Heritability

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Research on the heritability of IQ inquires into the degree of variation in IQ within a population that is due to 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.

Early twin studies of adult individuals have found a heritability of IQ between 57% and 73%,[6] with some recent studies showing heritability for IQ as high as 80%.[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][11]

Although IQ differences between individuals have been shown to have a large hereditary component, it does not follow that disparities in IQ between groups have a genetic basis.[12][13][14][15] The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups.[16][17][18][19][20][21]

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 that is due to genetic variation between individuals in that population.[22] 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?"[23]

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 in origin and a heritability estimate of 0 indicates that none of the variation is genetic. The determination of many traits can be considered primarily genetic under similar environmental backgrounds. For example, a 2006 study found that adult height has a heritability estimated at 0.80 when looking only at the height variation within families where the environment should be very similar.[24] Other traits have lower heritability estimates, which indicate a relatively larger environmental influence. For example, a twin study on the heritability of depression in men estimated it as 0.29, while it was 0.42 for women in the same study.[25]

Caveats

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

  • Heritability measures the proportion of variation in a trait that can be attributed to genes, and not the proportion of a trait caused by genes. Thus, if the environment relevant to a given trait changes in a way that affects all members of the population equally, the mean value of the trait will change without any change in its heritability (because the variation or 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.[26] Thus, even in developed nations, a high heritability of a trait does not necessarily mean that average group differences are due to genes.[26][27] Some have gone further, and used height as an example in order to argue that "even highly heritable traits can be strongly manipulated by the environment, so heritability has little if anything to do with controllability."[28]
  • A common error is to assume that a heritability figure is necessarily unchangeable. The value of heritability can change if the impact of environment (or of genes) in the population is substantially altered.[26] If the environmental variation encountered by different individuals increases, then the heritability figure would decrease. On the other hand, if everyone had the same environment, then heritability would be 100%. The population in developing nations often has more diverse environments than in developed nations. This would mean that heritability figures would be lower in developing nations. Another example is phenylketonuria which previously caused mental retardation for everyone who had this genetic disorder and thus had a heritability of 100%. Today, this can be prevented by following a modified diet, resulting in a lowered heritability.[29]
  • A high heritability of a trait does not mean that environmental effects such as learning are 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 and thus heritability is high.[26]
  • Since heritability increases during childhood and adolescence, and even increases greatly between 16 and 20 years of age and adulthood, one should be cautious drawing conclusions regarding the role of genetics and environment from studies where the participants are not followed until they are adults. Furthermore, there may be differences regarding the effects on the g-factor and on non-g factors, with g possibly being harder to affect and environmental interventions disproportionately affecting non-g factors.[30]
  • Polygenic traits often appear less heritable at the extremes. A heritable trait is definitionally more likely to appear in the offspring of two parents high in that trait than in the offspring of two randomly selected parents. However, the more extreme the expression of the trait in the parents, the less likely the child is to display the same extreme as the parents. At the same time, the more extreme the expression of the trait in the parents, the more likely the child is to express the trait at all. For example, the child of two extremely tall parents is likely to be taller than the average person (displaying the trait), but unlikely to be taller than the two parents (displaying the trait at the same extreme). See also regression toward the mean.[31][32]

Estimates

Various studies have estimated the heritability of IQ to be between 0.7 and 0.8 in adults and 0.45 in childhood in the United States.[26][33][34] It has been found that estimates of heritability increase as individuals age. Heritability estimates in infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.8 in adulthood.[7] The brain undergoes morphological changes in development which suggests that age-related physical changes could contribute to this effect.[35]

A 1994 article in Behavior Genetics based on a study of Swedish monozygotic and dizygotic twins found the heritability of the sample to be as high as 0.80 in general cognitive ability; however, it also varies by trait, with 0.60 for verbal tests, 0.50 for spatial and speed-of-processing tests, and 0.40 for memory tests. In contrast, studies of other populations estimate an average heritability of 0.50 for general cognitive ability.[33]

In 2006, David Kirp, writing in The New York Times Magazine, summarized a century's worth of research as follows, "about three-quarters of I.Q. differences between individuals are attributable to heredity."[36]

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,[37] 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.[34] Judith Rich Harris suggests that this might be due to biasing assumptions in the methodology of the classical twin and adoption studies.[38]

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.[26][37][39][40]

