ''Famous artefacts: Spearman's hypothesis. Author's reply'' researchers reported that positive correlations predicted by Spearman's hypothesis are likely only psychometric artefacts which also arise with measures which have nothing to do with 'general ability', for example, the number of toys or books a child has. In fact, these positive correlations will arise with any set of moderately correlated random data, once the sample is split into high and low groups. Zack Z. Cernovsky has said that "Hasty and eager acceptance of weak, biased, and unrepresentative data as scientific evidence of genetically based and relatively immutable racial differences in human potential amounts to psychological warfare on oppressed racial groups." and that it "causes major psychological harm to millions of black children and adults (with respect to self-esteem, career expectations, interracial relationships, etc.".

Gene-environment interactions
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). compared correlations between academic achievement and family environment. They reported 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,, , and , , ) reported 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 estimated the genetic architecture for Black and White siblings. They reported that the best-fitting model for the source of differences between and within races was the same: both genetic and environmental factors. (p. 465) reanalyzed a subset of this data. This analysis reported 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.

using differential heritabilities among Blacks and Whites and later using inbreeding depression calculated in Japan reported 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.

See also the table below for counter-arguments.

Null hypothesis
Discussion of alternative explanations in science, including alternative explanation of race differences in average IQ, often make reference to the concept of a null hypothesis against which an alternative hypothesis is being tested.

For example, when testing the hypothesis that genetic factors contribute to the Black-White gap, the null hypothesis according to Loring Brace is:

"'the same level of intellectual capability ought to have evolved in all human groups' (Brace, 1999a)"

Loring Brace writes, "Jensen, however, has labeled this the 'egalitarian fallacy,' adding that it is 'gratuitous' and 'scientifically unwarranted' (Jensen, 1980:370)".

Jensen responds, saying that "Brace's ad hominem criticism and nihilistic stance regarding key concepts in my book (Jensen 1998, 1999), particularly the g factor and race, as I have carefully defined these terms, can serve only one useful purpose: It gives present-day readers a view of one of the remote outposts of the 1970s style of attack by the ideologically committed opponents of my position 30 years ago."

Jensen states the proper null hypothesis should be the existence of differences along the same lines as those that exist within groups (Jensen, 1998:444): "the distribution of B-W differences in g results from the same mix of genetic and nongenetic factors that cause the distribution of individual differences in g within each group. Thus group differences in g are viewed simply as aggregated individual differences."

Another description of comparing alternative hypotheses relates to Bayesian probability, relating to the concept of a prior probability. According to David Rowe, "in science, viable, alternative hypotheses are ideally given equal Bayesian prior weights;... researchers should regard the prior probability of a genetic hypothesis being true as about the same as that of an environmental hypothesis being true."

Arguments against the genetic explanation
Although the vast majority of researchers who support a primarily environmental explanation for the test score gap between races acknowledge that abilities related to high performance on tests may be heritable for individuals they generally do not agree that this idea makes sense when comparing large groups or races. Many potential flaws in research that supports the genetic hypothesis have been named and there are objections to all the arguments raised above (see table below).

Nisbett (2005) argues that the most direct sort of evidence relates to the influence of European ancestry on Black intelligence. U.S. “Black” populations contain as much as 30% European genes. This means that an individual who is identified as Black could have anywhere from 100% African ancestry to mostly European ancestry (true of as much as 15% of some U.S “Black” subpopulations; Herskovits, 1930). This allows us to identify the extent to which percentage African ancestry, variously assessed, is associated with IQ. Such assessments include studies on skin color, self-reported ancestry, European blood groups, and studies on mixed race children. Nisbett states: "The most directly relevant research concerns degree of European ancestry in the Black population. 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." 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."

Another recent theory hypothesizes that fluid cognition (gF') may be separable from general intelligence, and that gF' may be very susceptible to environmental factors, in particular early childhood stress. Some IQ tests, especially those used with children, are poor measures of gF', which means that the effect of the environment on intelligence regarding racial differences, the Flynn effect, early childhood intervention, and life outcomes may have been underestimated in many studies. The article has received numerous peer commentaries for and against.

