Neuroscience and intelligence refers to the various neurological factors that are partly responsible for the variation of intelligence within species or between different species. A large amount of research in this area has been focused on the neural basis of human intelligence.
Historic approaches to studying the neuroscience of intelligence
consisted of correlating external head parameters, for example head
circumference, to intelligence. Post-mortem measures of brain weight and brain volume have also been used.
More recent methodologies focus on examining correlates of intelligence
within the living brain using techniques such as magnetic resonance
imaging (MRI), functional MRI (fMRI), electroencephalography (EEG), positron emission tomography and other non-invasive measures of brain structure and activity.
Researchers have been able to identify correlates of intelligence
within the brain and its functioning. These include overall brain
volume, grey matter volume, white matter volume, white matter integrity, cortical thickness and neural efficiency.
Analyses of the parameters of intellectual systems, patterns of
their emergence and evolution, distinctive features, and the constants
and limits of their structures and functions made it possible to measure
and compare the capacity of communications (~100 m/s), to quantify the
number of components in intellectual systems (~1011 neurons), and to calculate the number of successful links responsible for cooperation (~1014 synapses).
Although the evidence base for our understanding of the neural
basis of human intelligence has increased greatly over the past 30
years, even more research is needed to fully understand it.
The neural basis of intelligence has also been examined in animals such as primates, cetaceans, and rodents.
Humans
Brain volume
One of the main methods used to establish a relationship between intelligence and the brain is to use measures of brain volume.
The earliest attempts at estimating brain volume were done using
measures of external head parameters, such as head circumference as a
proxy for brain size.
More recent methodologies employed to study this relationship include
post-mortem measures of brain weight and volume. These have their own
limitations and strengths.
The advent of MRI as a non-invasive highly-accurate measure of living
brain structure and function (using fMRI) made this the pre-dominant and
preferred method for measuring brain volume.
Overall, larger brain size and volume is associated with better cognitive functioning and higher intelligence.
The specific regions that show the most robust correlation between
volume and intelligence are the frontal, temporal and parietal lobes of
the brain.
A large number of studies have been conducted with uniformly positive
correlations, leading to the generally safe conclusion that larger
brains predict greater intelligence. In healthy adults, the correlation of total brain volume and IQ is approximately 0.4 when high-quality tests are used. A large scale study (n = 29k) using the UK Biobank found a correlation of .275. The strength of this relationship did not depend on sex, contradicting some earlier studies. A study using a sibling-design in two medium sized samples found evidence of causality with an effect size of 0.19. This study design rules out confounders that vary between families, but not those that vary within families.
Less is known about variation on scales less than total brain
volume. A meta-analytic review by McDaniel found that the correlation
between intelligence and in vivo brain size was larger for females
(0.40) than for males (0.25).
The same study also found that the correlation between brain size and
Intelligence increased with age, with children showing smaller
correlations.
It has been suggested that the link between larger brain volumes and
higher intelligence is related to variation in specific brain regions: a
whole-brain measure would under-estimate these links.
For functions more specific than general intelligence, regional effects
may be more important. For instance evidence suggests that in
adolescents learning new words, vocabulary growth is associated with
gray matter density in bilateral posterior supramarginal gyri.
Small studies have shown transient changes in gray-matter associated
with developing a new physical skill (juggling) occipito-temporal cortex
Brain volume is not a perfect account of intelligence: the
relationship explains a modest amount of variance in intelligence – 12%
to 36% of the variance. The amount of variance explained by brain volume may also depend on the type of intelligence measured.
Up to 36% of variance in verbal intelligence can be explained by brain
volume, while only approximately 10% of variance in visuospatial
intelligence can be explained by brain volume. A 2015 study by researcher Stuart J. Ritchie found that brain size explained 12% of the variance in intelligence among individuals. These caveats imply that there are other major factors influencing how intelligent an individual is apart from brain size.
In a large meta-analysis consisting of 88 studies Pietschnig et al.
(2015) estimated the correlation between brain volume and intelligence
to be about correlation coefficient of 0.24 which equates to 6%
variance.
Taking into account measurement quality, and sample type and IQ-range,
the meta-analytic association of brain volume in appears to be ~ .4 in
normal adults.
Researcher Jakob Pietschnig argued that the strength of the positive
association of brain volume and IQ remains robust, but has been
overestimated in the literature. He has stated that "It is tempting
to interpret this association in the context of human cognitive
evolution and species differences in brain size and cognitive ability,
we show that it is not warranted to interpret brain size as an
isomorphic proxy of human intelligence differences".
Grey matter
Grey matter
has been examined as a potential biological foundation for differences
in intelligence. Similarly to brain volume, global grey matter volume is
positively associated with intelligence.
More specifically, higher intelligence has been associated with larger
cortical grey matter in the prefrontal and posterior temporal cortex in
adults.
Furthermore, both verbal and nonverbal intelligence have been shown to
be positively correlated with grey matter volume across the parietal,
temporal and occipital lobes in young healthy adults, implying that
intelligence is associated with a wide variety of structures within the
brain.
There appear to be sex differences between the relationship of grey matter to intelligence between men and women.
Men appear to show more intelligence to grey matter correlations in the
frontal and parietal lobes, while the strongest correlations between
intelligence and grey matter in women can be found in the frontal lobes
and Broca's area.
However, these differences do not seem to impact overall Intelligence,
implying that the same cognitive ability levels can be attained in
different ways.
One specific methodology used to study grey matter correlates of intelligence in areas of the brain is known as voxel-based morphometry
(VBM). VBM allows researchers to specify areas of interest with great
spatial resolution, allowing the examination of grey matter areas
correlated with intelligence with greater special resolution. VBM has
been used to correlate grey matter positively with intelligence in the
frontal, temporal, parietal, and occipital lobes in healthy adults.
VBM has also been used to show that grey matter volume in the medial
region of the prefrontal cortex and the dorsomedial prefrontal cortex
correlate positively with intelligence in a group of 55 healthy adults.
VBM has also been successfully used to establish a positive correlation
between grey matter volumes in the anterior cingulate and intelligence
in children aged 5 to 18 years old.
Grey matter has also been shown to positively correlate with intelligence in children. Reis and colleagues
have found that grey matter in the prefrontal cortex contributes most
robustly to variance in Intelligence in children between 5 and 17, while
subcortical grey matter is related to intelligence to a lesser extent.
Frangou and colleagues
examined the relationship between grey matter and intelligence in
children and young adults aged between 12 and 21, and found that grey
matter in the orbitofrontal cortex, cingulate gyrus, cerebellum and thalamus was positively correlated to intelligence, while grey matter in the caudate nucleus
is negatively correlated with intelligence. However, the relationship
between grey matter volume and intelligence only develops over time, as
no significant positive relationship can be found between grey matter
volume and intelligence in children under 11.
An underlying caveat to research into the relationship of grey
matter volume and intelligence is demonstrated by the hypothesis of neural efficiency.
The findings that more intelligent individuals are more efficient at
using their neurons might indicate that the correlation of grey matter
to intelligence reflects selective elimination of unused synapses, and
thus a better brain circuitry.
White matter
Similar to grey matter, white matter has been shown to correlate positively with intelligence in humans.
White matter consists mainly of myelinated neuronal axons, responsible
for delivering signals between neurons. The pinkish-white color of white
matter is actually a result of these myelin sheaths that electrically
insulate neurons that are transmitting signals to other neurons. White
matter connects different regions of grey matter in the cerebrum
together. These interconnections make transport more seamless and allow
us to perform tasks easier. Significant correlations between
intelligence and the corpus callosum have been found, as larger callosal areas have been positively correlated with cognitive performance.
However, there appear to be differences in importance for white matter
between verbal and nonverbal intelligence, as although both verbal and
nonverbal measures of intelligence correlate positively with the size of
the corpus callosum, the correlation for intelligence and corpus
callosum size was larger (.47) for nonverbal measures than that for
verbal measures (.18). Anatomical mesh-based geometrical modelling has also shown positive correlations between the thickness of the corpus callosum and Intelligence in healthy adults.
White matter integrity has also been found to be related to intelligence.
White matter tract integrity is important for information processing
speed, and therefore reduced white matter integrity is related to lower
intelligence. The effect of white matter integrity is mediate entirely through information processing speed.
These findings indicate that the brain is structurally interconnected
and that axonal fibres are integrally important for fast information
process, and thus general intelligence.
Contradicting the findings described above, VBM failed to find a
relationship between the corpus callosum and intelligence in healthy
adults.
This contradiction can be viewed to signify that the relationship
between white matter volume and intelligence is not as robust as that of
grey matter and intelligence.
