Heritability of IQ

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Research on the heritability of intelligence quotient (IQ) inquires into the degree of variation in IQ within a population that is associated with genetic variation between individuals in that population. There has been significant controversy in the academic community about the heritability of IQ since research on the issue began in the late nineteenth century. [1] [2] Intelligence in the normal range is a polygenic trait, meaning that it is influenced by more than one gene, [3] [4] and in the case of intelligence at least 500 genes. [5] Further, explaining the similarity in IQ of closely related persons requires careful study because environmental factors may be correlated with genetic factors. Outside the normal range, certain single gene genetic disorders, such as phenylketonuria, can negatively affect intelligence. [6]

Contents

Early twin studies of adult individuals have found a heritability of IQ between 57% and 73% [7] . IQ goes from being weakly correlated with genetics for children, to being strongly correlated with genetics for late teens and adults. The heritability of IQ increases with the child's age and reaches a plateau at 14–16 [8] years old, continuing at that level well into adulthood. However, poor prenatal environment, malnutrition and disease are known to have lifelong deleterious effects. [9] [10] Estimates in the academic research of the heritability of IQ have varied from below 0.5 [1] to a high of 0.8. [11] Eric Turkheimer and colleagues (2003) found that for children of low socioeconomic status heritability of IQ falls almost to zero. [12] These results have been challenged by other researchers. A 1996 statement by the American Psychological Association gave about 0.45 for children and about .75 during and after adolescence. [13] The general figure for heritability of IQ is about 0.5 across multiple studies in varying populations. [14]

Although IQ differences between individuals have been shown to have a hereditary component, it does not follow that disparities in IQ between groups have a genetic basis. [15] [16] [17] [18] The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups. [19] [20] [21] [22] [23] [24]

Heritability and caveats

Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of variation in a phenotypic trait in a population is explained by genetic variation between individuals in that population. [25] The concept of heritability can be expressed in the form of the following question: "What is the proportion of the variation in a given trait within a population that is not explained by the environment or random chance?" [26]

Estimates of heritability take values ranging from 0 to 1; a heritability estimate of 1 indicates that all variation in the trait in question is genetic and a heritability estimate of 0 indicates that none of the variation is genetic.

There are a number of caveats to consider when interpreting heritability:

Methodology

There are multiple types of studies that are designed to estimate heritability and other variance components, stemming from the field of biometrical genetics. [34] The variance, or more simply the differences in a trait within a population, can be partitioned into specific variance components that are based on particular decomposition models for the phenotype. First pointed out by Ronald Fisher and Sewall Wright, different sources of variance differ in their contribution to the resemblance between types of relatives. [35] : 131 By making certain assumptions about the relationship between genes and environment, the field of quantitative genetics has developed equations that describe the expected correlation between relatives in terms of specific variance components (shared environment, additive genetic, etc.). [35] : 163–167 

One simple case is presented by twin studies. By making assumptions about genetic additivity and sources of environmental variation, researchers can estimate the heritability of a phenotype by comparing the resemblance of twins. [35] : 581  Another method used to estimate heritability is adoption studies. Because adoptive parents and their adoptive children share a common environment, but not genetics, [14] : 80–82  researchers in behavioral genetics have used data from adoptive families to estimate the heritability of IQ. [36] [37] Based on quantitative genetic theory, some researchers have taken a broader approach to analyzing familial resemblance. These researchers typically use path analysis to fit models using larger sets of familiar correlations, incorporating cultural transmission [38] , adoption [39] , other complex living arrangements [40] , and generalized assortative mating. [41] [42] However, with the transition into the postgenomic era, issues with controversial assumptions in previous models analyzing familial resemblance may be superseded by genetic analysis. [15] Methods have developed in genetic analysis to estimate the total heritability of a trait using large datasets with millions of genetic variants. [43] [44]

