Heritability of IQ

Heritability of IQ

Study of the heritability of IQ is a controversial field of research that includes biology, psychology, philosophy, sociology and anthropology. Heritability is a measure of the contribution of genes to the variation of a phenotype on a given group in a specific environment."Commentary: Heritability estimates—long past their sell-by date" (2006), Steven Rose. International Journal of Epidemiology Volume 35, Number 3 pp. 525-527 doi|10.1093/ije/dyl064 ] Because heritability estimates can be high in situations where environments are known to be relatively uniform, or low when the amount of genetic variation in subjects is low, estimates for heritability are only applicable to the population being studied.

Throughout the developed world, the heritability of IQ is around three quartersFact|date=June 2008; the majority of the heritable variance in IQ appears to be carried by the general intelligence factor (or "g"). IQ is a polygenic trait under normal circumstances, though destructive mutation of individual genes associated with development can severely affect intelligence (for example see Phenylketonuria)

Methods and results

Heritability Calculations

:"See also: Heritability"

Heritability is defined as the proportion of variance in a trait which is attributable to genes within a defined population in a specific environment. Heritability takes a value ranging from 0 to 1, with a heritability of 1 indicating that all variation in the trait in question is genetic in origin, while a heritability of 0 indicates that "none" of the variation is genetic. The heritability of many traits can be considered primarily genetic under similar environmental backgrounds, for example Visscher "et al." (2006) found that adult height has a heritability estimated at 0.80, when a relatively uniform environmental background is present, to control for environment the study only looked at the contribution of heritability to variation within families "...one can never be sure that the estimates are correct, because nature and nurture can be confounded without one knowing it. The authors got around this problem by comparing the similarity between relatives as a function of the exact proportion of genes that they have in common, looking only within families." [Visscher PM, Medland SE, Ferreira MAR, Morley KI, Zhu G, et al. (2006) Assumption-free estimation of heritability from genome-wide identity-by-descent sharing between full siblings. PLoS Genet 2(3): e41.] Other traits have low heritabilities, which indicate a large relative environmental influence, for example in a twin study the heritability of depression in men was shown to be 0.29, while it was 0.42 for women in the same study. [Kendler KS, Gatz M, Gardner CO, Pedersen NL (2006). " [http://ajp.psychiatryonline.org/cgi/reprint/163/1/109.pdf A Swedish National Twin Study of Lifetime Major Depression] American Journal of Psychiatry, 163(1)": 109-14.]

Heritability for a trait is calculated by measuring how strongly traits covary in people of a given genetic and environmental similarity; the most common method is to consider identical twins reared apart, as any difference which exists between such twin pairs can only be attributed to the environment. In terms of correlation statistics, this means that theoretically the correlation of tests scores between monozygotic twins would be 1.00 if genetics alone accounted for variation in IQ scores; likewise, siblings and dizygotic twins share half of their alleles, and the correlation of their scores would be 0.50 if IQ were affected by genes alone. Practically, however, the upper bound of these correlations are given by the reliability of the test, which tends to be 0.90 to 0.95 for typical IQ tests [Jensen, Arthur (1998). The g Factor: The Science of Mental Ability. Westport, Connecticut: Praeger Publishers] Thus, the actual heritability of IQ will tend to be slightly higher than attained by estimates derived from studies of monozygotic twins, though this effect is small.

In the case of the inheritance of IQ or a certain degree of giftedness, the relatives of probands with a high IQ exhibit a comparably high IQ with a much higher probability than the general population. Bouchard and McGue (1981) have reviewed such correlations reported in 111 original studies in the United States.cite journal |author=Bouchard TJ, McGue M |title=Familial studies of intelligence: a review |journal=Science |volume=212 |issue=4498 |pages=1055–9 |year=1981 |pmid=7195071 |doi=] 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. From such data the heritability of IQ has been estimated at anywhere between 0.40 and 0.80 in the United States. The reason for this wide margin appears to be that the heritability of IQ rises through childhood and adolescence, peaking at 0.68 and 0.78 in adults, leaving the overwhelming majority of IQ differences between individuals to be explained genetically. [cite journal
last = Neisser et al.
first = Ulric
coauthors = Boodoo, Gwyneth; Bouchard, Thomas J. Jr.; Boykin, A. Wayde, Brody, Nathan; Ceci, Stephen J.; Halpern, Diane F.; Loehlin, John C.; Perloff, Robert; Sternberg, Robert J.; Urbina, Suzanna
journal = American Psychologist
year = 1996
volume = 51(2)
pages = 77–101
title = Intelligence: Knowns and Unknowns
doi = 10.1037/0003-066X.51.2.77