Cortical thickness
Cortical thickness has also been found to correlate positively with intelligence in humans. However, the rate of growth of cortical thickness is also related to intelligence.
In early childhood, cortical thickness displays a negative correlation
with intelligence, while by late childhood this correlation has shifted
to a positive one.
More intelligent children were found to develop cortical thickness more
steadily and over longer periods of time than less bright children. Studies have found cortical thickness to explain 5% in the variance of intelligence among individuals.
In a study conducted to find associations between cortical thickness
and general intelligence between different groups of people, sex did not
play a role in intelligence.
Although it is hard to pin intelligence on age based on cortical
thickness due to different socioeconomic circumstances and education
levels, older subjects (17 - 24) tended to have less variances in terms
of intelligence than when compared to younger subjects (19 - 17).
Cortical convolution
Cortical convolution
has increased the folding of the brain’s surface over the course of
human evolution. It has been hypothesized that the high degree of
cortical convolution may be a neurological substrate that supports some
of the human brain's most distinctive cognitive abilities. Consequently,
individual intelligence within the human species might be modulated by
the degree of cortical convolution.
An analysis published in 2019 found the contours of 677 children
and adolescent (mean age 12.72 years) brains had a genetic correlation
of almost 1 between IQ and surface area of the supramarginal gyrus on the left side of the brain.
Neural efficiency
The
neural efficiency hypothesis postulates that more intelligent
individuals display less activation in the brain during cognitive tasks,
as measured by Glucose metabolism.
A small sample of participants (N=8) displayed negative correlations
between intelligence and absolute regional metabolic rates ranging from
-0.48 to -0.84, as measured by PET scans, indicating that brighter
individuals were more effective processors of information, as they use
less energy. According to an extensive review by Neubauer & Fink a large number of studies (N=27) have confirmed this finding using methods such as PET scans, EEG and fMRI.
fMRI and EEG studies have revealed that task difficulty is an important factor affecting neural efficiency.
More intelligent individuals display neural efficiency only when faced
with tasks of subjectively easy to moderate difficulty, while no neural
efficiency can be found during difficult tasks. In fact, more able individuals appear to invest more cortical resources in tasks of high difficulty.
This appears to be especially true for the Prefrontal Cortex, as
individuals with higher intelligence displayed increased activation of
this area during difficult tasks compared to individuals with lower
intelligence.
It has been proposed that the main reason for the neural efficiency
phenomenon could be that individuals with high intelligence are better
at blocking out interfering information than individuals with low
intelligence.
Further research
Some
scientists prefer to look at more qualitative variables to relate to
the size of measurable regions of known function, for example relating
the size of the primary visual cortex to its corresponding functions, that of visual performance.
In a study of the head growth of 633 term-born children from the Avon Longitudinal Study of Parents and Children cohort,
it was shown that prenatal growth and growth during infancy were
associated with subsequent IQ. The study’s conclusion was that the brain
volume a child achieves by the age of 1 year helps determine later
intelligence. Growth in brain volume after infancy may not compensate
for poorer earlier growth.
There is an association between IQ and myopia. One suggested explanation is that one or several pleiotropic gene(s) affect the size of the neocortex part of the brain and eyes simultaneously.
A recent lesion mapping study conducted by Barbey and colleagues provides evidence to support the P-FIT theory of intelligence.
Brain injuries at an early age isolated to one side of the brain
typically results in relatively spared intellectual function and with IQ
in the normal range.
Primates
Brain size
Another theory of brain size in vertebrates is that it may relate to social rather than mechanical skills. Cortical size relates directly to pair-bonding lifestyle
and among primates, cerebral cortex size varies directly with the
demands of living in a large complex social network. Compared to other
mammals, primates have significantly larger brain sizes. Additionally,
most primates are found to be polygynandrous,
having many social relationships with others. Although inconclusive,
some studies have shown that this polygynandrous statue correlates to
brain size.
Intelligence in chimpanzees has been found to be related to brain
size, grey matter volume, and cortical thickness, as in humans.
Several environmental factors related to health can lead to
significant cognitive impairment, particularly if they occur during
pregnancy and childhood when the brain is growing and the blood–brain barrier
is less effective. Developed nations have implemented several health
policies regarding nutrients and toxins known to influence cognitive
function. These include laws requiring fortification of certain food products and laws establishing safe levels of pollutants (e.g. lead, mercury,
and organochlorides). Comprehensive policy recommendations targeting
reduction of cognitive impairment in children have been proposed.
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.
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.
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 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 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 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.
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. 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."
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.
A 2017 King's College London study suggests that genes account
for nearly 50 per cent of the differences between whether children are
socially mobile or not.
Maternal (fetal) environment
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. 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.[57][59]
Most of the improvements have allowed for better abstract reasoning,
spatial relations, and comprehension. Some scientists have suggested
that such enhancements are due to better nutrition, better parenting and
schooling, as well as exclusion of the least intelligent people from
reproduction. However, Flynn and a group of other scientists share the
viewpoint that modern life implies solving many abstract problems which
leads to a rise in their IQ scores.
Influence of genes on IQ stability
Recent research has illuminated genetic factors underlying IQ stability and change. Genome-wide association studies 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.
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.
The history of the race and intelligence controversy concerns the historical development of a debate about possible explanations of group differences encountered in the study of race and intelligence. Since the beginning of IQ testing around the time of World War I,
there have been observed differences between the average scores of
different population groups, and there have been debates over whether
this is mainly due to environmental and cultural factors, or mainly due
to some as yet undiscovered genetic factor, or whether such a dichotomy
between environmental and genetic factors is the appropriate framing of
the debate. Today, the scientific consensus is that genetics does not
explain differences in IQ test performance between racial groups.
Pseudoscientific claims of inherent differences in intelligence between races have played a central role in the history of scientific racism. In the late 19th and early 20th century, group differences in intelligence were often assumed to be racial in nature.
Apart from intelligence tests, research relied on measurements such as
brain size or reaction times. By the mid-1940s most psychologists had
adopted the view that environmental and cultural factors predominated.
In the mid-1960s, physicist William Shockley sparked controversy by claiming there might be genetic reasons that black people in the United States tended to score lower on IQ tests than white people. In 1969 the educational psychologist Arthur Jensen published a long article with the suggestion that compensatory education
could have failed to that date because of genetic group differences. A
similar debate among academics followed the publication in 1994 of The Bell Curve by Richard Herrnstein and Charles Murray. Their book prompted a renewal of debate on the issue and the publication of several interdisciplinary books on the issue. A 1995 report from the American Psychological Association
responded to the controversy, finding no conclusive explanation for the
observed differences between average IQ scores of racial groups. More
recent work by James Flynn, William Dickens and Richard Nisbett
has highlighted the narrowing gap between racial groups in IQ test
performance, along with other corroborating evidence that environmental
rather than genetic factors are the cause of these differences.
In the 18th century, debates surrounding the institution of slavery in the Americas
hinged on the question of whether innate differences in intellectual
capacity existed between races, in particular between black people and
white people. Some European philosophers and scientists, such as Voltaire, David Hume, Immanuel Kant, and Carl Linnaeus, either argued or simply presupposed that white people were intellectually superior. Others, such as Henri Gregoire and Constantin de Chasseboeuf, argued that ancient Egypt
had been a black civilization, and that it was therefore black people
who had "discovered the elements of science and art, at a time when all
other men were barbarous." During the French Revolution, Jean-Baptiste Belley, an elected member of the National Convention and the Council of Five Hundred who had been born in Senegal, became a leading proponent of the idea of racial intellectual equality.
In 1785, Thomas Jefferson wrote of his "suspicion" that black people were "inferior to... whites in endowments both of body and mind." However, in 1791, after corresponding with the free African-American polymath Benjamin Banneker,
Jefferson wrote that he hoped to see such "instances of moral eminence
so multiplied as to prove that the want of talents observed in them is
merely the effect of their degraded condition, and not proceeding from
any difference in the structure of the parts on which intellect
depends."
Samuel Morton, an American physician, used the study of human skulls to argue for racial differences in intelligence.
During the 19th and early 20th centuries, the idea that there are
differences in the brain structures and brain sizes of different races,
and that this implied differences in intelligence, was a popular topic,
inspiring numerous typological studies.Samuel Morton'sCrania Americana,
published in 1839, was one such study, arguing that intelligence was
correlated with brain size and that both of these metrics varied between
racial groups.
Francis Galton, an English eugenicist, argued that genius was unevenly distributed among racial groups.