In the case of IQ, heritability estimates based on direct observation of molecular genetics have been significantly lower (around 10%) than those employing traditional methods (40–80%). [45] [46] This discrepancy has been termed the "missing heritability problem." [45] While researchers such as Robert Plomin and Sophie von Stumm contend that this gap will likely disappear with the acquisition of more genetic data, Eric Turkheimer and Lucas J. Matthews argue that it reveals a deeper set of methodological problem inherent in the concept of heritability itself, which will not be easy to overcome. [45]

Estimates

Although individual family studies that estimate the heritability of IQ vary greatly [a] , most family studies estimate the heritability of IQ in the range from 0.4 to 0.8 [17] and reviews of the literature typically summarize classical family design research with an estimate of 0.5. [45] [46] Issues with the methodology of family studies have long plagued the research field, [48] [49] but many researchers now believe that genome-wide association studies can provide less biased estimates of heritability. [50] [51]

Twin and family research

Twin studies are the most common method used to estimate the heritability of most traits. [52] Because monozygotic twins derive from one fertilized egg, they are largely genetically identical [b] , while dizygotic twins are expected to share 50% of their DNA. [14] : 85–86  Twin studies compare the resemblance of monozygotic twins to the resemblance of dizygotic twins to estimate the heritability of trait using various models. [54]

Twin studies have shown the greater resemblance between monozygotic twins than dizygotic twins. [14] This has lead most researchers in behavioral genetics to conclude that the heritability of IQ is in a range between 0.4 to 0.8. [17] [55] : 143  However, it is not clear if all of the assumptions in twin studies hold true. [56] Critics typically focus on the equal environment assumption, which is the assumption that environmentally caused similarity is the same for both monozygotic and dizygotic twins. [14] : 86  [57] Genetic modeling has found that even in the complete absence of genetic factors for a trait, environmental similarity between monozygotic twins will cause heritability estimators to produce large estimates of genetic heritability. [58] [59] Researchers have produced different analyses of the equal environment assumption, some suggesting that the assumption is untenable for related traits like educational attainment. [60]

Twins reared apart have also been the subject of significant debate. [61] [62] [63] The 2006 edition of Assessing adolescent and adult intelligence by Alan S. Kaufman and Elizabeth O. Lichtenberger reports correlations of 0.86 for identical twins raised together compared to 0.76 for those raised apart and 0.47 for siblings. [64] Theoretically, the correlation between twins reared apart is a direct estimate of heritability, as the environments between twins is not correlated. [65] However, this makes an assumption of uncorrelated environments between twins, which may not be the case. [66] : 26  [67] : 18  For example, some research indicates that twins experience similar education and this causes increased similarity in IQ. [68] : 236–237  [69] : 67  [70] Thomas Bouchard claims that some previous analyses of the data on twins reared apart are an abuse of statistical theory that he calls "psuedoanalysis". [61] : 146–147 

Another study design researchers have used is adoption studies. In theory, "adoption creates sets of genetically related individuals who do not share a common family environment because they were adopted apart". [14] : 80  Adoption studies have broadly found that the IQs of adoptees are more similar to their biological parents than that of their adoptive parents, [71] in tandem with findings that adoption greatly increases IQ. [72] [73] For example, one analysis of the Texas Adoption Project estimated heritability at .78. [37] : 123  Some critics point to issues like assortative mating, range restriction [74] [75] and other complex family and social processes as providing issues in the interpretation of data from adoption studies. [36] [76] [77] Recent studies employing polygenic scores for educational attainment in adoptees have found a more complicated interaction between genes and environments. [78]

More broadly, researchers have analyzed the resemblance between all various familial classes. In 1982, Bouchard and McGue reviewed such correlations reported in 111 original studies in the United States. The mean correlation of IQ scores between monozygotic twins was 0.86, between siblings 0.47, between half-siblings 0.31, and between cousins 0.15. [79] Some more basic model fitting studies are found in the behavioral genetics literature. Loehlin fit a model to the Bouchard and McGue data which estimated broad heritability at .58 from 'direct' methods and .47 from 'indirect' methods. [80] [81] Chipuer et al. analyzed this data using a LISREL model and estimated broad sense heritability as .51. [82] Other models successively add more components. In 1997, Devlin et al. also used these correlations to fit and compare different models and estimated that heritability was less than 50% in their best fitting models, finding that maternal effects were particularly important in their analysis. [1] However, in a series of papers using models based on path analysis, a group of researchers from the University of Hawaii analyzed data collected on American families and estimated genetic heritability ranging from 0.30 to 0.34. [42] [83] [84] A later study by Otto et al. applied the path analysis method to the data collected by Bouchard and McGue and estimated heritability ranging from 0.29 to 0.42. [42] [85]