Many studies that attempt to test for heritability find results that do not support the genetic hypothesis. They include studies on IQ and skin color, self-reported European ancestry, children in post WWII Germany born to black and white American soldiers, blood groups, and mixed-race children born to either a black or a white mother. Many intervention and adoption studies also find results that do not support the genetic hypothesis. Non-hereditarians have argued that these are direct tests of the genetic hypothesis and of more value than indirect variables, such as skull size and reaction time. Hereditarians argue that these studies are flawed due to their age, lack of replication, problems with their sample population, or that they do in fact support the genetic hypothesis.

, 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". 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."

Comparison of explanations
The Black-White IQ gap in the U.S. may be explained by a variety of explanations on various axes of dispute. There are questions as to the magnitude, direction and causes of gaps, as well as questions regarding the fundamental assumptions of how to frame the question.

This table includes arguments put forward by a wide variety of sources on a wide variety of positions, and also includes some rebuttal and counter-rebuttal on those arguments. Neither column represents a single major point of view.

Possible explanations for how genetic differences could evolve
There are two mainstream theories of the evolution of contemporary humans. The Recent single-origin hypothesis proposes that modern humans evolved in Africa and later replaced hominids in other parts of the world. The multiregional hypothesis proposes that modern humans evolved to some degree from independent hominid populations. An emerging synthesis theory proposes that the genes of contemporary human are predominantly descended from a recent African origin, but that interbreeding with other hominids may have contributed genes to local populations. speculate that "as much as 80% of the nuclear genome is significantly affected by assimilation from archaic humans (i.e., 80% of loci may have some archaic admixture, not that the human genome is 80% archaic)."

Populations within continents are more closely related to one another than to populations on other continents. Thus, to the extent that racial labels correspond to ancient ancestry, racial groups (especially in the U.S.) are statistically distinguishable on the basis of genetics.

The Imperial examination system in China and similar systems in other East Asians nations have been proposed as an explanation for the higher average IQ, compared for example with the caste system in India which made if much more difficult for the intelligent but poor to gain SES. Celibacy for priests have been invoked as an explanation for claimed lower IQ in Catholic countries, although this also seems to be contradicted by the equal IQ in northern and southern Europe. The earlier mentioned comparative European IQ study reported that there was a larger variation in IQ scores in southern Europe. Possible explanations for the earlier mentioned difference for this include sample selection, larger environmental differences affecting IQ scores between urban and rural areas in southern Europe at the time of the test (1981), and/or that northern Europe became socially stratified later in history, causing less genetic variation in IQ.

Rushton has proposed a controversial theory in his book Race, Evolution and Behavior. He argues that r/K selection theory is applicable to humans and explains racial differences in intelligence, as well as claimed differences in many other traits. The theory has been severely criticized using various arguments.

Constant persecutions favoring a high IQ have been proposed as an explanation for the higher average Ashkenazi IQ, but other persecuted groups like the Romani do not score highly on IQ tests. Another theory suggests that there was selective breeding for Talmudic scholarship, but this seems unlikely to have been important because there weren't very many professional rabbis. A selective force that only affects a tiny fraction of the population can never be strong enough to cause important evolutionary change in tens of generations. A more plausible, but difficult to evaluate without detailed demographic information, variant of this is that achievement in Talmudic scholarship had high status and that rich families therefore preferred to marry their daughters to males who excelled in this. Yet another explanation, according to a 2005 study, the most likely, is that they mostly worked jobs in which increased IQ strongly favored economic success, in contrast with other populations, who were mostly peasant farmers. (See "Natural History of Ashkenazi Intelligence")

Significantly genetic view

 * Thirty Years of Research on Race Differences in Cognitive Ability J. Philippe Rushton & Arthur R. Jensen
 * Black-White-East Asian IQ differences at least 50% genetic, scientists conclude in major law journal

Significantly environmental view

 * Heredity, Environment, and Race Differences in IQ Richard E. Nisbett
 * Genetic Differences and School Readiness Dickens, William T. The Future of Children - Volume 15, Number 1, Spring 2005, pp. 55-69
 * Race and IQ: Molecular Genetics as Deus ex Machina Richard S. Cooper
 * Espectic, Friday, February 18th, 2005 Two book reviews, by Paul R. Gross and Alondra Oubre, of Sarich’s and Miele’s book, Race: The Reality of Human Differences.