Through the publication of his book Hereditary Genius in 1869, polymath Francis Galton spurred interest in the study of mental abilities, particularly as they relate to heredity and eugenics.Lacking the means to directly measure intellectual ability, Galton
attempted to estimate the intelligence of various racial and ethnic
groups. He based his estimations on observations from his and others'
travels, the number and quality of intellectual achievements of
different groups, and on the percentage of "eminent men" in each of
these groups. Galton hypothesized that intelligence was normally
distributed in all racial and ethnic groups, and that the means of these
distributions varied between the groups. In Galton's estimation,
ancient Attic Greeks had been the people with the highest incidence of genius intelligence, followed by contemporary Englishmen, with black Africans at a lower level, and Australian Aborigines lower still. He did not specifically study Jews, but remarked that "they appear to be rich in families of high intellectual breeds".
Autodidact and abolitionist Frederick Douglass served as a high-profile counterexample to myths of black intellectual inferiority.
Meanwhile, the American abolitionist and escaped slave Frederick Douglass had gained fame for his oratory and incisive writings, despite having learned to read as a child largely through surreptitious observation.
Accordingly, he had been described by abolitionists as a living
counter-example to slaveholders' arguments that people of African
descent lacked the intellectual capacity to function as independent
American citizens. His eloquence was so notable that some found it hard to believe he had once been a slave.
In the later years of his life, one newspaper described him as "a
bright example of the capability of the colored race, even under the
blighting influence of slavery, from which he emerged and became one of the distinguished citizens of the country."
Other abolitionists of the 19th century continued to advance the
theme of ancient Egypt as a black civilization as an argument against
racism. On this basis, scholar and diplomat Alexander Hill Everett argued in his 1927 book America:
"With regard to the intellectual capabilities of the African race, it
may be observed that Africa was once the nursery of science and
literature, and it was from thence that they were disseminated among the
Greeks and Romans." Similarly, the philosopher John Stuart Mill
posited in his 1849 essay "On the Negro Question" that "it was from
Negroes, therefore, that the Greeks learnt their first lessons in
civilization."
In 1895, R. Meade Bache of the University of Pennsylvania published an article in Psychological Review claiming that reaction time increases with evolution.
Bache supported this claim with data showing slower reaction times
among White Americans when compared with those of Native Americans and
African Americans, with Native Americans having the quickest reaction
time. He hypothesized that the slow reaction time of White Americans was
to be explained by their possessing more contemplative brains which did
not function well on tasks requiring automatic responses. This was one
of the first examples of modern scientific racism, in which a veneer of science was used to bolster belief in the superiority of a particular race.
Sociologist W. E. B. Du Bois argued that black populations just as much as white ones naturally give rise to what he termed a "talented tenth" of intellectually gifted individuals.
1900–1920
In 1903, the pioneering African-American sociologist W. E. B. Du Bois published his landmark collection of essays The Souls of Black Folk in defense of the inherent mental capacity and equal humanity of black people. According to Manning Marable,
this book "helped to create the intellectual argument for the black
freedom struggle in the twentieth century. 'Souls' justified the pursuit
of higher education for Negroes and thus contributed to the rise of the
black middle class." In contrast to other civil rights leaders like Booker T. Washington,
who advocated for incremental progress and vocational education as a
way for black Americans to demonstrate the virtues of "industry, thrift,
intelligence and property" to the white majority, Du Bois advocated for
black schools to focus more on liberal arts
and academic curriculum (including the classics, arts, and humanities),
because liberal arts were required to develop a leadership elite. Du Bois argued that black populations just as much as white ones naturally give rise to what he termed a "talented tenth" of intellectually gifted individuals.
At the same time, the discourse of scientific racism was accelerating. In 1910 the sociologist Howard W. Odum published his book Mental and Social Traits of the Negro,
which described African-American students as "lacking in filial
affection, strong migratory instincts, and tendencies; little sense of
veneration, integrity or honor; shiftless, indolent, untidy,
improvident, extravagant, lazy, lacking in persistence and initiative
and unwilling to work continuously at details. Indeed, experience with
the Negro in classrooms indicates that it is impossible to get the child
to do anything with continued accuracy, and similarly in industrial
pursuits, the Negro shows a woeful lack of power of sustained activity
and constructive conduct." As the historian of psychology Ludy T. Benjamin
explains, "with such prejudicial beliefs masquerading as facts," it was
at this time that educational segregation on the basis of race was
imposed in some states.
The first practical intelligence test was developed between 1905 and 1908 by Alfred Binet
in France for school placement of children. Binet warned that results
from his test should not be assumed to measure innate intelligence or
used to label individuals permanently. In 1916 Binet's test was translated into English and revised by Lewis Terman (who introduced IQ scoring for the test results) and published under the name Stanford–Binet Intelligence Scales.
Terman wrote that Mexican-Americans, African-Americans, and Native
Americans have a mental "dullness [that] seems to be racial, or at least
inherent in the family stocks from which they come." He also argued for a higher frequency of so-called "morons"
among non-white American racial groups, and concluded that there were
"enormously significant racial differences in general intelligence"
which could not be remedied by education.
Psychologist Robert Yerkes argued that immigration from Southern and Eastern Europe could decrease the average IQ of Americans.
In 1916 a team of psychologists, led by Robert Yerkes and including Terman and Henry H. Goddard,
adapted the Stanford-Binet tests as multiple-choice group tests for use
by the US army. In 1919, Yerkes devised a version of this test for
civilians, the National Intelligence Test, which was used in all levels
of education and in business. Like Terman, Goddard had argued in his book, Feeble-mindedness: Its Causes and Consequences (1914), that "feeble-mindedness" was hereditary; and in 1920 Yerkes in his book with Yoakum on the Army Mental Tests
described how they "were originally intended, and are now definitely
known, to measure native intellectual ability". Both Goddard and Terman
argued that the feeble-minded should not be allowed to reproduce. In the
US, however, independently and prior to the IQ tests, there had been
political pressure for such eugenic policies, to be enforced by sterilization; in due course IQ tests were later used as justification for sterilizing the mentally retarded.
Early IQ tests were also used to argue for limits to immigration
to the US. Already in 1917, Goddard reported on the low IQ scores of new
arrivals at Ellis Island.
Yerkes argued on the basis of his army test scores that there were
consistently lower IQ levels among those from Southern and Eastern
Europe, which he suggested could lead to a decline in the average IQ of
Americans if immigration from these regions were not limited.
1920–1960
In the 1920s, psychologists started questioning underlying
assumptions of racial differences in intelligence; although not
discounting them, the possibility was considered that they were on a
smaller scale than previously supposed and also due to factors other
than heredity. In 1924, Floyd Allport wrote in his book Social Psychology that the French sociologist Gustave Le Bon
was incorrect in asserting "a gap between inferior and superior
species", and pointed to "social inheritance" and "environmental
factors" as accounting for differences. Nevertheless, he suggested that:
"the intelligence of the white race is of a more versatile and complex
order than that of the black race. It is probably superior to that of
the red or yellow races."
In 1923, in his book A study of American intelligence, Carl Brigham
wrote that on the basis of the Yerkes army tests: "The decline in
intelligence is due to two factors, the change in races migrating to
this country, and to the additional factor of sending lower and lower
representatives of each race." He concluded that: "The steps that should
be taken to preserve or increase our present mental capacity must, of
course, be dictated by science and not by political expediency.
Immigration should not only be restrictive, but highly selective." The Immigration Act of 1924
put these recommendations into practice, introducing quotas based on
the 1890 census, prior to the waves of immigration from Poland and
Italy. While Gould and Kamin argued that the psychometric claims of
Nordic superiority had a profound influence on the institutionalization
of the 1924 immigration law, other scholars have argued that "the
eventual passage of the 'racist' immigration law of 1924 was not
crucially affected by the contributions of Yerkes or other
psychologists".
In 1929, Robert Woodworth, in his textbook Psychology: A Study of Mental Life,
made no claims about innate differences in intelligence between races,
pointing instead to environmental and cultural factors. He considered it
advisable to "suspend judgment and keep our eyes open from year to year
for fresh and more conclusive evidence that will probably be
discovered".
Raymond Cattell,
known for psychometric research into intrapersonal psychological
structure, advocated that supposedly inferior races should be
euthanized.