The finding of rising heritability with age is counterintuitive; it is reasonable to expect that genetic influences on traits like IQ should become less important as one gains experiences with age. However, that the opposite occurs is well documented. According to work by Robert Plomin, [R. Plomin et al.Behavioral Genetics (4th edn ed.), Worth Publishers (2001).] heritability estimates calculated on infant samples are as low as 20%, rising to around 40% in middle childhood, and ultimately as high as 80% in adult samples in the United States. This suggests that the underlying genes actually express themselves by affecting a person's predisposition to build, learn, and develop mental abilities throughout the lifespan.

Some studies find the heritability of IQ around 0.5 but the studies show ranges from 0.4 to 0.8. [cite journal
author = R. Plomin, N. L. Pedersen, P. Lichtenstein and G. E. McClearn
year = 1994
month = 05
title = Variability and stability in cognitive abilities are largely genetic later in life
journal = Behavior Genetics
volume = 24
issue = 3
url = http://www.springerlink.com/content/t0844nw244473143/
accessdate = 2006-08-06
doi = 10.1007/BF01067188
pages = 207

There are a number of points to consider when interpreting heritability:
*A high heritability does not mean that the environment has no effect 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. [Cite web
title=Intelligence: Knowns and Unknowns
accessmonthday=August 6 |accessyear=2006
date=August 7, 1995
author=Neisser "et al."
publisher=Board of Scientific Affairs of the American Psychological Association
*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. [Cite web
title=Intelligence: Knowns and Unknowns
accessmonthday=August 6 |accessyear=2006
date=August 7, 1995
author=Neisser "et al."
publisher=Board of Scientific Affairs of the American Psychological Association
] Another example is Phenylketonuria which previously caused mental retardation for everyone who had this genetic disorder. Today, this can be prevented by 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. [Cite web
title=Intelligence: Knowns and Unknowns
accessmonthday=August 6 |accessyear=2006
date=August 7, 1995
author=Neisser "et al."
publisher=Board of Scientific Affairs of the American Psychological Association
*Even in developed nations, high heritability of a trait within a given group has no necessary implications for the source of a difference between groups. [See: "Ethnic Differences in Children's Intelligence Test Scores: Role of Economic Deprivation, Home Environment, and Maternal Characteristics"] [Cite web
title=Intelligence: Knowns and Unknowns
accessmonthday=August 6 |accessyear=2006
date=August 7, 1995
author=Neisser "et al."
publisher=Board of Scientific Affairs of the American Psychological Association

Developing nations

:"See also: Health and intelligence"Almost all studies on heritability have been in the developed world, mostly in the United States. In developing nations there are many environmental factors affecting IQ which are much less important in developed nations. Examples include nutrition, diseases, environmental toxins, and health care. This likely affects heritability.

Family environment

In the developed world, nearly all traits show that, contrary to expectations, environmental effects actually cause non-related children raised in the same family ("adoptive siblings") to be as different as children raised in different families (Harris, 1998; Plomin & Daniels, 1987). There are some family effects on the IQ of children, accounting for up to a quarter of the variance. However, by adulthood, this correlation disappears, such that adoptive siblings are not more similar in IQ than strangers. [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9549239&dopt=Citation Genetic and environmental influences on adult intelligence and special mental abilities] . Human Biology, 70, 257–279. 1998] For IQ, adoption studies show that, after adolescence, adoptive siblings are no more similar in IQ than strangers (IQ correlation near zero), while full siblings show an IQ correlation of 0.6. Twin studies reinforce this pattern: monozygotic (identical) twins raised separately are highly similar in IQ (0.86), more so than dizygotic (fraternal) twins raised together (0.6) and much more than adoptive siblings (~0.0). [Plomin "et al." (2001, 2003)]

The American Psychological Association's report "Intelligence: Knowns and Unknowns" (1995) 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. Do differences between children's family environments (within the normal range) produce differences in their intelligence test performance? The problem here is to disentangle causation from correlation. 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 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. It also stated "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 re-mains possible that, across the full range of income and ethnicity, between-family differences have more lasting consequences for psychometric intelligence." [Cite web |url=http://www.lrainc.com/swtaboo/taboos/apa_01.html |title=Intelligence: Knowns and Unknowns |accessmonthday=August 6 |accessyear=2006 |date=August 7, 1995 |author=Neisser "et al." |publisher=Board of Scientific Affairs of the American Psychological Association]

