Inheritance of intelligence
Encyclopedia
The study of the heritability of IQ investigates the relative importance of genetics and environment for variation in intelligence quotient
(IQ) in a population. "Heritability
", in this sense, "refers to the genetic contribution to variance
within a population and in a specific environment" There has been significant controversy in the academic community about the heritability of IQ ever since research began in the 19th century.
IQ is a polygenic trait
under normal circumstances according to recent research. However, certain single gene genetic disorder
s can severely affect intelligence, with phenylketonuria
as an example.
Estimates in the academic research of the heritability of IQ have varied from below 0.5 to a high of 0.9. A 1996 statement by the American Psychological Association
gave about .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 .85 for 18-year-olds and older. The New York Times Magazine
has listed about three quarters as a figure held by the majority of studies.
in a trait which is attributable to genotype within a defined population in a specific environment. Heritability takes a value ranging from 0 to 1; a heritability of 1 indicates that all variation in the trait in question is genetic in origin and a heritability 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 heritabilities, which indicate a relatively larger environmental influence. For example, a twin study
on the heritability of depression in men calculated it as 0.29, while it was 0.42 for women in the same study.
. It may seem 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. Heritability measures in infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.8 in adulthood. One proposed explanation is that people with different genes tend to seek out different environments that reinforce the effects of those genes.
A 1994 review in Behavior Genetics based on identical/fraternal twin studies found that heritability is as high as 0.80 in general cognitive ability but it also varies based on the trait, with .60 for verbal tests, .50 for spatial and speed-of-processing tests, and only .40 for memory tests.
In 2006, The New York Times Magazine
listed about three quarters as a figure held by the majority of studies, while a 2004 meta-analysis
of reports in Current Directions in Psychological Science gave an overall estimate of around .85 for 18-year-olds and older.
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). 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. JR 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 the effects of extreme environments such as in abusive families.
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. There is no doubt that such variables as resources of the home and parents' use of language are correlated with children's IQ scores, but such correlations may be mediated by genetic as well as (or instead of) environmental factors. But how much of that variance in IQ results from differences between families, as contrasted with the varying experiences of different children in the same family? Recent twin and adoption studies suggest that while the effect of the shared family environment is substantial in early childhood, it becomes quite small by late adolescence. These findings suggest that differences in the life styles of families whatever their importance may be for many aspects of children's lives make little long-term difference for the skills measured by intelligence tests.
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 were 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.
A study by Nagoshi and Johnson (2005) failed to replicate Turkheimer and colleagues' findings. They 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 and colleagues (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."
Rushton and Jensen (2010) criticized many of these studies for being done on children or adolescents. They argued that heritability increases during childhood and adolescence, and even increases greatly between 16–20 years of age and adulthood, so one should be cautious drawing conclusions regarding the role of genetics from studies where the participants are not adults. Furthermore, the studies typically did not examine if IQ gains due to adoption were on the general intelligence factor
(g). When the studies by Capron and Duyme were re-examined, IQ gains from being adopted into high SES homes were on non-g factors. By contrast, the adopted children's g mainly depended on their biological parents SES, which implied that g is more difficult to environmentally change.
A 2011 study by Tucker-Drob and colleagues reported that at age 2 years, 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.
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.
Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of heritability is not substantially different than previous reports and that their conclusions regarding prenatal effects stands in contradiction to many previous reports. They write that:
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 (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.
using groups of around 100 people investigated 1,842 DNA markers in a high-IQ group and in an average-IQ control group. The study used a five-step replication process to eliminate false positives, and no gene met this rigid criterion for replicability.
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. The more genes that contribute to a trait the more the trait will be continuous instead of discrete. A 2008 study of 500,000 single nucleotide polymorphism
s (SNPs) from 7,089 children did not substantially improve on earlier studies. The study did not find any SNPs that accounted for more than 0.5% of the variance in general intelligence.
Mutations in GDI1
have been linked to X-linked nonspecific mental retardation.
A 2006 Dutch family study found that there was "a highly significant association" between the CHRM2 gene
and intelligence. 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.
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.
However, a 2009 study attempting to replicate this claim instead found no significant association between the CHRM2 gene and intelligence in a Scottish cohort
of 2,091 individuals over age 50, nor an English cohort of 758 non-cognitively-impaired elderly individuals, nor an Australian family-based sample of 1,537 adolescents.
Microcephalin
and ASPM are two genes that are associated with brain development. Mutations in these genes are associated with microcephaly
, and hence they were initially associated with general intelligence. However recent studies have found no association with general cognitive abilities.
