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 relative contribution of genes to the variation of a phenotype on a given group in a specific environment.^[1] 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 quarters; 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.^[1] 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."^[2] 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.^[3]

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^[4] 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.^[5] 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.^[6]

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,^[7] 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.^[8]

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.^[9]
• 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.^[10] 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.^[11]
• 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.^[12][13]

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 personality 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.^[14] 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).^[15]

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."^[16]

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."^[17]

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.^[18] 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".^[19]

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.^[20] They suggest that the role of shared environmental factors may have been underestimated in older studies which often only studied affluent middle class families.^[21]

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.^[22]

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.^[23] 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).^[24]

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.^[25][26]

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 = \bar x + h^2 \left ( \frac{m + f}{2} - \bar x \right), where

• \hat{y} is the expected IQ of the child,
• \bar{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.^[27]

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)