A study of French children adopted between the ages of 4 and 6 shows the continuing interplay of nature and nurture. The children came from poor backgrounds with IQs that initially averaged 77, putting them near retardation. Nine years later after adoption, they retook the I.Q. tests, and all of them did better. The amount they improved was directly related to the adopting family’s status. "Children adopted by farmers and laborers had average I.Q. scores of 85.5; those placed with middle-class families had average scores of 92. The average I.Q. scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98." [Cite web
title=After the Bell Curve
accessmonthday=August 6 |accessyear=2006
date=July 23, 2006
author=David L. Kirp
publisher=New York Times Magazine

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 underestimated 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. [Stoolmiller, M. (1999). [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10414224&dopt=Citation Implications of the restricted range of family environments for estimates of heritability and nonshared environment in behavior-genetic adoption studies.] Psychological Bulletin, 125, 392-409.] 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". [ [http://dx.doi.org/10.1007/s10519-007-9142-7 SpringerLink Home - Main ] ]

Eric Turkheimer and colleagues (2003), not using an adoption study, included impoverished US families. Results demonstrated that the proportions of IQ variance attributable to genes and environment vary nonlinearly with SES. The models suggest that in impoverished families, 60% of the variance in IQ is 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. [ [http://www.blackwell-synergy.com/doi/abs/10.1046/j.0956-7976.2003.psci_1475.x?cookieSet=1 Socioeconomic status modifies heritability of iq in young children] Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D'Onofrio, Irving I. Gottesman. Psychological Science 14 (6), 623–628. 2003] They suggest that the role of shared environmental factors may have been underestimated in older studies which often only studied affluent middle class families. [ [http://www.connectforkids.org/node/516 New Thinking on Children, Poverty & IQ] November 10, 2003 Connect for Kids]

Maternal (fetal) environment

A meta-analysis, by Devlin and colleagues 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. [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9242404&dopt=Citation The heritability of IQ.] Devlin B, Daniels M, Roeder K. Nature. 1997 Jul 31;388(6641):417-8.]

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. [doi|10.1002/neu.10160] 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). [doi|10.1002/neu.10160]

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 too 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 effecting 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 (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. [William T. Dickens and James R. Flynn, [http://www.apa.org/journals/features/rev1082346.pdf Heritability Estimates Versus Large Environmental Effects:The IQ Paradox Resolved] , "Psychological Review" 2001. Vol. 108, No. 2. 346-369.] [William T. Dickens and James R. Flynn, " [http://www.brookings.edu/views/papers/dickens/20020205.pdf The IQ Paradox: Still Resolved] ," "Psychological Review" 109, no. 4 (2002).]

Regression towards the mean

The heritability of IQ measures the extent to which the IQ of a child is measurably influenced by the IQ its parents. As IQ is a quantifiable phenotype, one can estimate the expected IQ of child using the equation hat y = ar x + h^2 left ( frac{m + f}{2} - ar x ight), where

*hat{y} is the expected IQ of the child,
*ar{x} is the mean IQ of the population to which the parents belong,
*h^2 is the heritability of IQ,
*m and f are the IQs of the mother and father, respectively. [Cite web
title=Estimating the Offspring Phenotype
accessmonthday=August 6 |accessyear=2006
author=Phillip McClean
publisher=Quantitative Genetics

The equation asserts that, on average, the IQ of a child tends to the mean IQ of the population. For instance, if the heritability of IQ is 50% and the mean IQ of a population is 100, then a couple with an average IQ of 120 will, on average, have a child with an IQ of 110. Similarly, a couple with an average IQ of 80 will, on average, have a child with an IQ of 90.

It is noted that the above equation relates only statistical averages and is not deterministic. Furthermore, the equation is a general equation based in the inheritance of genetically-based characteristics (in this case, phenotypes), and so it is implicitly assumed that environmental factors are, for the sake of correctly assessing the genetic contribution to IQ, the same across the population.