In the 1930s, the English psychologist Raymond Cattell wrote three tracts, Psychology and Social Progress (1933), The Fight for Our National Intelligence (1937) and Psychology and the Religious Quest (1938). The second was published by the Eugenics Society,
of which he had been a research fellow; it predicted the disastrous
consequences of not stopping the decline in the average intelligence in
Britain by one point per decade. In 1933, Cattell wrote that, of all the
European races, the "Nordic race was the most evolved in intelligence
and stability of temperament". He argued for "no mixture of bloods
between racial groups" because "the resulting re-shuffling of impulses
and psychic units throws together in each individual a number of forces
which may be incompatible". He rationalised the "hatred and
abhorrence ... for the Jewish practice of living in other nations
instead of forming an independent self-sustained group of their own",
referring to them as "intruders" with a "crafty spirit of calculation".
He recommended a rigid division of races, referring to those suggesting
that individuals be judged on their merits, irrespective of racial
background, as "race-slumpers". He wrote that in the past, "the backward
branches of the tree of mankind" had been lopped off as "the American
Indians, the Black Australians, the Mauris and the negroes had been
driven by bloodshed from their lands", unaware of "the biological
rationality of that destiny". He advocated what he saw as a more
enlightened solution: by birth control, by sterilization, and by "life
in adapted reserves and asylums", where the "races which have served
their turn [should] be brought to euthanasia." He considered blacks to
be naturally inferior, on account of their supposedly "small skull
capacity". In 1937, he praised the Third Reich
for their eugenic laws and for "being the first to adopt sterilization
together with a policy of racial improvement". In 1938, after
newspapers had reported on the segregation of Jews into ghettos and
concentration camps, he commented that the rise of Germany "should be
welcomed by the religious man as reassuring evidence that in spite of
modern wealth and ease, we shall not be allowed ... to adopt foolish
social practices in fatal detachment from the stream of evolution". In
late 1937, Cattell moved to the US on the invitation of the psychologist
Edward Thorndike of Columbia University,
also involved in eugenics. He spent the rest of his life there as a
research psychologist, devoting himself after retirement to devising and
publicising a refined version of his ideology from the 1930s that he
called beyondism.
In 1935, Otto Klineberg wrote two books, Negro Intelligence and Selective Migration and Race Differences,
dismissing claims that African Americans in the northern states were
more intelligent than those in the south. He argued that there was no
scientific proof of racial differences in intelligence and that this
should not therefore be used as a justification for policies in
education or employment.
The hereditarian view began to change in the 1920s in reaction to
excessive eugenicist claims regarding abilities and moral character,
and also due to the development of convincing environmental arguments.
In the 1940s many psychologists, particularly social psychologists,
began to argue that environmental and cultural factors, as well as
discrimination and prejudice, provided a more probable explanation of
disparities in intelligence. According to Franz Samelson, this change in attitude had become widespread by then,
with very few studies in race differences in intelligence, a change
brought out by an increase in the number of psychologists not from a
"lily-white ... Anglo-Saxon" background but from Jewish backgrounds.
Other factors that influenced American psychologists were the economic
changes brought about by the depression and the reluctance of
psychologists to risk being associated with the Nazi claims of a master
race. The 1950 race statement of UNESCO, prepared in consultation with scientists including Klineberg, created a further taboo against conducting scientific research on issues related to race. Adolf Hitler banned IQ testing for being "Jewish" as did Joseph Stalin for being "bourgeois".
1960–1980
William
Shockley, the Nobel laureate in physics, suggested that the decline in
the average IQ in the US could be solved by eugenics.
In 1965 William Shockley, Nobel laureate in physics and professor at Stanford University,
made a public statement at the Nobel conference on "Genetics and the
Future of Man" about the problems of "genetic deterioration" in humans
caused by "evolution in reverse". He claimed social support systems
designed to help the disadvantaged had a regressive effect. Shockley
subsequently claimed the most competent American population group were
the descendants of original European settlers, because of the extreme selective pressures imposed by the harsh conditions of early colonialism.
Speaking of the "genetic enslavement" of African Americans, owing to an
abnormally high birth rate, Shockley discouraged improved education as a
remedy, suggesting instead sterilization and birth control. In the
following ten years he continued to argue in favor of this position,
claiming it was not based on prejudice but "on sound statistics".
Shockley's outspoken public statements and lobbying brought him into
contact with those running the Pioneer Fund who subsequently, through the intermediary Carleton Putnam,
provided financial support for his extensive lobbying activities in
this area, reported widely in the press. With the psychologist and segregationistR. Travis Osborne as adviser, he formed the Foundation for Research and Education on Eugenics and Dysgenics
(FREED). Although its stated purpose was "solely for scientific and
educational purposes related to human population and quality problems",
FREED mostly acted as a lobbying agency for spreading Shockley's ideas
on eugenics.
Wickliffe Draper, founder of the Pioneer Fund
The Pioneer Fund had been set up by Wickliffe Draper
in 1937 with one of its two charitable purposes being to provide aid
for "study and research into the problems of heredity and eugenics in
the human race" and "into the problems of race betterment with special
reference to the people of the United States". From the late fifties
onwards, following the 1954 Supreme Court decision
on segregation in schools, it supported psychologists and other
scientists in favour of segregation. All of these ultimately held
academic positions in the Southern states, notably Henry E. Garrett (head of psychology at Columbia University until 1955), Wesley Critz George, Frank C.J. McGurk, R. Travis Osborne and Audrey Shuey, who in 1958 wrote The Testing of Negro Intelligence, demonstrating "the presence of native differences between Negroes and whites as determined by intelligence tests". In 1959 Garrett helped to found the International Association for the Advancement of Ethnology and Eugenics,
an organisation promoting segregation. In 1961 he blamed the shift away
from hereditarianism, which he described as the "scientific hoax of the
century", on the school of thought –the "Boas cult" – promoted by his
former colleagues at Columbia, notably Franz Boas and Otto Klineberg,
and more generally "Jewish organizations", most of whom "belligerently
support the egalitarian dogma which they accept as having been
'scientifically' proved". He also pointed to Marxist origins in this
shift, writing in a pamphlet, Desegregation: Fact and hokum,
that: "It is certain that the Communists have aided in the acceptance
and spread of egalitarianism although the extent and method of their
help is difficult to assess. Egalitarianism is good Marxist doctrine,
not likely to change with gyrations in the Kremlin line." In 1951
Garrett had even gone as far as reporting Klineberg to the FBI for advocating "many Communistic theories", including the idea that "there are no differences in the races of mankind".
One of Shockley's lobbying campaigns involved the educational psychologist, Arthur Jensen, of the University of California, Berkeley
(UC Berkeley). Although earlier in his career Jensen had favored
environmental rather than genetic factors as the explanation of race
differences in intelligence, he had changed his mind during 1966-1967
when he was at the Center for Advanced Study in the Behavioral Sciences at Stanford. Here Jensen met Shockley and through him received support for his research from the Pioneer Fund. Although Shockley and Jensen's names were later to become linked in the media, Jensen does not mention Shockley as an important influence on his thought in his subsequent writings; rather he describes as decisive his work with Hans Eysenck. He also mentions his interest in the behaviorist theories of Clark L. Hull
which he says he abandoned largely because he found them to be
incompatible with experimental findings during his years at Berkeley.
Arthur Jensen,
professor of educational psychology at UC Berkeley, wrote the 1969
article on intelligence that became one of the most controversial
articles in the history of psychology.
In a 1968 article published in Disadvantaged Child, Jensen
questioned the effectiveness of child development and antipoverty
programs, writing: "As a social policy, avoidance of the issue could be
harmful to everyone in the long run, especially to future generations of
Negroes, who could suffer the most from well-meaning but misguided and
ineffective attempts to improve their lot." In 1969 Jensen wrote a long article in the Harvard Educational Review, "How Much Can We Boost IQ and Scholastic Achievement?"
In his article, 123 pages long, Jensen insisted on the accuracy
and lack of bias in intelligence tests, stating that the absolute
quantity g that they measured, the general intelligence factor, first introduced by the English psychologist Charles Spearman
in 1904, "stood like a Rock of Gibraltar in psychometrics". He
stressed the importance of biological considerations in intelligence,
commenting that "the belief in the almost infinite plasticity of
intellect, the ostrich-like denial of biological factors in individual
differences, and the slighting of the role of genetics in the study of
intelligence can only hinder investigation and understanding of the
conditions, processes, and limits through which the social environment
influences human behavior." He argued at length that, contrary to
environmentalist orthodoxy, intelligence was partly dependent on the
same genetic factors that influence other physical attributes. More
controversially, he briefly speculated that the difference in
performance at school between blacks and whites might have a partly
genetic explanation, commenting that there were "various lines of
evidence, no one of which is definitive alone, but which, viewed all
together, make it a not unreasonable hypothesis that genetic factors are
strongly implicated in the average Negro-white
intelligence difference. The preponderance of the evidence is, in my
opinion, less consistent with a strictly environmental hypothesis than
with a genetic hypothesis, which, of course, does not exclude the
influence of environment or its interaction with genetic factors."
