Heritability and caveats
Heritability is aCaveats
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. Thus, even in developed nations, a high heritability of a trait does not necessarily mean that average group differences are due to genes. 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." * 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. 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 isEstimates
Various studies have estimated the heritability of IQ to be between 0.7 and 0.8 in adults and 0.45 in childhood in theShared 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, 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.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. 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.Malnutrition and diseases
Certain single-geneHeritability and socioeconomic status
The APA report "Intelligence: Knowns and Unknowns" (1996) also stated that:"We should note, however, thatlow-income Poverty is the state of having few material possessions or little 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." A study (1999) by Capron and Duyme of French children adopted between the ages of four and six examined the influence ofsocioeconomic status Socioeconomic status (SES) is an economic and sociological combined total measure of a person's work experience and of an individual's or family's economic access to resources and social position in relation to others. When analyzing a family's ...(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 withmiddle-class The middle class refers to a class of people in the middle of a social hierarchy, often defined by occupation, income, education, or social status. The term has historically been associated with modernity, capitalism and political debate. Comm ...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." 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. On the other hand, the effect of this was examined byMatt McGue Matt McGue is an American behavior geneticist and Regents Professor of psychology at the University of Minnesota, where he co-directs the Minnesota Center for Twin and Family Research. Career McGue received his B.A. in psychology (minor: mathe ...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 nIQ" 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. 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. 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. 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." 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. The argument here rests on a strong form ofSpearman's hypothesis Spearman's hypothesis has two formulations. The original formulation was that the magnitudes of the black-white differences on tests of cognitive ability positively correlate with the tests' g-loading.Jensen, A. R. (1985). The nature of the blackâ ..., that the hereditability of different kinds of IQ test can vary according to how closely they correlate to thegeneral intelligence factor The ''g'' factor (also known as general intelligence, general mental ability or general intelligence factor) is a construct developed in psychometric investigations of cognitive abilities and human intelligence. It is a variable that summarizes ...(''g''); both the empirical data and statistical methodology bearing on this question are matters of active controversy. 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. A 2012 study based on a representative sample of twins from theUnited Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ..., 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. 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.
Maternal (fetal) environment
Ameta-analysis A meta-analysis is a statistical analysis that combines the results of multiple scientific studies. Meta-analyses can be performed when there are multiple scientific studies addressing the same question, with each individual study reporting me ...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 A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal develo ...) 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. IQ heritability increases during early childhood, but whether it stabilizes thereafter remains unclear. 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. 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. 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).
Dickens and Flynn model
Dickens and Flynn (2001) argued that the "heritability" figure includes both a direct effect of thegenotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...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 butfeedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...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. This model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects. TheFlynn effect The Flynn effect is the substantial and long-sustained increase in both fluid and crystallized intelligence test scores that were measured in many parts of the world over the 20th century. When intelligence quotient (IQ) tests are initially standa ...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. This 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. Most of the improvements have allowed for better abstract reasoning, spatial relations, and comprehension. 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.
Influence of genes on IQ stability
Recent research has illuminated genetic factors underlying IQ stability and change.Genome-wide association studies In genomics, a genome-wide association study (GWA study, or GWAS), also known as whole genome association study (WGA study, or WGAS), is an observational study of a genome-wide set of genetic variants in different individuals to see if any varian ...have demonstrated that the genes involved in intelligence remain fairly stable over time. Specifically, in terms of IQ stability, "genetic factors mediated phenotypic stability throughout this entire period ge 0 to 16 whereas most age-to-age instability appeared to be due to non-shared environmental influences". These findings have been replicated extensively and observed in the United Kingdom, the United States, and the Netherlands. Additionally, researchers have shown that naturalistic changes in IQ occur in individuals at variable times.
Influence of parents genes that are not inherited
Kong 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 ofeducational attainment Educational attainment is a term commonly used by statisticians to refer to the highest degree of education an individual has completed as defined by the US Census Bureau Glossary. See also *Academic achievement *Academic degree *Bachelor's degree ...andpolygenic A polygene is a member of a group of non-epistatic genes that interact additively to influence a phenotypic trait, thus contributing to multiple-gene inheritance (polygenic inheritance, multigenic inheritance, quantitative inheritance), a type of ...scores of non-transmitted alleles. Although the study deals with educational attainment and not IQ, these two are strongly linked.
Spatial ability component of IQ
Spatial ability has been shown to be unifactorial (a single score accounts well for all spatial abilities), and is 69% heritable in a sample of 1,367 pairs of twins from the ages 19 through 21. Further only 8% of spatial ability can be accounted for by shared environmental factors like school and family. Of the genetically determined portion of spatial ability, 24% is shared with verbal ability (general intelligence) and 43% was specific to spatial ability alone.