Historical Research

As early as 1869, Francis Galton replaced mere speculations by statistical data through his book, "Hereditary Genius":

Highly Gifted Men and the Percentage of their Highly Gifted Male Relatives

(classified by occupation and achievement)

Galton Terman Brimhall Weiss
% % % % "n" (Weiss)
Probands 100 84+ 100 97+ 1972: "1329"
1994: "357"
Fathers 26 41 29 40 "346"
Brothers 47 - 49 49 "220"
Sons 60 64* - 55 "77"
Grandfathers 14 - 9 9 "681"
Uncles 16 - 13 14 "615"
Nephews 23 - - 22 "76"
Grandsons 14 - - - -
Greatgrandfathers 0 - - 4 "1290"
Uncles of the parents 5 - - 5 "1996"
Cousins 16 - 9# 18 "570"
Greatgrandsons 7 - - - -
Cousins of parents - - - 11 "2250"
"+": classified by occupation; 100%, if classified by test
"*": classified only by IQ; classification by occupation gives about 55%; "n = 820".
"#": some cousins were still too young and did not have full opportunity to become distinguished
"-": no data

Sources:" "
  • Francis Galton: Hereditary Genius. London 1869, p. 195. [http://galton.org/books/hereditary-genius/] .
    "100 famous Famous men (n = 43) of science and the percentage of their famous male relatives."
  • M. H. Oden: The fulfillment of promise: 40-year follow-up of the Terman gifted group.
    Genetical Psychology Monographs 77 (1968) 3-93.
    "The mean IQ (transformed to 100;15) of the sample of probands was 146 (n = 724); the cut-off score IQ 137."
  • Dean R. Brimhall: Family resemblances among American men of science.
    The American Naturalist 56 (1922) 504-547; 57 (1923) 74-88, 137-152, and 326-344.
    " In 1915 questionnaires were filled in by 956 distinguished American men of science and their relatives."
  • Volkmar Weiss: Mathematical giftedness and family relationship. European Journal for High Ability 5 (1994) 58-67. [http://www.volkmar-weiss.de/ability.html]
    "Highly gifted males (mean IQ 135 +/- 9) and their relatives in professions and occupational positions, typically associated with an IQ above 123."
  • Despite the differences in methods and societies, there is a notable parallelism in the published statistics. The ITO-method by Li and Sacks (1954) allows from this set of data the estimation of the underlying number of genes and their allele frequencies.

    The inheritance of cognitive deficits

    There are many genetic variants known to cause lower IQ. The number of such mutations already known is in the hundreds. For example, an allele of the gene GDI1 is associated with an IQ below 70.Fact|date=August 2008

    Copy number variation has also been associated with idiopathic learning disability. [cite journal
    title= Subtle chromosomal rearrangements in children with unexplained mental retardation
    doi= 10.1016/S0140-6736(99)03070-6
    year= 1999
    author= Knight, S., et al.
    journal= The Lancet
    volume= 354
    pages= 1676

    There are number of known cases where the homozygotes have severe cognitive deficits and the heterozygotes show a small decrease of IQ. In such cases further alleles are investigated to estimate their influence on IQ. For example, one minor allele of the gene ALDH5A1 is associated with an IQ difference of around 1.5 points.cite journal |author=Plomin R, Turic DM, Hill L, "et al" |title=A functional polymorphism in the succinate-semialdehyde dehydrogenase (aldehyde dehydrogenase 5 family, member A1) gene is associated with cognitive ability |journal=Mol. Psychiatry |volume=9 |issue=6 |pages=582–6 |year=2004 |pmid=14981524 |doi=10.1038/sj.mp.4001441]

    Interindividual (between individuals) differences in learning ability are also known in mice, dogs and other animals, and the achievements of pure strains can be improved by selective breeding.clarifyme In such a way also behaviour genetics is contributing to our knowledge of the inheritance of mental traits. There is an open question to which degree differences of animal behaviour have any meaning for differences in human intelligence.clarifyme

    The Search for Specific Genes

    Many studies attempting to find loci in the genome relating to IQ have had little success. Using several hundreds of people a study of 1842 DNA markers from a high IQ group with an IQ of 160 and a control group with an IQ of 102. The study used a five step inspection process to eliminate false positives. By the fifth step the study could not find a single gene that was related to IQ. [ [http://web.archive.org/http://www.meb.ki.se/education/epi/descriptions/gen_mol_epi/cognitive_paper.pdf A Genome-Wide Scan of 1842 DNA Markers for Allelic Associations With General Cognitive Ability: A Five-Stage Design Using DNA Pooling and Extreme Selected Groups] ] The failure to find a specific gene associated with IQ indicates that cognitive abilities are very complex and are likely to involve several genes (polygenic). Some estimate that as much as 40% of all genes may contribute to IQ. [ [http://www.amazon.com/gp/reader/0525948252/ The race myth p178] ISBN 0452286581] The more genes that contribute to a trait the more the trait will be continuous instead of discrete.