Molecular genetics

A novel molecular genetic method for estimating heritability calculates the overall genetic similarity (as indexed by the cumulative effects of all genotyped single nucleotide polymorphisms) between all pairs of individuals in a sample of unrelated individuals and then correlates this genetic similarity with phenotypic similarity across all the pairs. A study using this method estimated that the lower bounds for the narrow-sense heritability of crystallized and fluid intelligence are 40% and 51%, respectively. A replication study in an independent sample confirmed these results, reporting a heritability estimate of 47%. [4] These findings are compatible with the view that a large number of genes, each with only a small effect, contribute to differences in intelligence. [86]

Molecular genetic studies have been central to several major scientific advances relevant to heritability. The first is the advent of polygenic scores, which take the thousands of single-nucleotide polymorphisms (SNPs) that influence a given trait to form an estimated "genetic value" or "genetic propensity". [87] Most genome-wide association studies (GWAS) estimate a polygenic score (PGS) and describe its overall statistical contribution to variance for the population in question, i.e. providing a R2 (coefficient of determination) for the polygenic score. [88] : 251  [89] This measures only the predictive power of the genetic variants in the study in question, a future study with larger sample size or more genetic variants tagged may produce a larger amount. [90] Consequently, a given polygenic score only captures "a fraction of SNP-heritability". [91] The second is the concept of SNP heritability, which is the total proportion of phenotypic variance explained by SNPs. [92] [44]

More recent estimates of the heritability of IQ based on single-nucleotide polymorphism, derived from genome-wide association studies (GWAS), put the figure much lower, at around 10%. [45]

Environmental effects

Shared family environment

There are some family effects on the IQ of children, accounting for up to a quarter of the variance. However, adoption studies show that by adulthood adoptive siblings aren't more similar in IQ than strangers, [93] while adult full siblings show an IQ correlation of 0.24. However, some studies of twins reared apart (e.g. Bouchard, 1990) find a significant shared environmental influence, of at least 10% going into late adulthood. [94] Judith Rich Harris suggests that this might be due to biasing assumptions in the methodology of the classical twin and adoption studies. [95]

There are aspects of environments that family members have in common (for example, characteristics of the home). This shared family environment accounts for 0.25-0.35 of the variation in IQ in childhood. By late adolescence it is quite low (zero in some studies). There is a similar effect for several other psychological traits. These studies have not looked into the effects of extreme environments such as in abusive families. [13] [93] [96] [97]

The American Psychological Association's report "Intelligence: Knowns and Unknowns" (1996) asserts the necessity of a certain minimum level of responsible care for normal child development. Environments that are severely deprived, neglectful, or abusive negatively affect various developmental aspects, including intellectual growth. Beyond this minimum threshold, the influence of family experience on child development is contentious. Variables such as home resources and parents' use of language are correlated with children's IQ scores; however, these correlations may be influenced by genetic as well as environmental factors.[ citation needed ] The extent to which variance in IQ results from differences between families, compared to the varying experiences of different children within the same family, is a subject of debate. Recent twin and adoption studies indicate that the effect of the shared family environment is significant in early childhood but diminishes substantially by late adolescence.[ citation needed ] These findings suggest that differences in family lifestyles, while potentially important for many aspects of children's lives, have little long-term impact on the skills measured by intelligence tests.[ citation needed ]

Non-shared family environment and environment outside the family

Although parents treat their children differently, such differential treatment explains only a small amount of non-shared environmental influence.[ citation needed ] One suggestion is that children react differently to the same environment due to different genes. More likely influences may be the impact of peers and other experiences outside the family. [13] [96] For example, siblings grown up in the same household may have different friends and teachers and even contract different illnesses. This factor may be one of the reasons why IQ score correlations between siblings decreases as they get older. [98]