He advocated the allocation of educational resources according to merit
and insisted on the close correlation between intelligence and
occupational status, arguing that "in a society that values and rewards
individual talent and merit, genetic factors inevitably take on
considerable importance." Concerned that the average IQ in the US was
inadequate to answer the increasing needs of an industrialised society,
he predicted that people with lower IQs would become unemployable while
at the same time there would be an insufficient number with higher IQs
to fill professional posts. He felt that eugenic reform would prevent
this more effectively than compensatory education, surmising that "the
technique for raising intelligence per se in the sense of g,
probably lie more in the province of biological science than in
psychology or education". He pointed out that intelligence and family
size were inversely correlated, particularly amongst the black
population, so that the current trend in average national intelligence
was dysgenic
rather than eugenic. As he wrote, "Is there a danger that current
welfare policies, unaided by eugenic foresight, could lead to the
genetic enslavement of a substantial segment of our population? The
fuller consequences of our failure seriously to study these questions
may well be judged by future generations as our society's greatest
injustice to Negro Americans." He concluded by emphasizing the
importance of child-centered education. Although a tradition had
developed for the exclusive use of cognitive learning in schools, Jensen
argued that it was not suited to "these children's genetic and cultural
heritage": although capable of associative learning and memorization
("Level I" ability), they had difficulties with abstract conceptual
reasoning ("Level II" ability). He felt that in these circumstances the
success of education depended on exploiting "the actual potential
learning that is latent in these children's patterns of abilities". He
suggested that, in order to ensure equality of opportunity, "schools and
society must provide a range and diversity of educational methods,
programs and goals, and of occupational opportunities, just as wide as
the range of human abilities."
Later, writing about how the article came into being, Jensen said
that the editors of the Review had specifically asked him to include
his view on the heritability of race differences, which he had not
previously published. He also maintains that only five percent of the
article touched on the topic of race difference in IQ. Cronbach (1975) also gave a detailed account of how the student editors of Harvard Educational Review commissioned and negotiated the content of Jensen's article.
Many academics have given commentaries on what they considered to
be the main points of Jensen's article and the subsequent books in the
early 1970s that expanded on its content. According to Jencks & Phillips (1998),
in his article Jensen had argued "that educational programs for
disadvantaged children initiated as the War on Poverty had failed, and
the black-white race gap probably had a substantial genetic component."
They summarised Jensen's argument as follows:
"Most of the variation in black-white scores is genetic"
"No one has advanced a plausible environmental explanation for the black-white gap"
"Therefore it is more reasonable to assume that part of the black-white gap is genetic in origin"
IQ tests provide accurate measurements of a real human ability that is relevant in many aspects of life.
Intelligence, as measured by IQ tests, is highly (about 80%) heritable and parents with low IQs are much more likely to have children with low IQs
Educational programs have been unable to significantly change the intelligence of individuals or groups.
According to Webster (1997),
the article claimed "a correlation between intelligence, measured by
IQ tests, and racial genes". He wrote that Jensen, based on empirical
evidence, had concluded that "black intelligence was congenitally
inferior to that of whites"; that "this partly explains unequal
educational achievements"; and that, "because a certain level of
underachievement was due to the inferior genetic attributes of blacks,
compensatory and enrichment programs are bound to be ineffective in
closing the racial gap in educational achievements." Several commentators mention Jensen's recommendations for schooling: according to Barry Nurcombe,
Jensen's own research suggests that
IQ tests amalgamate two forms of thinking which are hierarchically
related but which become differentially distributed in the population
according to SES: level 1 and level 2, associative learning and abstract
thinking (g), respectively. Blacks do as well as whites on tests
of associative learning, but they fall behind on abstract thinking. The
educational system should attend to this discrepancy and derive a more
pluralistic approach. The current system puts minority groups at a
marked disadvantage, since it overemphasizes g-type thinking.
Jensen had already suggested in the article that initiatives like the Head Start Program were ineffective, writing in the opening sentence, "Compensatory education has been tried and it apparently has failed." Other experts in psychometrics, such as Flynn (1980) and Mackintosh (1998),
have given accounts of Jensen's theory of Level I and Level II
abilities, which originated in this and earlier articles. As the historian of psychologyWilliam H. Tucker
commented, Jensen's question is leading: "Is there a danger that
current welfare policies, unaided by eugenic foresight, could lead to
the genetic enslavement of a substantial segment of our population? The
fuller consequences of our failure seriously to study these questions
may well be judged by future generations as our society's greatest
injustice to Negro Americans". Tucker noted that it repeats Shockley's
phrase "genetic enslavement", which proved later to be one of the most
inflammatory statements in the article.
Shockley conducted a widespread publicity campaign for Jensen's
article, supported by the Pioneer Fund. Jensen's views became widely
known in many spheres. As a result, there was renewed academic interest
in the hereditarian viewpoint and in intelligence tests. Jensen's
original article was widely circulated and often cited; the material was
taught in university courses over a range of academic disciplines. In
response to his critics, Jensen wrote a series of books on all aspects
of psychometrics. There was also a widespread positive response from the
popular press — with The New York Times Magazine dubbing the topic "Jensenism" — and amongst politicians and policy makers.
In 1971 Richard Herrnstein wrote a long article on intelligence tests in The Atlantic
for a general readership. Undecided on the issues of race and
intelligence, he discussed instead score differences between social
classes. Like Jensen he took a firmly hereditarian point of view. He
also commented that the policy of equal opportunity would result in
making social classes more rigid, separated by biological differences,
resulting in a downward trend in average intelligence that would
conflict with the growing needs of a technological society.
Jensen and Herrnstein's articles were widely discussed. Hans Eysenck
defended the hereditarian point of view and the use of intelligence
tests in "Race, Intelligence and Education" (1971), a pamphlet
presenting Jensenism to a popular audience, and "The Inequality of Man"
(1973). He was severely critical of anti-hereditarians whose policies he
blamed for many of the problems in society. In the first book he wrote
that, "All the evidence to date suggests the strong and indeed
overwhelming importance of genetic factors in producing the great
variety of intellectual differences which [are] observed between certain
racial groups", adding in the second, that "for anyone wishing to
perpetuate class or caste differences, genetics is the real foe". "Race, Intelligence and Education" was immediately criticized in strong terms by IQ researcher Sandra Scarr
as an "uncritical popularization of Jensen's ideas without the nuances
and qualifiers that make much of Jensen's writing credible or at least
responsible." Later scholars have identified errors and suspected data manipulation in Eysenck's work. An inquiry on behalf of King's College London found 26 of his papers to be "incompatible with modern clinical science". Rod Buchanan, a biographer of Eysenck, has argued that 87 publications by Eysenck should be retracted.[
Student groups and faculty at Berkeley and Harvard protested
Jensen and Herrnstein with charges of racism. Two weeks after the
appearance of Jensen's article, Students for a Democratic Society staged protests against Arthur Jensen on the campus of the University of California, Berkeley, chanting "Fight racism. Fire Jensen!" Jensen himself states that he even lost his employment at Berkeley because of the controversy. Similar campaigns were waged in London against Eysenck and in Boston against sociobiologistEdward Wilson. The attacks on Wilson were orchestrated by the Sociobiology Study Group, part of the left wing organization Science for the People, formed of 35 scientists and students, including the Harvard biologists Stephen J. Gould and Richard Lewontin, who both became prominent critics of hereditarian research in race and intelligence.
In 1972 50 academics, including the psychologists Jensen, Eysenck, and
Herrnstein as well as five Nobel laureates, signed a statement entitled
"Resolution on Scientific Freedom Regarding Human Behavior and
Heredity", criticizing the climate of "suppression, punishment and
defamation of scientists who emphasized the role of heredity in human
behavior". In October 1973 a half-page advertisement entitled
"Resolution Against Racism" appeared in The New York Times.
With over 1000 academic signatories, including Lewontin, it condemned
"racist research", denouncing in particular Jensen, Shockley and
Herrnstein.
This was accompanied by commentaries, criticisms and denouncements from the academic community. Two issues of the Harvard Educational Review were devoted to critiques of Jensen's work by psychologists, biologists and educationalists. As documented by Wooldridge (1995), the main commentaries involved: population genetics (Richard Lewontin, Luigi Cavalli-Sforza, Walter Bodmer); the heritability of intelligence (Christopher Jencks, Mary Jo Bane, Leon Kamin, David Layzer); the possible inaccuracy of IQ tests as measures of intelligence (summarised in Jensen 1980, pp. 20–21); and sociological assumptions about the relationship between intelligence and income (Jencks and Bane).