STX1A
correlates significantly with intelligence in Williams syndrome
patients.
A 2007 study by Caspi and colleagues found 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. The findings could however not be replicated in two studies.
Copy number variation has also been associated with idiopathic learning disability
.
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 allele
s 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.
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 of their allele
s 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
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 number 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.
Another summary:
, illustrates this point:
Arthur Jensen
agrees that this is technically correct but argues that a high heritability increases the probability that genetics play a role in average group differences.
Heritability
Intelligence quotient
An intelligence quotient, or IQ, is a score derived from one of several different standardized tests designed to assess intelligence. When modern IQ tests are constructed, the mean score within an age group is set to 100 and the standard deviation to 15...
(IQ) in a population. "Heritability
Heritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...
", in this sense, "refers to the genetic contribution to variance
Variance
In probability theory and statistics, the variance is a measure of how far a set of numbers is spread out. It is one of several descriptors of a probability distribution, describing how far the numbers lie from the mean . In particular, the variance is one of the moments of a distribution...
within a population and in a specific environment" There has been significant controversy in the academic community about the heritability of IQ ever since research began in the 19th century.
IQ is a polygenic trait
Quantitative trait locus
Quantitative traits refer to phenotypes that vary in degree and can be attributed to polygenic effects, i.e., product of two or more genes, and their environment. Quantitative trait loci are stretches of DNA containing or linked to the genes that underlie a quantitative trait...
under normal circumstances according to recent research. However, certain single gene genetic disorder
Genetic disorder
A genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....
s can severely affect intelligence, with phenylketonuria
Phenylketonuria
Phenylketonuria is an autosomal recessive metabolic genetic disorder characterized by a mutation in the gene for the hepatic enzyme phenylalanine hydroxylase , rendering it nonfunctional. This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine...
as an example.
Estimates in the academic research of the heritability of IQ have varied from below 0.5 to a high of 0.9. A 1996 statement by the American Psychological Association
American Psychological Association
The American Psychological Association is the largest scientific and professional organization of psychologists in the United States. It is the world's largest association of psychologists with around 154,000 members including scientists, educators, clinicians, consultants and students. The APA...
gave about .45 for children and about .75 during and after adolescence
Adolescence
Adolescence is a transitional stage of physical and mental human development generally occurring between puberty and legal adulthood , but largely characterized as beginning and ending with the teenage stage...
. A 2004 meta-analysis
Meta-analysis
In statistics, a meta-analysis combines the results of several studies that address a set of related research hypotheses. In its simplest form, this is normally by identification of a common measure of effect size, for which a weighted average might be the output of a meta-analyses. Here the...
of reports in Current Directions in Psychological Science gave an overall estimate of around .85 for 18-year-olds and older. The New York Times Magazine
The New York Times Magazine
The New York Times Magazine is a Sunday magazine supplement included with the Sunday edition of The New York Times. It is host to feature articles longer than those typically in the newspaper and has attracted many notable contributors...
has listed about three quarters as a figure held by the majority of studies.
Heritability and caveats
"Heritability" is defined as the proportion of varianceVariance
In probability theory and statistics, the variance is a measure of how far a set of numbers is spread out. It is one of several descriptors of a probability distribution, describing how far the numbers lie from the mean . In particular, the variance is one of the moments of a distribution...
in a trait which is attributable to genotype within a defined population in a specific environment. Heritability takes a value ranging from 0 to 1; a heritability of 1 indicates that all variation in the trait in question is genetic in origin and a heritability of 0 indicates that none of the variation is genetic. The determination of many traits can be considered primarily genetic
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
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 heritabilities, which indicate a relatively larger environmental influence. For example, a twin study
Twin study
Twin studies help disentangle the relative importance of environmental and genetic influences on individual traits and behaviors. Twin research is considered a key tool in behavioral genetics and related fields...
on the heritability of depression in men calculated 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." However, others argue that IQ is highly stable during life and has been largely resistant to interventions aimed to change it long-term and substantially.
- 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 phenylketonuriaPhenylketonuriaPhenylketonuria is an autosomal recessive metabolic genetic disorder characterized by a mutation in the gene for the hepatic enzyme phenylalanine hydroxylase , rendering it nonfunctional. This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine...
which previously caused mental retardation for everyone who had this genetic disorder and thus had a heritability of 100%. Today, this can be prevented by following a modified diet, resulting in a lowered heritability. - 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 16–20 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.
Estimates of the heritability of IQ
Various studies have found the heritability of IQ to be between 0.7 and 0.8 in adults and 0.45 in childhood in the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. It may seem 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. Heritability measures in infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.8 in adulthood. One proposed explanation is that people with different genes tend to seek out different environments that reinforce the effects of those genes.