Maternal (fetal) environment

A meta-analysis by Devlin and colleagues (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 (fetal) 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. [1] IQ heritability increases during early childhood, but whether it stabilizes thereafter remains unclear. [1] [ needs update ] These results have two implications: a new model may be required regarding the influence of genes and environment on cognitive function; and interventions aimed at improving the prenatal environment could lead to a significant boost in the population's IQ. [1]

Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of heritability is not substantially different from previous reports and that their conclusions regarding prenatal effects stands in contradiction to many previous reports. [7] 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). [7]

Dickens and Flynn model

Dickens and Flynn (2001) argued that the "heritability" figure 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 decays over time unless the stimulus continues. [99] This model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects.

The Flynn effect is the increase in average intelligence test scores by about 0.3% annually, resulting in the average person today scoring 15 points higher in IQ compared to the generation 50 years ago. [100] This effect can be explained by a generally more stimulating environment for all people. Some scientists have suggested that such enhancements are due to better nutrition, better parenting and schooling, as well as exclusion of the least intelligent people from reproduction. However, Flynn and a group of other scientists share the viewpoint that modern life implies solving many abstract problems which leads to a rise in their IQ scores. [100]

Influence of genes on IQ stability

Genome-wide association studies have demonstrated that the genes involved in intelligence remain fairly stable over time. [101] Specifically, in terms of IQ stability, "genetic factors mediated phenotypic stability throughout this entire period [age 0 to 16], whereas most age-to-age instability appeared to be due to non-shared environmental influences". [102] [103] Additionally, researchers have shown that naturalistic changes in IQ occur in individuals at variable times. [104]

Influence of parents genes that are not inherited

Kong [105] reports that, "Nurture has a genetic component, i.e. alleles in the parents affect the parents' phenotypes and through that influence the outcomes of the child." These results were obtained through a meta-analysis of educational attainment and polygenic scores of non-transmitted alleles. Although the study deals with educational attainment and not IQ, these two are strongly linked. [106]

Influences

Heritability and socioeconomic status

The APA report "Intelligence: Knowns and Unknowns" (1996) also stated that:

"We should note, however, that low-income and non-white families are poorly represented in existing adoption studies as well as in most twin samples. Thus it is not yet clear whether these studies apply to the population as a whole. It remains possible that, across the full range of income and ethnicity, between-family differences have more lasting consequences for psychometric intelligence." [13]

A study (1999) by Capron and Duyme of French children adopted between the ages of four and six examined the influence of socioeconomic status (SES). The children's IQs initially averaged 77, putting them near retardation. Most were abused or neglected as infants, then shunted from one foster home or institution to the next. Nine years later after adoption, when they were on average 14 years old, they retook the IQ tests, and all of them did better. The amount they improved was directly related to the adopting family's socioeconomic status. "Children adopted by farmers and laborers had average IQ scores of 85.5; those placed with middle-class families had average scores of 92. The average IQ scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98." [107] [108]

Stoolmiller (1999) argued that the range of environments in previous adoption studies was restricted. Adopting families tend to be more similar on, for example, socio-economic status than the general population, which suggests a possible underestimation of the role of the shared family environment in previous studies. Corrections for range restriction to adoption studies indicated that socio-economic status could account for as much as 50% of the variance in IQ. [109]

On the other hand, the effect of this was examined by Matt McGue and colleagues (2007), who wrote that "restriction in range in parent disinhibitory psychopathology and family socio-economic status had no effect on adoptive-sibling correlations [in] IQ" [110]

Turkheimer and colleagues (2003) argued that the proportions of IQ variance attributable to genes and environment vary with socioeconomic status. They found that in a study on seven-year-old twins, in impoverished families, 60% of the variance in early childhood IQ was accounted for by the shared family environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse. [12]