More specifically, the Harvard biologist Richard Lewontin commented on
Jensen's use of population genetics, writing that, "The fundamental
error of Jensen's argument is to confuse heritability of character
within a population with heritability between two populations."
Jensen denied making such a claim, saying that his argument was that
high within-group heritability increased the probability of non-zero
between-group heritability.
The political scientists Christopher Jencks and Mary Jo Bane, also from
Harvard, recalculated the heritability of intelligence as 45% instead
of Jensen's estimate of 80%; and they calculated that only about 12% of
variation in income was due to IQ, so that in their view the connections
between IQ and occupation were less clear than Jensen had suggested.
Ideological differences also emerged in the controversy. The
circle of scientists around Lewontin and Gould rejected the research of
Jensen and Herrnstein as "bad science". While not objecting to research
into intelligence per se, they felt that this research was politically motivated and objected to the reification of intelligence: the treatment of the numerical quantity g
as a physical attribute like skin color that could be meaningfully
averaged over a population group. They claimed that this was contrary to
the scientific method, which required explanations at a molecular
level, rather than the analysis of a statistical artifact in terms of
undiscovered processes in biology or genetics. In response to this
criticism, Jensen later wrote: "... what Gould has mistaken for
'reification' is neither more nor less than the common practice in every
science of hypothesizing explanatory models to account for the observed
relationships within a given domain. Well known examples include the
heliocentric theory of planetary motion, the Bohr atom, the
electromagnetic field, the kinetic theory of gases, gravitation, quarks,
Mendelian genes, mass, velocity, etc. None of these constructs exists
as a palpable entity occupying physical space." He asked why psychology
should be denied "the common right of every science to the use of
hypothetical constructs or any theoretical speculation concerning causal
explanations of its observable phenomena".
Cyril Burt, the English educationalist whose disputed twin studies were used as data by Jensen in some of his early articles and books
The academic debate also became entangled with the so-called "Burt Affair", because Jensen's article had partially relied on the 1966 twin studies of the British educational psychologist Sir Cyril Burt:
shortly after Burt's death in 1971, there were allegations, prompted by
research of Leon Kamin, that Burt had fabricated parts of his data,
charges which have never been fully resolved.
Franz Samelson documents how Jensen's views on Burt's work varied over
the years: Jensen was Burt's main defender in the US during the 1970s.
In 1983, following the publication in 1978 of Leslie Hearnshaw's
official biography of Burt, Jensen changed his mind, "fully accept[ing]
as valid ... Hearnshaw's biography" and stating that "of course [Burt]
will never be exonerated for his empirical deceptions".
However, in 1992, he wrote that "the essence of the Burt affair ...
[was] a cabal of motivated opponents, avidly aided by the mass media, to
bash [Burt's] reputation completely", a view repeated in an invited address on Burt before the American Psychological Association, when he called into question Hearnshaw's scholarship.
Trofim Lysenko who, as director of Soviet research in biology under Joseph Stalin, blocked research into genetics for ideological reasons
Similar charges of a politically motivated campaign to stifle scientific research on racial differences, later dubbed "Neo-Lysenkoism", were frequently repeated by Jensen and his supporters. Jensen (1972)
bemoaned the fact that "a block has been raised because of the obvious
implications for the understanding of racial differences in ability and
achievement. Serious considerations of whether genetic as well as
environmental factors are involved has been taboo in academic circles",
adding that: "In the bizarre racist theories of the Nazis and the
disastrous Lysenkoism of the Soviet Union under Stalin, we have seen
clear examples of what happens when science is corrupted by subservience
to political dogma."
After the appearance of his 1969 article, Jensen was later more
explicit about racial differences in intelligence, stating in 1973 "that
something between one-half and three-fourths of the average IQ
differences between American Negroes and whites is attributable to
genetic factors." He even speculated that the underlying mechanism was a
"biochemical connection between skin pigmentation and intelligence"
linked to their joint development in the ectoderm
of the embryo. Although Jensen avoided any personal involvement with
segregationists in the US, he did not distance himself from the
approaches of journals of the far right in Europe, many of whom viewed
his research as justifying their political ends. In an interview with Nation Europa,
he said that some human races differed from one another even more than
some animal species, claiming that a measurement of "genetic distance"
between blacks and whites showed that they had diverged over 46,000
years ago. He also granted interviews to Alain de Benoist's French journal Nouvelle École and Jürgen Rieger's German journal Neue Anthropologie of which he later became a regular contributor and editor.
The debate was further exacerbated by issues of racial bias that
had already intensified through the 1960s because of civil rights
concerns and changes in the social climate. In 1968 the Association of Black Psychologists
(ABP) had demanded a moratorium on IQ tests for children from minority
groups.
After a committee set up by the American Psychological Association drew
up guidelines for assessing minority groups, failing to confirm the
claims of racial bias, Jackson (1975) wrote the following as part of a response on behalf of the ABP:
Psychological testing historically
has been a quasi-scientific tool in the perpetuation of racism on all
levels of scientific objectivity, it [testing] has provided a cesspool
of intrinsically and inferentially fallacious data which inflates the
egos of whites by demeaning Black people and threatens to potentiate Black genocide.
Other professional academic bodies reacted to the dispute differently. The Society for the Psychological Study of Social Issues, a division of the American Psychological Society,
issued a public statement in 1969 criticizing Jensen's research,
declaring that, "To construct questions about complex behavior in terms
of heredity versus environment is to oversimplify the essence and nature of human development and behavior." The American Anthropological Association
convened a panel discussion in 1969 at its annual general meeting,
shortly after the appearance of Jensen's paper, where several
participants labelled his research as "racist".
Subsequently, the association issued an official clarification, stating
that, "The shabby misuse of IQ testing in the support of past American
racist policies has created understandable anxiety over current research
on the inheritance of human intelligence. But the resulting personal
attacks on a few scientists with unpopular views has had a chilling
effect on the entire field of behavioral genetics and clouds public
discussion of its implications." In 1975 the Genetics Society of America
made a similarly cautious statement: "The application of the techniques
of quantitative genetics to the analysis of human behavior is fraught
with human complications and potential biases, but well-designed
research on the genetic and environmental components of human
psychological traits may yield valid and socially useful results and
should not be discouraged."
1980–2000
James Flynn, the New Zealand political scientist who has studied changes in group-level IQ averages over time
In the 1980s, political scientist James Flynn
compared the results of groups who took both older and newer versions
of specific IQ tests. His research led him to the discovery of what is
now called the Flynn effect:
a substantial increase in average IQ scores over the years across all
groups tested. His discovery was confirmed later by many other studies.
While trying to understand these remarkable test score increases, Flynn
had postulated in 1987 that "IQ tests do not measure intelligence but rather a correlate with a weak causal link to intelligence". By 2009, however, Flynn felt that the IQ test score changes are real.
He suggests that our fast-changing world has faced successive
generations with new cognitive challenges that have considerably
stimulated intellectual ability. "Our brains as presently constructed
probably have much excess capacity ready to be used if needed. That was
certainly the case in 1900."
Flynn notes that "Our ancestors in 1900 were not mentally retarded.
Their intelligence was anchored in everyday reality. We differ from them
in that we can use abstractions and logic and the hypothetical to
attack the formal problems that arise when science liberates thought
from concrete situations. Since 1950, we have become more ingenious in
going beyond previously learned rules to solve problems on the spot."
Richard Lynn, the controversial English psychologist who has argued for global group differences in intelligence
From the 1980s onwards, the Pioneer Fund continued to fund
hereditarian research on race and intelligence, in particular the two
English-born psychologists Richard Lynn of the University of Ulster and J. Philippe Rushton of the University of Western Ontario,
who became president of the fund in 2002. Rushton returned to the
cranial measurements of the 19th century, using brain size as an extra
factor determining intelligence; in collaboration with Jensen, he most
recently developed updated arguments for the genetic explanation of race
differences in intelligence.
Lynn, longtime editor of and contributor to Mankind Quarterly,
has concentrated his research in race and intelligence on gathering and
tabulating data purporting to show race differences in intelligence
across the world. He has also made suggestions about the political
implications of his data, including the revival of older theories of
eugenics.
Snyderman & Rothman (1987)
announced the results of a survey conducted in 1984 on a sample of over
a thousand psychologists, sociologists and educationalists in a
multiple choice questionnaire, and expanded in 1988 into the book The IQ Controversy, the Media, and Public Policy.