The American Psychological Association's report "Intelligence: Knowns and Unknowns" (1996) states that there is no doubt that normal child development requires a certain minimum level of responsible care. Severely deprived, neglectful, or abusive environments must have negative effects on a great many aspects of development, including intellectual aspects. Beyond that minimum, however, the role of family experience is in serious dispute. There is no doubt that such variables as resources of the home and parents' use of language are correlated with children's IQ scores, but such correlations may be mediated by genetic as well as (or instead of) environmental factors. But how much of that variance in IQ results from differences between families, as contrasted with the varying experiences of different children in the same family? Recent twin and adoption studies suggest that while the effect of the shared family environment is substantial in early childhood, it becomes quite small by late adolescence. These findings suggest that differences in the life styles of families whatever their importance may be for many aspects of children's lives make little long-term difference for the skills measured by intelligence tests.

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. 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.[26][39] 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.[41]

Malnutrition and diseases

Certain single-gene metabolic disorders can severely affect intelligence. Phenylketonuria is an example,[42] with publications demonstrating the capacity of phenylketonuria to produce a reduction of 10 IQ points on average.[43] Meta-analyses have found that environmental factors, such as iodine deficiency, can result in large reductions in average IQ; iodine deficiency has been shown to produce a reduction of 12.5 IQ points on average.[44]

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."[26]

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."[36][45]

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.[46]

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"[47]

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.[48]

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.[49]

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.[50]

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."[51]

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.[30] 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.[52][53][54]

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.[55]

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.[56]

A 2017 King's College London study suggests that genes account for nearly 50 per cent of the differences between whether children are socially mobile or not.[57]

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 –– despite the fact that greater variation in IQ scores exists within each ethnic group than between them.[58] 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.[13][14][20] The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups.[16][18][19][20][21] Growing evidence indicates that environmental factors, not genetic ones, explain the racial IQ gap.[20][21]

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.[13] 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.[59] 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.[60][61][62] Further reviews of both adoption studies and racial admixture studies have also found no evidence for a genetic component behind group-level IQ differences.[63][64][65] 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."[15] 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."[15]

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.[66] 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.[67] In the same period, the educational achievement disparity also diminished.[68] Reviews by Flynn and Dickens, Mackintosh, and Nisbett et al. all accept the gradual closing of the gap as a fact.[67][69]

