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. Intelligence in the normal range is a polygenic trait, meaning that it is influenced by more than one gene, and in the case of intelligence at least 500 genes.
Further, explaining the similarity in IQ of closely related persons
requires careful study because environmental factors may be correlated
with genetic factors. Outside the normal range, certain single gene genetic disorders, such as phenylketonuria, can negatively affect intelligence.
Early twin studies of adult individuals have found a heritability of IQ between 57% and 73%, with some recent studies showing heritability for IQ as high as 80%.
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
years old, continuing at that level well into adulthood. However, poor
prenatal environment, malnutrition and disease are known to have
lifelong deleterious effects. Estimates in the academic research of the heritability of IQ have varied from below 0.5
to a high of 0.8 (where 1.0 indicates that monozygotic twins have no
variance in IQ and 0 indicates that their IQs are completely
uncorrelated).
Eric Turkheimer and colleagues (2003) found that for children of low
socioeconomic status heritability of IQ falls almost to zero.
These results have been challenged by other researchers. IQ
heritability increases during early childhood, but it is unclear whether
it stabilizes thereafter. A 1996 statement by the American Psychological Association gave about 0.45 for children and about .75 during and after adolescence. A 2004 meta-analysis of reports in Current Directions in Psychological Science gave an overall estimate of around 0.85 for 18-year-olds and older. The general figure for heritability of IQ is about 0.5 across multiple studies in varying populations.
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. The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups.
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.
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?"
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. 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.
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. 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 is phenylketonuria which previously caused intellectual disabilities in 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.
- 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.
- 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.
- Contrary to popular belief, two parents of higher IQ will not necessarily produce offspring of equal or higher intelligence. 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. In fact, parents whose IQ is at either extreme are more likely to produce offspring with IQ closer to the mean (or average) than they are to produce offspring with high IQ. 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.
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.
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.
The brain undergoes morphological changes in development which suggests
that age-related physical changes could contribute to this effect.
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.
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" while also highlighting that "much of what is labeled
'hereditary' becomes meaningful only in the context of experience."
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,
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. Judith Rich Harris suggests that this might be due to biasing assumptions in the methodology of the classical twin and adoption studies.
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.
The American Psychological Association's report "Intelligence: Knowns and Unknowns" (1996) asserts the necessity of a certain minimum level of responsible care for normal child development. Environments that are severely deprived, neglectful, or abusive negatively affect various developmental aspects, including intellectual growth. Beyond this minimum threshold, the influence of family experience on child development is contentious. Variables such as home resources and parents' use of language are correlated with children's IQ scores; however, these correlations may be influenced by genetic as well as environmental factors. The extent to which variance in IQ results from differences between families, compared to the varying experiences of different children within the same family, is a subject of debate. Recent twin and adoption studies indicate that the effect of the shared family environment is significant in early childhood but diminishes substantially by late adolescence. These findings suggest that differences in family lifestyles, while potentially important for many aspects of children's lives, have little long-term impact on the skills measured by intelligence tests.
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-gene metabolic disorders can severely affect intelligence. Phenylketonuria is an example, with publications documenting the capacity of treated phenylketonuria to produce a reduction of 10 IQ points on average. 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.
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."
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."
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 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"
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 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.
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 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.
Maternal (fetal) environment
A meta-analysis
by Devlin and colleagues (1997) of 212 previous studies evaluated an
alternative model for environmental influence and found that it fits the
data better than the 'family-environments' model commonly used. The
shared maternal (fetal)
environment effects, often assumed to be negligible, account for 20% of
covariance between twins and 5% between siblings, and the effects of
genes are correspondingly reduced, with two measures of heritability
being less than 50%. They argue that the shared maternal environment may
explain the striking correlation between the IQs of twins, especially
those of adult twins that were reared apart. 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 the genotype on IQ and also indirect effects where the genotype changes the environment, in turn affecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but feedback loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decays over time unless the stimulus continues. This model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects.
The Flynn effect
is the increase in average intelligence test scores by about 0.3%
annually, resulting in the average person today scoring 15 points higher
in IQ compared to the generation 50 years ago.
This effect can be explained by a generally more stimulating
environment for all people.
Some scientists have suggested that such enhancements are due to better
nutrition, better parenting and schooling, as well as exclusion of the
least intelligent people from reproduction. However, Flynn and a group
of other scientists share the viewpoint that modern life implies solving
many abstract problems which leads to a rise in their IQ scores.
Influence of genes on IQ stability
Recent research has illuminated genetic factors underlying IQ stability and change. Genome-wide association studies 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 [age 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 of educational attainment and polygenic
scores of non-transmitted alleles. Although the study deals with
educational attainment and not IQ, these two are strongly linked.
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 50 genetic polymorphisms
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 probably false positives 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 genotyped single nucleotide polymorphisms)
between all pairs of individuals in a sample of unrelated individuals
and then correlates this genetic similarity with phenotypic similarity
across all the pairs. A study using this method estimated that the lower
bounds for the narrow-sense heritability of crystallized and fluid
intelligence are 40% and 51%, respectively. A replication study in an
independent sample confirmed these results, reporting a heritability
estimate of 47%.
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 consider identical twins reared apart, with any similarities that exist between such twin pairs attributed to genotype. 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 on average half alleles
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,
the upper bound of these correlations are given by the reliability of the test, which is 0.90 to 0.95 for typical IQ tests.
If there is biological inheritance
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, researchers 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. 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. In fact, greater variation in IQ scores exists within each ethnic group than between them. 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.