The book claimed to document a liberal bias in the media coverage of
scientific findings regarding IQ. The survey included the question,
"Which of the following best characterizes your opinion of the
heritability of black-white differences in IQ?" 661 researchers returned
the questionnaire, and of these, 14% declined to answer the question,
24% voted that there was insufficient evidence to give an answer, 1%
voted that the gap was purely "due entirely to genetic variation", 15%
voted that it "due entirely due to environmental variation" and 45%
voted that it was a "product of genetic and environmental variation".
Jencks & Phillips (1998)
have pointed out that those who replied "both" did not have the
opportunity to specify whether genetics played a large role. There has
been no agreement amongst psychometricians on the significance of this particular answer. Scientists supporting the hereditarian point of view have seen it as a vindication of their position.
In 1989, Rushton was placed under police investigation by the Attorney General of Ontario, after complaints that he had promoted racism in one of his publications on race differences. In the same year, Linda Gottfredson of the University of Delaware
had an extended battle with her university over the legitimacy of
grants from the Pioneer Fund, eventually settled in her favour.
Both later responded with an updated version of Henry E.
Garrett's "egalitarian dogma", labelling the claim that all races were
equal in cognitive ability as an "egalitarian fiction" and a "scientific
hoax". Gottfredson (1994)
spoke of a "great fraud", a "collective falsehood" and a "scientific
lie", citing the findings of Snyderman and Rothman as justification. Rushton (1996)
wrote that there was a "taboo on race" in scientific research that had
"no parallel ... not the Inquisition, not Stalin, not Hitler". In his 1998 book "The g Factor: The Science of Mental Ability", Jensen reiterated his earlier claims of Neo-Lysenkoism,
writing that "The concept of human races [as] a fiction" has various
"different sources, none of them scientific", one of them being
"Neo-Marxist philosophy", which "excludes consideration of genetic or
biological factors ... from any part in explaining behavioral
differences amongst humans". In the same year, the evolutionary psychologistKevin B. MacDonald
went much further, reviving Garrett's claim of the "Boas cult" as a
Jewish conspiracy, after which "research on racial differences ceased,
and the profession completely excluded eugenicists like Madison Grant and Charles Davenport".
In 1994, the debate on race and intelligence was reignited by the publication of the book
The Bell Curve: Intelligence and Class Structure in American Life by Richard Herrnstein and Charles Murray. The book was received positively by the media, with prominent coverage in Newsweek, Time, The New York Times and The Wall Street Journal.
Although only two chapters of the book were devoted to race differences
in intelligence, treated from the same hereditarian standpoint as
Jensen's 1969 paper, it nevertheless caused a similar furore in the
academic community to Jensen's article. Many critics, including Stephen
J. Gould and Leon Kamin, asserted that the book contained unwarranted
simplifications and flaws in its analysis; in particular there were
criticisms of its reliance on Lynn's estimates of average IQ scores in South Africa,
where data had been used selectively, and on Rushton's work on brain
size and intelligence, which was controversial and disputed. These
criticisms were subsequently presented in books, most notably The Bell Curve Debate (1995), Inequality by Design: Cracking the Bell Curve Myth (1996) and an expanded edition of Gould's The Mismeasure of Man (1996).
In 1994 a group of 52 scientists, including Rushton, Lynn, Jensen and Eysenck, were cosignatories of an op-ed article in The Wall Street Journal written by Linda Gottfredson entitled "Mainstream Science on Intelligence". The article, supporting the conclusions of The Bell Curve, was later republished in an expanded version in the journal Intelligence. The editorial included the statements:
Genetics plays a bigger role than
environment in creating IQ differences among individuals ... The bell
curve for whites is centred roughly around IQ 100; the bell curve for
American blacks roughly around 85 ... black 17-year olds perform, on the
average, more like white 13-year olds in reading, math and science,
with Hispanics in between.
Another early criticism was that Herrnstein and Murray did not submit their work to academic peer review before publication. There were also three books written from the hereditarian point of view: Why race matters: race differences and what they mean (1997) by Michael Levin; The g Factor: The Science of Mental Ability (1998) by Jensen; and Intelligence; a new look by Hans Eysenck. Various other books of collected contributions appeared at the same time, including The black-white test gap (1998) edited by Christopher Jencks and Meredith Phillips, Intelligence, heredity and environment (1997) edited by Robert Sternberg and Elena Grigorenko. A section in IQ and human intelligence (1998) by Nicholas Mackintosh discussed ethnic groups and Race and intelligence: separating science from myth (2002) edited by Jefferson Fish presented further commentary on The Bell Curve by anthropologists, psychologists, sociologists, historians, biologists and statisticians.
In 1999 the same journal Intelligence reprinted as an invited editorial a long article by the attorneyHarry F. Weyher Jr.
defending the integrity of the Pioneer Fund, of which he was then
president and of which several editors, including Gottfredson, Jensen,
Lynn and Rushton, were grantees. In 1994 the Pioneer-financed journal Mankind Quarterly, of which Roger Pearson was the manager and pseudonymous contributor, had been described by Charles Lane in a review of The Bell Curve in the New York Review of Books
as "a notorious journal of 'racial history' founded, and funded, by men
who believe in the genetic superiority of the white race"; he had
called the fund and its journal "scientific racism's keepers of the
flame." Gottfredson had previously defended the fund in 1989–1990,
asserting that Mankind Quarterly was a "multicultural journal"
dedicated to "diversity ... as an object of dispassionate study" and
that Pearson did not approve of membership of the American Nazi Party. Pearson (1991) had himself defended the fund in his book Race, Intelligence and Bias in Academe.
In response to the debate on The Bell Curve, the American Psychological Association set up a ten-person taskforce, chaired by Ulrich Neisser, to report on the book and its findings. In its report, "Intelligence: Knowns and Unknowns", published in February 1996, the committee made the following comments on race differences in intelligence:
African American IQ scores have
long averaged about 15 points below those of Whites, with
correspondingly lower scores on academic achievement tests. In recent
years the achievement-test gap has narrowed appreciably. It is possible
that the IQ-score differential is narrowing as well, but this has not
been clearly established. The cause of that differential is not known;
it is apparently not due to any simple form of bias in the content or
administration of the tests themselves. The Flynn effect shows that
environmental factors can produce differences of at least this
magnitude, but that effect is mysterious in its own right. Several
culturally-based explanations of the Black/White IQ differential have
been proposed; some are plausible, but so far none has been conclusively
supported. There is even less empirical support for a genetic
interpretation. In short, no adequate explanation of the differential
between the IQ means of Blacks and Whites is presently available.
Jensen commented:
As I read the APA statement, [...] I
didn't feel it was contradicting my position, but rather merely
sidestepping it. It seems more evasive of my position than
contradictory. The committee did acknowledge the factual status of what I
have termed the Spearman Effect, the reality of g, the inadequacy of test bias and socioeconomic status
as causal explanations, and many other conclusions that don't differ at
all from my own position. [...] Considering that the report was
commissioned by the APA, I was surprised it went as far as it did.
Viewed in that light, I am not especially displeased by it.
Rushton found himself at the centre of another controversy in 1999
when unsolicited copies of a special abridged version of his 1995 book Race, Evolution and Behavior,
aimed at a general readership, were mass mailed to psychologists,
sociologists and anthropologists in North American universities. As a
result, Transaction Publishers
withdrew from publishing the pamphlet, financed by the Pioneer Fund,
and issued an apology in the January 2000 edition of the journal Society.
In the pamphlet Rushton recounted how black Africans had been seen by
outside observers through the centuries as naked, insanitary,
impoverished and unintelligent. In modern times he remarked that their
average IQ of 70 "is the lowest ever recorded", due to smaller average
brain size. He explained these differences in terms of evolutionary
history: those that had migrated to colder climates in the north to
evolve into whites and Asians had adapted genetically to have more
self-control, lower levels of sex hormones, greater intelligence, more
complex social structures, and more stable families. He concluded that
whites and Asians are more disposed to "invest time and energy in their
children rather than the pursuit of sexual thrills. They are 'dads'
rather than 'cads.'" J. Philippe Rushton did not distance himself from
groups on the far right in the US. He was a regular contributor to the
newsletters of American Renaissance and spoke at many of their biennial conferences, in 2006 sharing the platform with Nick Griffin, leader of the British National Party.
2000–present
In 2002, Richard Lynn and Tatu Vanhanen, published IQ and the Wealth of Nations.