A 1994 review in Behavior Genetics based on identical/fraternal twin studies found that heritability is as high as 0.80 in general cognitive ability but it also varies based on the trait, with .60 for verbal tests, .50 for spatial and speed-of-processing tests, and only .40 for memory tests.
In 2006, The New York Times Magazine
The New York Times Magazine
The New York Times Magazine is a Sunday magazine supplement included with the Sunday edition of The New York Times. It is host to feature articles longer than those typically in the newspaper and has attracted many notable contributors...
listed about three quarters as a figure held by the majority of studies, while a 2004 meta-analysis
Meta-analysis
In statistics, a meta-analysis combines the results of several studies that address a set of related research hypotheses. In its simplest form, this is normally by identification of a common measure of effect size, for which a weighted average might be the output of a meta-analyses. Here the...
of reports in Current Directions in Psychological Science gave an overall estimate of around .85 for 18-year-olds and older.
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 0.6. Conventional twin studies reinforce this pattern: monozygotic (identical) twinsTwin
A twin is one of two offspring produced in the same pregnancy. Twins can either be monozygotic , meaning that they develop from one zygote that splits and forms two embryos, or dizygotic because they develop from two separate eggs that are fertilized by two separate sperm.In contrast, a fetus...
raised separately are highly similar in IQ (0.86), more so than dizygotic (fraternal) twins
Twin
A twin is one of two offspring produced in the same pregnancy. Twins can either be monozygotic , meaning that they develop from one zygote that splits and forms two embryos, or dizygotic because they develop from two separate eggs that are fertilized by two separate sperm.In contrast, a fetus...
raised together (0.6) and much more than adoptive siblings (~0.0). 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. JR 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 the effects of extreme environments such as in abusive families.
The American Psychological Association
American Psychological Association
The American Psychological Association is the largest scientific and professional organization of psychologists in the United States. It is the world's largest association of psychologists with around 154,000 members including scientists, educators, clinicians, consultants and students. The APA...
's report "Intelligence: Knowns and Unknowns
Intelligence: Knowns and Unknowns
Intelligence: Knowns and Unknowns was a 1995 report issued by a Task Force created by the Board of Scientific Affairs of the American Psychological Association.- Background :...
" (1995) states that there is no doubt that normal child development
Child development
Child development stages describe theoretical milestones of child development. Many stage models of development have been proposed, used as working concepts and in some cases asserted as nativist theories....
requires a certain minimum level of responsible care. Severely deprived, neglectful, or abusive environments must have negative effects on a great many aspects of development, including intellectual aspects. Beyond that minimum, however, the role of family experience is in serious dispute. There is no doubt that such variables as resources of the home and parents' use of language are correlated with children's IQ scores, but such correlations may be mediated by genetic as well as (or instead of) environmental factors. But how much of that variance in IQ results from differences between families, as contrasted with the varying experiences of different children in the same family? Recent twin and adoption studies suggest that while the effect of the shared family environment is substantial in early childhood, it becomes quite small by late adolescence. These findings suggest that differences in the life styles of families whatever their importance may be for many aspects of children's lives make little long-term difference for the skills measured by intelligence tests.
Non-shared family environment and environment outside the family
Although parents treat their children differently, such differential treatment explains only a small amount of non-shared environmental influence. One suggestions 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.Lower heritability with lower socioeconomic status?
The APA report "Intelligence: Knowns and Unknowns" (1995) also stated that:
"We should note, however, that low-incomePovertyPoverty is the lack of a certain amount of material possessions or money. Absolute poverty or destitution is inability to afford basic human needs, which commonly includes clean and fresh water, nutrition, health care, education, clothing and shelter. About 1.7 billion people are estimated to live...
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
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
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
Middle class
The middle class is any class of people in the middle of a societal hierarchy. In Weberian socio-economic terms, the middle class is the broad group of people in contemporary society who fall socio-economically between the working class and upper 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 were 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.
A study by Nagoshi and Johnson (2005) failed to replicate Turkheimer and colleagues' findings. They 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 and colleagues (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."
Rushton and Jensen (2010) criticized many of these studies for being done on children or adolescents. They argued that heritability increases during childhood and adolescence, and even increases greatly between 16–20 years of age and adulthood, so one should be cautious drawing conclusions regarding the role of genetics from studies where the participants are not adults. Furthermore, the studies typically did not examine if IQ gains due to adoption were on the general intelligence factor
General intelligence factor
The g factor, where g stands for general intelligence, is a statistic used in psychometrics to model the mental ability underlying results of various tests of cognitive ability...