Notes and references

  1. ↑ Devlin, B.; Daniels, Michael; Roeder, Kathryn (1997). "The heritability of IQ". Nature. 388 (6641): 468–71. Bibcode:1997Natur.388..468D. doi:10.1038/41319. PMID 9242404. S2CID 4313884
  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
  4. ↑ Davies, G.; Tenesa, A.; Payton, A.; Yang, J.; Harris, S. E.; Liewald, D.; Deary, I. J. (2011). "Genome-wide association studies establish that human intelligence is highly heritable and polygenic". Molecular Psychiatry. 16 (10): 996–1005. doi:10.1038/mp.2011.85. PMC 3182557. PMID 21826061
  5. ↑ Association, New Scientist staff and Press. "Found: more than 500 genes that are linked to intelligence". New Scientist. Archived from the original on 2019-12-13. Retrieved 2018-11-29.
  6. ↑ Bouchard, Thomas J.; McGue, Matt (January 2003). "Genetic and environmental influences on human psychological differences". Journal of Neurobiology. 54 (1): 4–45. doi:10.1002/neu.10160. PMID 12486697
  7. ↑ 7.0 7.1 Bouchard, Thomas J. (7 August 2013). "The Wilson Effect: The Increase in Heritability of IQ With Age". Twin Research and Human Genetics. 16 (5): 923–930. doi:10.1017/thg.2013.54. PMID 23919982. S2CID 13747480
  8. ↑ Icenogle, G.; Steinberg, L.; Duell, N.; Chein, J.; Chang, L.; Chaudhary, N.; Di Giunta, L.; Dodge, K. A.; Fanti, K. A.; Lansford, J. E.; Oburu, P.; Pastorelli, C.; Skinner, A. T.; Sorbring, E.; Tapanya, S.; Tirado, L. M.; Alampay, L. P.; Al-Hassan, S. M.; Takash, H. M.; Bacchini, D. (2019). "Adolescents' Cognitive Capacity Reaches Adult Levels Prior to Their Psychosocial Maturity: Evidence for a "Maturity Gap" in a Multinational, Cross-Sectional Sample". Law and Human Behavior. 43 (1): 69–85. doi:10.1037/lhb0000315. PMC 6551607. PMID 30762417
  9. ↑ Eppig, C. (2010). "Parasite prevalence and the worldwide distribution of cognitive ability". Proceedings of the Royal Society of London B: Biological Sciences. 277 (1701): 3801–3808. doi:10.1098/rspb.2010.0973. PMC 2992705. PMID 20591860
  10. ↑ Lutter, C. K.; Lutter, R. (2012). "Fetal and Early Childhood Undernutrition, Mortality, and Lifelong Health". Science. 337 (6101): 1495–1499. Bibcode:2012Sci...337.1495L. doi:10.1126/science.1224616. hdl:1903/24471. PMID 22997328. S2CID 45170027
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  12. ↑ Visscher, Peter M.; Hill, William G.; Wray, Naomi R. (2008). "Heritability in the genomics era – concepts and misconceptions". Nature Reviews Genetics. 9 (4): 255–266. doi:10.1038/nrg2322. PMID 18319743. S2CID 690431
  13. ↑ 13.0 13.1 13.2 Mackenzie, Brian (1984). "Explaining race differences in IQ: The logic, the methodology, and the evidence". American Psychologist. 39 (11): 1214–1233. doi:10.1037/0003-066X.39.11.1214. Archived from the original on 2021-02-02. Retrieved 2021-01-29.
  14. ↑ 14.0 14.1 Nisbett, Richard E.; Aronson, Joshua; Blair, Clancy; Dickens, William; Flynn, James; Halpern, Diane F.; Turkheimer, Eric (2012). "Intelligence: New findings and theoretical developments". American Psychologist. 67 (2): 130–159. doi:10.1037/a0026699. ISSN 1935-990X. PMID 22233090
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  16. ↑ 16.0 16.1 Ceci, Stephen; Williams, Wendy M. (1 February 2009). "Should scientists study race and IQ? YES: The scientific truth must be pursued". Nature. 457 (7231): 788–789. Bibcode:2009Natur.457..788C. doi:10.1038/457788a. PMID 19212385. S2CID 205044224. There is an emerging consensus about racial and gender equality in genetic determinants of intelligence; most researchers, including ourselves, agree that genes do not explain between-group differences.
  17. ↑ Panofsky, Aaron; Dasgupta, Kushan; Iturriaga, Nicole (28 September 2020). "How White nationalists mobilize genetics: From genetic ancestry and human biodiversity to counterscience and metapolitics". American Journal of Physical Anthropology. 175 (2): 387–398.
  18. ↑ 18.0 18.1 Hunt, Earl (2010). Human Intelligence. Cambridge University Press. p. 447. ISBN 978-0-521-70781-7. OL 24384631M – via Open Library.
  19. ↑ 19.0 19.1 Mackintosh, N. J. (2011). IQ and human intelligence (2nd ed.). Oxford: Oxford University Press. pp. 334–338, 344. ISBN 978-0-19-958559-5. OCLC 669754008. OL 25211083M – via Open Library.
  20. ↑ 20.0 20.1 20.2 20.3 Nisbett, Richard E.; Aronson, Joshua; Blair, Clancy; Dickens, William; Flynn, James; Halpern, Diane F.; Turkheimer, Eric (2012). "Group differences in IQ are best understood as environmental in origin"
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  36. ↑ 36.0 36.1 Kirp, David L. (July 23, 2006). "After the Bell Curve". New York Times Magazine. Archived from the original on December 31, 2019. Retrieved August 6, 2006.
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  42. ↑ Robert J. Sternberg; Elena Grigorenko (2002). The general factor of intelligence. Lawrence Erlbaum Associates. pp. 260–261. ISBN 978-0-8058-3675-2.
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  53. ↑ Dickens, William T.; Flynn, James R. (2006). "Black Americans Reduce the Racial IQ Gap: Evidence from Standardization Samples" (PDF). Psychological Science. 17 (10): 913–920. doi:10.1111/j.1467-9280.2006.01802.x. PMID 17100793. S2CID 6593169. Archived (PDF) from the original on 2020-11-05. Retrieved 2020-06-24.
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  68. ↑ Neisser, Ulric (Ed). 1998. The rising curve: Long-term gains in IQ and related measures. Washington, DC, US: American Psychological Association
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