Vanhanen claimed "Whereas the average IQ of Finns is 97, in Africa it
is between 60 and 70. Differences in intelligence are the most
significant factor in explaining poverty." A complaint by Finland's
"Ombudsman for Minorities", Mikko Puumalainen, resulted in Vanhanen
being considered to be investigated for incitement of "racial hatred" by
the Finnish National Bureau of Investigations. In 2004, the police stated they found no reason to suspect he incited racial hatred and decided not to launch an investigation. Several negative reviews of the book have been published in the scholarly literature. Susan Barnett and Wendy M. Williams wrote that "we see an edifice built on layer upon layer of arbitrary assumptions and selective data manipulation.
The data on which the entire book is based are of questionable validity
and are used in ways that cannot be justified." They also wrote that
cross country comparisons are "virtually meaningless."
Richardson (2004) argued, citing the Flynn effect
as the best evidence, that Lynn has the causal connection backwards and
suggested that "the average IQ of a population is simply an index of
the size of its middle class, both of which are results of industrial
development". The review concludes that "This is not so much science,
then, as a social crusade."
A review by Michael Palairet criticized the book's methodology,
particularly the imprecise estimates of GDP and the fact that IQ data
were only available for 81 of the 185 countries studied. Kamin (2006) has also criticized Lynn and Vanhanen's work on methodological grounds.
On July 27, 2020, the European Human Behavior and Evolution Association
issued a formal statement opposing the utilization of Lynn's national
IQ dataset, citing various methodological concerns. They concluded "Any
conclusions drawn from analyses which use these data are therefore
unsound, and no reliable evolutionary work should be using these data."
In 2005 the journal Psychology, Public Policy and Law of the American Psychological Association (APA) published a review article by Rushton and Jensen, "Thirty Years of Research on Race Differences in Cognitive Ability". The article was followed by a series of responses, some in support, some critical. Psychologist Richard Nisbett later included an amplified version of his critique as part of the book Intelligence and How to Get It: Why Schools and Cultures Count (2009).
Rushton and Jensen in 2010 made a point-for-point reply to this and
again summarized the hereditarian position in "Race and IQ: A
theory-based review of the research in Richard Nisbett's Intelligence
and How to Get It".
In 2006, a systematic analysis by James Flynn and William Dickens
showed the gap between black and white Americans to have closed
dramatically during the period between 1972 and 2002, suggesting that,
in their words, the "constancy of the Black-White IQ gap is a myth."
They argued that their results refute the possibility of a genetic
origin, concluding that "the environment has been responsible" for
observed differences.
A subsequent review led by Richard Nisbett and co-authored by Flynn,
published in 2012, reached a similar conclusion, stating that the weight
of evidence presented in all prior research literature shows that group
differences in IQ are best understood as environmental in origin.
On the other hand, a 2007 meta-analysis by Rindermann found many
of the same groupings and correlations found by Lynn and Vanhanen, with
the lowest scores in sub-Saharan Africa, and a correlation of .60
between cognitive skill and GDP per capita. By measuring the
relationship between educational data and social well-being over time,
this study also performed a causal analysis, finding that nations
investing in education leads to increased well-being later on. Hunt (2010,
pp. 437–439) considers Rindermann's analysis to be much more reliable
than Lynn and Vanhanen's. However, a 2017 systematic review notes that
other researchers have dismissed Rindermann's findings on the basis that
"the meaning of variables shifts when you aggregate to different
levels; a conceptual, methodological point that is well-established in
the field of multi-level modelling."
In particular, James Flynn writes that "Rindermann's results suggest
that different factors lie behind the emergence of g in international
comparisons and the emergence of g when we compare the differential
performance of individuals. This renders g(l) and g(ID) so unlike that
they have little significance in common."
Similarly, Martin Brunner and Romain Martin argue that Rindermann's
identification of "a common factor underlying measures of intelligence
and student achievement on the cross-national level" is methodologically
flawed, stating that given "the level of analysis applied . . . this
factor cannot be interpreted as general cognitive ability (g). Rather it
is an indicator of a nation's prosperity."
In 2007, James D. Watson, Nobel laureate in biology, gave a controversial interview to the Sunday Times Magazine during a book tour in the United Kingdom.
Watson stated he was "inherently gloomy about the prospect of Africa"
because "all our social policies are based on the fact that their
intelligence is the same as ours – whereas all the testing says not
really." He also wrote that "there is no firm reason to anticipate that
the intellectual capacities of peoples geographically separated in their
evolution should prove to have evolved identically. Our wanting to
reserve equal powers of reason as some universal heritage of humanity
will not be enough to make it so." This resulted in the cancellation of a
Royal Society lecture, along with other public engagements, and his suspension from his administrative duties at Cold Spring Harbor Laboratory.
He subsequently cancelled the tour and resigned from his position at
CSHL, where he had served as either director, president or chancellor
since 1968. However, Watson was later appointed chancellor emeritus of
CSHL, and, as of 2009, he continued to advise and guide project work at
the laboratory.
A 2009 debate in the journal Nature on the question "Should scientists study race and IQ?" involved position papers by Stephen Ceci and Wendy M. Williams arguing "yes" and Steven Rose
arguing "no". It is notable that both sides agreed that, as Ceci and
Williams put it, "There is an emerging consensus about racial and gender
equality in genetic determinants of intelligence; most researchers,
including ourselves, agree that genes do not explain between-group
differences." Subsequent editorials in Nature
have affirmed this view, for example the 2017 statement by the
editorial board that "the (genuine but closing) gap between the average
IQ scores of groups of black and white people in the United States has
been falsely attributed to genetic differences between the races."
In a meta-analysis of studies of IQ estimates in Sub-Saharan Africa, Wicherts, Dolan & van der Maas (2010,
p. 10) concluded that Lynn and Vanhanen had relied on unsystematic
methodology by failing to publish their criteria for including or
excluding studies. They found that Lynn and Vanhanen's exclusion of
studies had depressed their IQ estimate for sub-Saharan Africa, and that
including studies excluded in IQ and Global Inequality
resulted in average IQ of 82 for sub-Saharan Africa, lower than the
average in Western countries, but higher than Lynn and Vanhanen's
estimate of 67. Wicherts at al. conclude that this difference is likely
due to sub-Saharan Africa having limited access to modern advances in
education, nutrition and health care. A 2010 systematic review by the same research team, along with Jerry S. Carlson,
found that compared to American norms, the average IQ of sub-Saharan
Africans was about 80. The same review concluded that the Flynn effect
had not yet taken hold in sub-Saharan Africa.
Wicherts, Borsboom, and Dolan (2010) argued that studies
reporting support for evolutionary theories of intelligence based on
national IQ data suffer from multiple fatal methodological flaws. For
example, they state that such studies "... assume that the Flynn Effect
is either nonexistent or invariant with respect to different regions of
the world, that there have been no migrations and climatic changes over
the course of evolution, and that there have been no trends over the
last century in indicators of reproductive strategies (e.g., declines in
fertility and infant mortality)." They also showed that a strong degree
of confounding exists between national IQs and current national
development status.
Similarly, Pesta & Poznanski (2014) showed that the average
temperature of a given US state is strongly associated with that state's
average IQ and other well-being variables, despite the fact that
evolution has not had enough time to operate on non-Native American
residents of the United States. They also noted that this association
persisted even after controlling for race, and concluded that "Evolution
is therefore not necessary for temperature and IQ/well-being to co-vary
meaningfully across geographic space."
In 2016, Rindermann, Becker & Coyle (2016) attempted to replicate the findings of Snyderman & Rothman (1987)
by surveying 71 self-identified psychology experts on the causes of
international differences in cognitive test scores; only 20% of those
invited participated. They found that the experts surveyed ranked
education as the most important factor of these differences, with
genetics in second place (accounting on average for 15% of the gap, with
high variability in estimates among experts) and health, wealth,
geography, climate, and politics as the next most important factors.
About 90% of experts in the survey believed there was a genetic
component to international IQ gaps. The authors emphasized, however,
that their study serves as an "opinion instrument" rather than "an
indicator of truth." Notably, the study relied on "self-selection of
experts," which the authors acknowledge as a limitation, and focused on
self-identified experts in psychology rather than genetics.
In 2018, in response to a resurgence of public controversy over
race and intelligence, the geneticist and neuroscientist Kevin Mitchell
made a statement in The Guardian
that described the idea of genetic IQ differences between races as
"inherently and deeply implausible" because it goes against basic
principles of population genetics.
There he argued, "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." Mitchell concluded
that, "While genetic variation may help to explain why one person is
more intelligent than another, there are unlikely to be stable and
systematic genetic differences that make one population more intelligent
than the next."
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