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For example, childhood exposure to lead, associated with homes in poorer areas[81] causes an average IQ drop of 7 points,[82] and iodine deficiency causes a fall, on average, of 12 IQ points.
[83][84] Such impairments may sometimes be permanent, sometimes be partially or wholly compensated for by later growth.
The first two years of life is the critical time for malnutrition, the consequences of which are often irreversible and include poor cognitive development, educability, and future economic productivity.
[85] The African American population of the United States is statistically more likely to be exposed to many detrimental environmental factors such as poorer neighborhoods, schools, nutrition, and prenatal and postnatal health care.
[86][87] Mackintosh points out that for American blacks infant mortality is about twice as high as for whites, and low birthweight is twice as prevalent.
At the same time white mothers are twice as likely to breastfeed their infants, and breastfeeding is highly correlated with IQ for low birthweight infants.
In this way a wide number of health related factors that influence IQ are unequally distributed between the two groups.
The Copenhagen consensus in 2004 stated that lack of both iodine and iron has been implicated in impaired brain development, and this can affect enormous numbers of people: it is estimated that one-third of the total global population are affected by iodine deficiency.
In developing countries, it is estimated that 40% of children aged four and under suffer from anaemia because of insufficient iron in their diets.
[89] Other scholars have found that simply the standard of nutrition has a significant effect on population intelligence, and that the Flynn effect may be caused by increasing nutrition standards across the world.
[90] James Flynn has himself argued against this view.
[91] Some recent research has argued that the retardation caused in brain development by infectious diseases, many of which are more prevalent in non-white populations, may be an important factor in explaining the differences in IQ between different regions of the world.
[92] The findings of this research, showing the correlation between IQ, race and infectious diseases was also shown to apply to the IQ gap in the US, suggesting that this may be an important environmental factor.
[93] A 2013 meta-analysis by the World Health Organization found that, after controlling for maternal IQ, breastfeeding was associated with IQ gains of 2.19 points.
The authors suggest that this relationship is causal but state that the practical significance of this gain is debatable; however, they highlight one study suggesting an association between breastfeeding and academic performance in Brazil, where "breastfeeding duration does not present marked variability by socioeconomic position.
"[94] Colen and Ramey (2014) similarly find that controlling for sibling comparisons within families, rather than between families, reduces the correlation between breastfeeding status and WISC IQ scores by nearly a third, but further find the relationship between breastfeeding duration and WISC IQ scores to be insignificant.
They suggest that "much of the beneficial long-term effects typically attributed to breastfeeding, per say, may primarily be due to selection pressures into infant feeding practices along key demographic characteristics such as race and socioeconomic status.
"[95] Reichman estimates that no more than 3 to 4% of the black-white IQ gap can be explained by black-white disparities in low birth weight.
[96] Education Several studies have proposed that a large part of the gap can be attributed to differences in quality of education.
[97] Racial discrimination in education has been proposed as one possible cause of differences in educational quality between races.
[98] According to a paper by Hala Elhoweris, Kagendo Mutua, Negmeldin Alsheikh and Pauline Holloway, teachers' referral decisions for students to participate in gifted and talented educational programs were influenced in part by the students' ethnicity.
[99] The Abecedarian Early Intervention Project, an intensive early childhood education project, was also able to bring about an average IQ gain of 4.4 points at age 21 in the black children who participated in it compared to controls.
[78] Arthur Jensen agreed that the Abecedarian project demonstrates that education can have a significant effect on IQ, but also said that no educational program thus far has been able to reduce the black-white IQ gap by more than a third, and that differences in education are thus unlikely to be its only cause.
[100] Rushton and Jensen argue that long-term follow-up of the Head Start Program found large immediate gains for blacks and whites but that these were quickly lost for the blacks although some remained for whites.
They argue that also other more intensive and prolonged educational interventions have not produced lasting effects on IQ or scholastic performance.
[47] Nisbett argues that they ignore studies such as Campbell & Ramey (1994) which found that at the age 12, 87% of black infants exposed to an intervention had IQs in the normal range (above 85) compared to 56% of controls, and none of the intervention-exposed children were mildly retarded compared to 7% of controls.
Other early intervention programs have shown IQ effects in the range of 4–5 points, which are sustained until at least age 8–15.
Effects on academic achievement can also be substantial.
Nisbett also argues that not only early age intervention can be effective, citing other successful intervention studies from infancy to college.
[101] A series of studies by Joseph Fagan and Cynthia Holland measured the effect of prior exposure to the kind of cognitive tasks posed in IQ tests on test performance.
Assuming that the IQ gap was the result of lower exposure to tasks using the cognitive functions usually found in IQ tests among African American test takes, they prepared a group of African Americans in this type of tasks before taking an IQ test.
The researchers found that there was no subsequent difference in performance between the African-Americans and white test takers.