(g). When the studies by Capron and Duyme were re-examined, IQ gains from being adopted into high SES homes were on non-g factors. By contrast, the adopted children's g mainly depended on their biological parents SES, which implied that g is more difficult to environmentally change.
A 2011 study by Tucker-Drob and colleagues reported that at age 2 years, 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.
Maternal (fetal) environment
A meta-analysisMeta-analysis
In statistics, a meta-analysis combines the results of several studies that address a set of related research hypotheses. In its simplest form, this is normally by identification of a common measure of effect size, for which a weighted average might be the output of a meta-analyses. Here the...
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.
Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of heritability is not substantially different than 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 genotypeGenotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
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
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
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.
Regression toward the mean
Regression towards the mean is a statistical phenomenon that occurs when an outcome is determined by many independent factors. If an outcome is extreme, then this occurred because most of the independent factors agreed by chance. This is unlikely to occur again so to the next outcome is likely to be less extreme. If IQ is determined by many factors, genetic and/or environmental, then they must mostly agree in the same direction in order to produce an extreme IQ. The child of a person with an extreme IQ is unlikely to have all the factors agree so similarly so the child is on average likely to have a less extreme IQ.Heritability of giftedness
High intelligence is a form of intellectual giftedness. The study of giftedness, whether it has multiple forms, their heritability and their relation to each other is also a subject of major study.The search for specific genes
Many studies attempting to find loci in the genome relating to IQ have had little success. For example, a study by Robert PlominRobert Plomin
Robert Plomin is an American psychologist best known for his work in twin studies and behavior genetics. Plomin has made two of the most important discoveries in that field. First, he has shown the importance of non-shared environment, a term that he coined to refer to the environmental reasons...
using groups of around 100 people investigated 1,842 DNA markers in a high-IQ group and in an average-IQ control group. The study used a five-step replication process to eliminate false positives, and no gene met this rigid criterion for replicability.
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. The more genes that contribute to a trait the more the trait will be continuous instead of discrete. A 2008 study of 500,000 single nucleotide polymorphism
Single nucleotide polymorphism
A single-nucleotide polymorphism is a DNA sequence variation occurring when a single nucleotide — A, T, C or G — in the genome differs between members of a biological species or paired chromosomes in an individual...
s (SNPs) from 7,089 children did not substantially improve on earlier studies. The study did not find any SNPs that accounted for more than 0.5% of the variance in general intelligence.
Mutations in GDI1
GDI1
Rab GDP dissociation inhibitor alpha is a protein that in humans is encoded by the GDI1 gene.Rab GTPases cycles between the cytosolic compartment, where it is bound to a protein called GDI , and the membrane, where it interacts with a receptor, a nucleotide exchange factor, a GAP and probably...
have been linked to X-linked nonspecific mental retardation.
A 2006 Dutch family study found that there was "a highly significant association" between the CHRM2 gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
and intelligence. 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.
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.
However, a 2009 study attempting to replicate this claim instead found no significant association between the CHRM2 gene and intelligence in a Scottish cohort
Cohort (statistics)
In statistics and demography, a cohort is a group of subjects who have shared a particular time together during a particular time span . Cohorts may be tracked over extended periods in a cohort study. The cohort can be modified by censoring, i.e...
of 2,091 individuals over age 50, nor an English cohort of 758 non-cognitively-impaired elderly individuals, nor an Australian family-based sample of 1,537 adolescents.
Microcephalin
Microcephalin
Microcephalin is one of six genes causing primary microcephaly when non-functional mutations exist in the homozygous state...
and ASPM are two genes that are associated with brain development. Mutations in these genes are associated with microcephaly
Microcephaly
Microcephaly is a neurodevelopmental disorder in which the circumference of the head is more than two standard deviations smaller than average for the person's age and sex. Microcephaly may be congenital or it may develop in the first few years of life...
, and hence they were initially associated with general intelligence. However recent studies have found no association with general cognitive abilities.
STX1A
STX1A
Syntaxin-1A is a protein that in humans is encoded by the STX1A gene.- Function :Synaptic vesicles store neurotransmitters that are released during calcium-regulated exocytosis. The specificity of neurotransmitter release requires the localization of both synaptic vesicles and calcium channels to...
correlates significantly with intelligence in Williams syndrome
Williams syndrome
Williams syndrome is a rare neurodevelopmental disorder characterized by a distinctive, "elfin" facial appearance, along with a low nasal bridge; an unusually cheerful demeanor and ease with strangers; developmental delay coupled with strong language skills; and cardiovascular problems, such as...
patients.