In contrast to Turkheimer (2003), a study by Nagoshi and Johnson (2005) concluded that the heritability of IQ did not vary as a function of parental socioeconomic status in the 949 families of Caucasian and 400 families of Japanese ancestry who took part in the Hawaii Family Study of Cognition. [111]

Asbury and colleagues (2005) studied the effect of environmental risk factors on verbal and non-verbal ability in a nationally representative sample of 4-year-old British twins. There was not any statistically significant interaction for non-verbal ability, but the heritability of verbal ability was found to be higher in low-SES and high-risk environments. [112]

Harden, Turkheimer, and Loehlin (2007) investigated adolescents, most 17 years old, and found that, among higher income families, genetic influences accounted for approximately 55% of the variance in cognitive aptitude and shared environmental influences about 35%. Among lower income families, the proportions were in the reverse direction, 39% genetic and 45% shared environment." [113]

In the course of a substantial review, Rushton and Jensen (2010) criticized the study of Capron and Duyme, arguing their choice of IQ test and selection of child and adolescent subjects were a poor choice because this gives a relatively less hereditable measure. [114] The argument here rests on a strong form of Spearman's hypothesis, that the hereditability of different kinds of IQ test can vary according to how closely they correlate to the general intelligence factor (g); both the empirical data and statistical methodology bearing on this question are matters of active controversy. [115] [116] [117]

A 2011 study by Tucker-Drob and colleagues reported that at age 2, genes accounted for approximately 50% of the variation in mental ability for children being raised in high socioeconomic status families, but genes accounted for negligible variation in mental ability for children being raised in low socioeconomic status families. This gene–environment interaction was not apparent at age 10 months, suggesting that the effect emerges over the course of early development. [118]

A 2012 study based on a representative sample of twins from the United Kingdom, with longitudinal data on IQ from age two to age fourteen, did not find evidence for lower heritability in low-SES families. However, the study indicated that the effects of shared family environment on IQ were generally greater in low-SES families than in high-SES families, resulting in greater variance in IQ in low-SES families. The authors noted that previous research had produced inconsistent results on whether or not SES moderates the heritability of IQ. They suggested three explanations for the inconsistency. First, some studies may have lacked statistical power to detect interactions. Second, the age range investigated has varied between studies. Third, the effect of SES may vary in different demographics and different countries. [119]

Heritability and age

Some research from classical family studies (adoption, twin studies) indicates that estimates of heritability increase as individuals age. According to Thomas Bouchard, heritability estimates in infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.8 in adulthood. [120] The brain undergoes morphological changes in development which suggests that age-related physical changes could contribute to this effect. [121]

Implications

Some researchers, especially those that work in fields like developmental systems theory, have criticized the concept of heritability as misleading or meaningless. Douglas Wahlsten and Gilbert Gottlieb argue that the prevailing models of behavioral genetics are too simplistic by not accounting for gene-environment interactions. [122] Stephen Ceci also highlights the issues with this assumption, noting that they were raised by Jane Loevinger in 1943. [55] : 139–141  [123] They assert that the idea of partitioning variance makes no sense when environments and genes interact and argue that such interaction is ubiquitous in human development. [124] [125] They highlight their belief that heritability analysis requires a hidden assumption they call the "separation of causes", which isn't borne out by biological reality or experimental research. [126] Such researchers argue that the notion of heritability gives the false impression that "genes have some direct and isolated influence on traits", rather than another developmental resource that a complex system uses over the course of ontogeny. [124]

Between-group heritability

In the US, individuals identifying themselves as Asian generally tend to score higher on IQ tests than Caucasians, who tend to score higher than Hispanics, who tend to score higher than African Americans. [127] Yet, although IQ differences between individuals have been shown to have a large hereditary component, it does not follow that between-group differences in average IQ have a genetic basis. [16] [17] [23] In fact, greater variation in IQ scores exists within each ethnic group than between them. [127] The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups. [19] [21] [22] [23] [24] Growing evidence indicates that environmental factors, not genetic ones, explain the racial IQ gap. [23] [24] [128]