[102][103] Daley and Onwuegbuzie conclude that Fagan and Holland demonstrate that "differences in knowledge between blacks and whites for intelligence test items can be erased when equal opportunity is provided for exposure to the information to be tested".
A similar argument is made by David Marks who argues that IQ differences correlate well with differences in literacy suggesting that developing literacy skills through education causes an increase in IQ test performance.
[105][106] A 2003 study found that two variables — stereotype threat and the degree of educational attainment of children's fathers — partially explained the black-white gap in cognitive ability test scores, undermining the hereditarian view that they stemmed from immutable genetic factors.
[107] Socioeconomic environment Different aspects of the socioeconomic environment in which children are raised have been shown to correlate with part of the IQ gap, but they do not account for the entire gap.
According to a 2006 review, these factors account for slightly less than half of one standard deviation of the gap.
[109] Generally the difference between mean test scores of blacks and whites is not eliminated when individuals and groups are matched on socioeconomic status (SES), suggesting that the relationship between IQ and SES is not simply one in which SES determines IQ.
Rather it may be the case that differences in intelligence, particularly parental intelligence, may also cause differences in SES, making separating the two factors difficult.
[48] Hunt (2010, p. 428) summarises data[clarification needed] showing that, jointly, SES and parental IQ account for the full gap (in populations of young children, after controlling parental IQ and parental SES, the gap is not statistically different from zero).
He argues the SES-linked components reflect parental occupation status, mother's verbal comprehension score and parent-child interaction quality.
Hunt also reviews data showing that the correlation between home environment and IQ becomes weaker with age.
[citation needed] Hart and Risley argue that in welfare, working-class, and professional families, children hear a large disparity in the amount of language (between 13 million and 45 million words) in the age range of 0–3, and that by age 9 these differences led to large differences in child outcomes.
[110] Other research has focussed on different causes of variation within low SES and high SES groups.
[111][112][113] In the US, among low-SES groups, genetic differences account for a smaller proportion variance in IQ than among higher SES populations.
[114] Such effects are predicted by the bioecological hypothesis – that genotypes are transformed into phenotypes through nonadditive synergistic effects of the environment.
[115] Nisbett et al.
(2012) suggest that high SES individuals are more likely to be able to develop their full biological potential, whereas low SES individuals are likely to be hindered in their development by adverse environmental conditions.
The same review also points out that adoption studies generally are biased towards including only high and high middle SES adoptive families, meaning that they will tend to overestimate average genetic effects.
They also note that studies of adoption from lower-class homes to middle-class homes have shown that such children experience a 12–18 pt gain in IQ relative to children who remain in low SES homes.
A 2015 study found that environmental factors (namely, family income, maternal education, maternal verbal ability/knowledge, learning materials in the home, parenting factors (maternal sensitivity, maternal warmth and acceptance, and safe physical environment), child birth order, and child birth weight) accounted for the black-white gap in cognitive ability test scores.
Test bias A number of studies have reached the conclusion that IQ tests may be biased against certain groups.
[117][118][119][120] The validity and reliability of IQ scores obtained from outside the United States and Europe have been questioned, in part because of the inherent difficulty of comparing IQ scores between cultures.
[121][122] Several researchers have argued that cultural differences limit the appropriateness of standard IQ tests in non-industrialized communities.
[123][124] A 1996 report by the American Psychological Association states that intelligence can be difficult to compare across culture, and notes that differing familiarity with test materials can produce substantial difference in test results; it also says that tests are accurate predictors of future achievement for black and white Americans, and are in that sense unbiased.
[48] The view that tests accurately predict future educational attainment is reinforced by Nicholas Mackintosh in his 1998 book IQ and Human Intelligence,[125] and by a 1999 literature review by Brown, Reynolds & Whitaker (1999).
James R. Flynn, surveying studies on the topic, notes that the weight and presence of many test questions depends on what sorts of information and modes of thinking are culturally-valued.
[126] Stereotype threat and minority status Stereotype threat is the fear that one's behavior will confirm an existing stereotype of a group with which one identifies or by which one is defined; this fear may in turn lead to an impairment of performance.
[127] Testing situations that highlight the fact that intelligence is being measured tend to lower the scores of individuals from racial-ethnic groups who already score lower on average or are expected to score lower.
Stereotype threat conditions cause larger than expected IQ differences among groups.
[128] Psychometrician Nicholas Mackintosh considers that there is little doubt that the effects of stereotype threat contribute to the IQ gap between blacks and whites.
A large number of studies have shown that systemically disadvantaged minorities, such as the African American minority of the United States generally perform worse in the educational system and in intelligence tests than the majority groups or less disadvantaged minorities such as immigrant or "voluntary" minorities.
[48] The explanation of these findings may be that children of caste-like minorities, due to the systemic limitations of their prospects of social advancement, do not have "effort optimism", i.e.
they do not have the confidence that acquiring the skills valued by majority society, such as those skills measured by IQ tests, is worthwhile.