A 2007 study by Caspi and colleagues found that a gene called FADS2
FADS2
Fatty acid desaturase 2 also known as delta fatty acid desaturase is an enzyme that in humans is encoded by the FADS2 gene.- Function :Fatty acid desaturase 2 is a member of the fatty acid desaturase gene family...
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. The findings could however not be replicated in two studies.
Copy number variation has also been associated with idiopathic learning disability
Learning disability
Learning disability is a classification including several disorders in which a person has difficulty learning in a typical manner, usually caused by an unknown factor or factors...
.
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 allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s 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.
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. The most common method is to consider identical twins reared apart, with any similarities which exists between such twin pairs attributed to genotype. In terms of correlationCorrelation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
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 of their allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s and the correlation of their scores would be 0.50 if IQ were affected by genes alone. Practically, however, the upper bound
Upper bound
In mathematics, especially in order theory, an upper bound of a subset S of some partially ordered set is an element of P which is greater than or equal to every element of S. The term lower bound is defined dually as an element of P which is lesser than or equal to every element of S...
of these correlations are given by the reliability
Reliability (statistics)
In statistics, reliability is the consistency of a set of measurements or of a measuring instrument, often used to describe a test. Reliability is inversely related to random error.-Types:There are several general classes of reliability estimates:...
of the test, which tends to be 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
Alan S. Kaufman
Alan S. Kaufman is an American psychology professor known for his work on intelligence testing.- Early life and career :Born in Brooklyn and raised on Long Island, Kaufman earned his bachelor's degree from the University of Pennsylvania in 1965; M.A. in Educational Psychology from Columbia...
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 number 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.
Another summary:
- Same person (tested twice) .95
- 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
- Unrelated children—Reared together .30
- Parent-child—Living together .42
- Parent-child—Living apart .22
- Adoptive parent–child—Living together .19
Between-group heritability
Although IQ differences between individuals are shown to have a large hereditary component, it does not automatically follow that mean group-level disparities (between-group differences) in IQ can be assumed to have a genetic basis. An analogy, attributed to Richard LewontinRichard Lewontin
Richard Charles "Dick" Lewontin is an American evolutionary biologist, geneticist and social commentator. A leader in developing the mathematical basis of population genetics and evolutionary theory, he pioneered the notion of using techniques from molecular biology such as gel electrophoresis to...
, illustrates this point:
Suppose two handfuls are taken from a sack containing a genetically diverse variety of corn, and each grown under carefully controlled and standardized conditions, except that one batch is lacking in certain nutrients that are supplied to the other. After several weeks, the plants are measured. There is variability of growth within each batch, due to the genetic variability of the corn. Given that the growing conditions are closely controlled, nearly all the variation in the height of the plants within a batch will be due to differences in their genes. Thus, within populations, heritabilities will be very high. Nevertheless, the difference between the two groups is due entirely to an environmental factor - differential nutrition. Lewontin didn't go so far as to have the one set of pots painted white and the other set black, but you get the idea. The point of the example, in any case, is that the causes of between-group differences may in principle be quite different from the causes of within-group variation.
Arthur Jensen
Arthur Jensen
Arthur Robert Jensen is a Professor Emeritus of educational psychology at the University of California, Berkeley. Jensen is known for his work in psychometrics and differential psychology, which is concerned with how and why individuals differ behaviorally from one another.He is a major proponent...
agrees that this is technically correct but argues that a high heritability increases the probability that genetics play a role in average group differences.
See also
- GeneticsGeneticsGenetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
- Cyril BurtCyril BurtSir Cyril Lodowic Burt was an English educational psychologist who made contributions to educational psychology and statistics....
- Genetic Studies of GeniusGenetic Studies of GeniusThe Genetic Studies of Genius, today known as the Terman Study of the Gifted, is a still-running longitudinal study begun in 1921 to examine the development and characteristics of gifted children into adulthood...
Heritability
- HeritabilityHeritabilityThe Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...
- Heritability of autismHeritability of autismThe heritability of autism is the proportion of autism that can be explained by genetic variation; if the heritability of a condition is high, then the condition is considered to be primarily genetic...
Further reading
- G. Meisenberg: Genes for intelligence. A review of recent progress. Mankind QuarterlyMankind QuarterlyThe Mankind Quarterly is a peer-reviewed academic journal dedicated to physical and cultural anthropology and is currently published by the Council for Social and Economic Studies in Washington, D.C. It contains articles on human evolution, intelligence, ethnography, linguistics, mythology,...
36 (Winter 2005) 139-164. abstract