Arguments in support of a genetic explanation of racial differences in average IQ are sometimes fallacious. For instance, some hereditarians have cited as evidence the failure of known environmental factors to account for such differences, or the high heritability of intelligence within races. [16] Jensen and Rushton, in their formulation of Spearman's Hypothesis, argued that cognitive tasks that have the highest g-load are the tasks in which the gap between black and white test takers is greatest, and that this supports their view that racial IQ gaps are in large part genetic. [129] However, in separate reviews, Mackintosh, Nisbett et al. and Flynn have all concluded that the slight correlation between g-loading and the test score gap offers no clue to the cause of the gap. [130] [131] [132] Further reviews of both adoption studies and racial admixture studies have also found no evidence for a genetic component behind group-level IQ differences. [133] [134] [135] [136] [ page needed ] Hereditarian arguments for racial differences in IQ have been criticized from a theoretical point of view as well. For example, the geneticist and neuroscientist Kevin Mitchell has argued that "systematic genetic differences in intelligence between large, ancient populations" are "inherently and deeply implausible" because the "constant churn of genetic variation works against any long-term rise or fall in intelligence." [18] As he argues, "To end up with systematic genetic differences in intelligence between large, ancient populations, the selective forces driving those differences would need to have been enormous. What's more, those forces would have to have acted across entire continents, with wildly different environments, and have been persistent over tens of thousands of years of tremendous cultural change." [18]

In favor of an environmental explanation, on the other hand, numerous studies and reviews have shown promising results. Among these, some focus on the gradual closing of the black–white IQ gap over the last decades of the 20th century, as black test-takers increased their average scores relative to white test-takers. For instance, Vincent reported in 1991 that the black–white IQ gap was decreasing among children, but that it was remaining constant among adults. [137] Similarly, a 2006 study by Dickens and Flynn estimated that the difference between mean scores of black people and white people closed by about 5 or 6 IQ points between 1972 and 2002, a reduction of about one-third. [138] In the same period, the educational achievement disparity also diminished. [139] Reviews by Flynn and Dickens, Mackintosh, and Nisbett et al. all accept the gradual closing of the gap as a fact. [138] [140] [131] Other recent studies have focused on disparities in nutrition and prenatal care, as well as other health-related environmental disparities, and have found that these disparities may account for significant IQ gaps between population groups. [141] [142] [143] [144] Still other studies have focused on educational disparities, and have found that intensive early childhood education and test preparation can diminish or eliminate the black–white IQ test gap. [145] [146] [147] [148] In light of these and similar findings, a consensus has formed that genetics does not explain differences in average IQ test performance between racial groups. [19] [23]

See also

Footnotes

  1. ↑ For example, individual studies have estimated values as low as "not significantly different from zero" [47] to as high as over .8.
  2. ↑ Strictly speaking, monozygotic twins are not purely genetically identical due to somatic mosaicism and mutations. [53]

References

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  2. ↑ Rose, Steven P R (June 2006). "Commentary: Heritability estimates—long past their sell-by date". International Journal of Epidemiology. 35 (3): 525–527. doi: 10.1093/ije/dyl064 . PMID   16645027.
  3. ↑ Alice Marcus. 2010. Human Genetics: An Overview. Alpha Science section 14.5
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  12. 1 2 Turkheimer, Eric; Haley, Andreana; Waldron, Mary; d'Onofrio, Brian; Gottesman, Irving I. (2003). "Socioeconomic status modifies heritability of iq in young children". Psychological Science. 14 (6): 623–8. doi:10.1046/j.0956-7976.2003.psci_1475.x. PMID   14629696. S2CID   11265284.
  13. 1 2 3 4 5 6 7 8 Neisser, Ulric; Boodoo, Gwyneth; Bouchard, Thomas J. Jr.; Boykin, A. Wade; Brody, Nathan; Ceci, Stephen J.; Halpern, Diane F.; Loehlin, John C.; et al. (1996). "Intelligence: Knowns and unknowns". American Psychologist. 51 (2): 77–101. doi:10.1037/0003-066X.51.2.77.
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