Molecular genetic investigations
A 2009 review article identified over 50genetic polymorphism A gene is said to be polymorphic if more than one allele occupies that gene's locus within a population. In addition to having more than one allele at a specific locus, each allele must also occur in the population at a rate of at least 1% to gen ...s that have been reported to be associated with cognitive ability in various studies, but noted that the discovery of small effect sizes and lack of replication have characterized this research so far. Another study attempted to replicate 12 reported associations between specific genetic variants and general cognitive ability in three large datasets, but found that only one of the genotypes was significantly associated with general intelligence in one of the samples, a result expected by chance alone. The authors concluded that most reported genetic associations with general intelligence are probablyfalse positive A false positive is an error in binary classification in which a test result incorrectly indicates the presence of a condition (such as a disease when the disease is not present), while a false negative is the opposite error, where the test result ...s brought about by inadequate sample sizes. Arguing that common genetic variants explain much of the variation in general intelligence, they suggested that the effects of individual variants are so small that very large samples are required to reliably detect them. Genetic diversity within individuals is heavily correlated with IQ. A novel molecular genetic method for estimating heritability calculates the overall genetic similarity (as indexed by the cumulative effects of all genotypedsingle nucleotide polymorphism In genetics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a sufficiently larg ...s) 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%. These findings are compatible with the view that a large number of genes, each with only a small effect, contribute to differences in intelligence.
Correlations between IQ and degree of genetic relatedness
The relative influence of genetics and environment for a trait can be calculated by measuring how strongly traits covary in people of a given genetic (unrelated, siblings, fraternal twins, or identical twins) and environmental (reared in the same family or not) relationship. One method is to consideridentical twin Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...s reared apart, with any similarities that exist between such twin pairs attributed to genotype. In terms ofcorrelation In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics ...statistics, this means that theoretically the correlation of tests scores betweenmonozygotic Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two ...twins would be 1.00 if genetics alone accounted for variation in IQ scores; likewise, siblings anddizygotic Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...twins share on average halfallele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...s and the correlation of their scores would be 0.50 if IQ were affected by genes alone (or greater if there is a positive correlation between the IQs of spouses in the parental generation). Practically, however, theupper bound In mathematics, particularly in order theory, an upper bound or majorant of a subset of some preordered set is an element of that is greater than or equal to every element of . Dually, a lower bound or minorant of is defined to be an element ...of these correlations are given by thereliability Reliability, reliable, or unreliable may refer to: Science, technology, and mathematics Computing * Data reliability (disambiguation), a property of some disk arrays in computer storage * High availability * Reliability (computer networking), a ...of the test, which is 0.90 to 0.95 for typical IQ tests If there isbiological inheritance Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic informa ...of IQ, then the relatives of a person with a high IQ should exhibit a comparably high IQ with a much higher probability than the general population. 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. 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. These numbers are not necessarily static. When comparing pre-1963 to late 1970s data, researches DeFries and Plomin found that the IQ correlation between parent and child living together fell significantly, from 0.50 to 0.35. The opposite occurred for fraternal twins. Every one of these studies presented next contains estimates of only two of the three factors which are relevant. The three factors are G, E, and GxE. Since there is no possibility of studying equal environments in a manner comparable to using identical twins for equal genetics, the GxE factor can not be isolated. Thus the estimates are actually of G+GxE and E. Although this may seem like nonsense, it is justified by the unstated assumption that GxE=0. It is also the case that the values shown below are r correlations and not r(squared), proportions of variance. Numbers less than one are smaller when squared. The next to last number in the list below refers to less than 5% shared variance between a parent and child living apart. Another summary: * Same person (tested twice over time) .85 or above * Identical twins—Reared together .86 * Identical twins—Reared apart .76 * Fraternal twins—Reared together .55 * Fraternal twins—Reared apart .35 * Biological siblings—Reared together .47 * Biological siblings—Reared apart .24 * Biological siblings—Reared together—Adults .24 * Unrelated children—Reared together—Children .28 * Unrelated children—Reared together—Adults .04 * Cousins .15 * Parent-child—Living together .42 * Parent-child—Living apart .22 * Adoptive parent–child—Living together .19
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. 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. The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups. Growing evidence indicates that environmental factors, not genetic ones, explain the racial IQ gap. 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. 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. 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. Further reviews of both adoption studies and racial admixture studies have also found no evidence for a genetic component behind group-level IQ differences. 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." 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." 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. 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. In the same period, the educational achievement disparity also diminished. Reviews by Flynn and Dickens, Mackintosh, and Nisbett ''et al.'' all accept the gradual closing of the gap as a fact. 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. 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. 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.
See also
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Notes and references
Further reading
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External links
* Free Massively Open Online Course on human behavior genetics by Matt McGue of the University of Minnesota, including unit on genetics of human intelligence {{DEFAULTSORT:Heritability Of Iq Human genetics Race and intelligence controversy Factors related to intelligence Medical genetics Behavioural genetics Intelligence quotient Quantitative genetics