    A recent study did find that a gene called FADS2 along with breastfeeding adds about 7 IQ points to those with the "C" version of the gene. Those with the "G" version see no advantage. [ [http://www.physorg.com/news113505546.html Gene governs IQ boost from breastfeeding] ] cite journal |author=Caspi A, Williams B, Kim-Cohen J, "et al" |title=Moderation of breastfeeding effects on the IQ by genetic variation in fatty acid metabolism |journal= |volume= |issue= |pages= |year=2007 |pmid=17984066 |doi=10.1073/pnas.0704292104]

    There is "a highly significant association" between the CHRM2 gene and intelligence according to a 2006 Dutch family study. The study concluded that there was an association between the CHRM2 gene on chromosome 7 and Performance IQ, as measured by the Wechsler Adult Intelligence Scale-Revised. The Dutch family study used a sample of 667 individuals from 304 families. [cite journal
    last = Gosso
    first = M. F.
    authorlink =
    coauthors = van Belzen M.; de Geus E. J.; Polderman J. C.; Heutink P.; Boomsma D. I.; Posthuma D.
    title = Association between the CHRM2 gene and intelligence in a sample of 304 Dutch families
    journal = Genes, Brain and Behavior
    volume = 5
    issue = 8
    pages = 577-584
    publisher =
    location =
    date = 2006-03-03
    url = http://www3.interscience.wiley.com/journal/118618277/abstract?CRETRY=1&SRETRY=0
    format =
    issn =
    accessdate = 2008-09-03
    ] A similar association was found independently in the "Minnesota Twin and Family Study" (Comings et al. 2003) and by the Department of Psychiatry at the Washington University. [Cite web
    title=Association of CHRM2 with IQ: converging evidence for a gene influencing intelligence.
    author=Dick DM, Aliev F, Kramer J, Wang JC, Hinrichs A, Bertelsen S, Kuperman S, Schuckit M, Nurnberger J Jr, Edenberg HJ, Porjesz B, Begleiter H, Hesselbrock V, Goate A, Bierut L.
    publisher=Department of Psychiatry, Washington University

    Between-group heritability

    The fact that IQ differences between individuals are found to have a genetic component does not imply that mean group-level disparities in IQ must likewise have a genetic basis. An analogy, attributed to Richard Lewontin, illustrates this point: if a random sample of pea seeds (drawn from many varieties) is planted in a plot of good soil, and another random sample of pea seeds is planted in a plot of poor soil, the adult plants will show a great deal of variation in features such as pea size. "Within" each plot, the variation is entirely genetic, since the plants share the same environment. But when one compares the means of the two plots, the variation is entirely environmental, since the between-group genetic differences are not significant. Thus, the mere fact that IQ has a high heritability "within" groups says nothing by itself about the heritability of "between"-group differences.

    Some researchers such as Arthur Jensen maintain that environmental differences between groups are too small to account for between-group IQ differences. Many others, such as Joseph Graves, have argued that as long as social and environmental disparities between these groups exist, it will be impossible to scientifically test whether there are any genetic differences in IQ between the various populations.



    *G. Meisenberg: Genes for intelligence. A review of recent progress. Mankind Quarterly 36 (Winter 2005) 139-164. [http://cat.inist.fr/?aModele=afficheN&cpsidt=17460719 abstract]



    External links

    * [https://genepi.qimr.edu.au/staff/nick_pdf/CV411.pdf The first genome-wide scan for linkage of genes underlying IQ] American Journal of Human Genetics 77 (2005) 318-326 ; identifies intelligence-related genes on chromosome areas 2q and 6p
    *cite journal |author=Dick DM, Aliev F, Kramer J, "et al" |title=Association of CHRM2 with IQ: converging evidence for a gene influencing intelligence |journal=Behav. Genet. |volume=37 |issue=2 |pages=265–72 |year=2007 |pmid=17160701 |doi=10.1007/s10519-006-9131-2

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    • heritability — heritable ► ADJECTIVE ▪ able to be inherited. DERIVATIVES heritability noun heritably adverb …   English terms dictionary

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