They may even deliberately reject certain behaviors that are seen as "acting white."
Research published in 1997 indicates that part of the black-white gap in cognitive ability test scores is due to racial differences in test motivation.
[132] Attempts to replicate studies evincing significant effects of stereotype threat however have not yielded the same results.
In 2004 Sackett et al.
found that eliminating stereotype threat does not eliminate the racial test performance gap, and in 2005 Tyson et al.
found African Americans to have motivation similar to or even better than that of white Americans.
[133][134] Self-affirmation exercises promoted by research scientists such as Geoffrey L. Cohen have not been shown to be effective by attempts to replicate his studies purporting them to be successful.
[135] A 2015 meta-analysis conducted by Flore & Wicherts of studies on the relationship between gender and stereotype threat found the observed estimates to be inflated by publication bias, arguing the true effect to be most likely near zero.
[136] Research into the possible genetic influences on test score differences Ongoing research aims to understand the contribution of genes to differences in intelligence.
Currently there is no non-circumstantial evidence that the test score gap has a genetic component, although some researchers believe that the existing circumstantial evidence makes it plausible to believe that hard evidence for a genetic component will eventually appear.
Growing evidence indicates that environmental factors, not genetic ones, are more important in explaining the racial IQ gap.
Several lines of investigation have been followed in the attempt to ascertain whether there is a genetic component to the test score gap as well as its relative contribution to the magnitude of the gap.
Genetics of race and intelligence Geneticist Alan R. Templeton argues that the question about the possible genetic effects on the test score gap is muddled by the general focus on "race" rather than on populations defined by gene frequency or by geographical proximity, and by the general insistence on phrasing the question in terms of heritability.
[141] Templeton points out that racial groups neither represent sub-species nor distinct evolutionary lineages, and that therefore there is no basis for making claims about the general intelligence of races.
[141] From this point of view the search for possible genetic influences on the black-white test score gap is a priori flawed, because there is no genetic material shared by all Africans or by all Europeans.
Mackintosh (2011) points out that by using genetic cluster analysis to correlate gene frequencies with continental populations it could possibly be the case that African populations had a higher frequency of certain genetic variants that contribute to an average lower intelligence.
Such a hypothetical situation could hold without all Africans carrying the same genes or belonging to a single Evolutionary lineage.
According to Mackintosh, a biological basis for the gap thus cannot be ruled out on a priori grounds.
Intelligence is a polygenic trait.
This means that intelligence is under the influence of several genes, possibly several thousand.
The effect of most individual genetic variants on intelligence is thought to be very small, well below 1% of the variance in g. Current studies using quantitative trait loci have yielded little success in the search for genes influencing intelligence.
Robert Plomin is confident that QTLs responsible for the variation in IQ scores exist, but due to their small effect sizes, more powerful tools of analysis will be required to detect them.
[142] Others assert that no useful answers can be reasonably expected from such research before an understanding of the relation between DNA and human phenotypes emerges.
[87] Several candidate genes have been proposed to have a relationship with intelligence.
[143][144] However, a review of candidate genes for intelligence published in Deary, Johnson & Houlihan (2009) failed to find evidence of an association between these genes and general intelligence, stating "there is still almost no replicated evidence concerning the individual genes, which have variants that contribute to intelligence differences".
[145] In 2001, a review in the Journal of Black Psychology refuted eight major premises on which the hereditarian view regarding race and intelligence is based.
[146] A 2005 literature review article by Sternberg, Grigorenko and Kidd stated that no gene has been shown to be linked to intelligence, "so attempts to provide a compelling genetic link of race to intelligence are not feasible at this time".
Hunt (2010, p. 447) and Mackintosh (2011, p. 344) concurred, both scholars noting that while several environmental factors have been shown to influence the IQ gap, the evidence for a genetic influence has been circumstantial, and according to Mackintosh negligible.
Mackintosh however suggests that it may never become possible to account satisfyingly for the relative contributions of genetic and environmental factors.
The 2012 review by Nisbett et al.
(2012) concluded that "Almost no genetic polymorphisms have been discovered that are consistently associated with variation in IQ in the normal range".
Hunt and several other researchers however maintain that genetic causes cannot be ruled out, and that new evidence may yet show a genetic contribution to the gap.
Hunt concurs with Rushton and Jensen who considered the 100% environmental hypothesis to be impossible.
Nonetheless, Nisbett and colleagues (2012) consider the entire IQ gap to be explained by the environmental factors that have thus far been demonstrated to influence it, and Mackintosh does not find this view to be unreasonable.
Heritability within and between groups An environmental factor that varies between groups but not within groups can cause group differences in a trait that is otherwise 100 percent heritable Twin studies of intelligence have reported high heritability values.
However, these studies are based on questionable assumptions.