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EVOLUTION OF INTELLIGENCE
To read up on the evolution of intelligence, refer to pages 305–322 of Eysenck’s A2
Level Psychology.
ASK YOURSELF
 Why did our evolutionary ancestors develop bigger brains and so greater
intelligence?
 Why might fluke genetic mutation account for our bigger brains?
 When it comes to intelligence, size does not matter; bigger brain does not
equal greater intelligence. Why do you think this is?
WHAT YOU NEED TO KNOW
EVOLUTIONARY FACTORS IN THE
DEVELOPMENT OF HUMAN
INTELLIGENCE
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Ecological and social factors
The relationship between brain
size and intelligence
Measures of brain size
ROLE OF GENETIC AND
ENVIRONMENTAL FACTORS IN
INTELLIGENCE TEST PERFORMANCE
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Genetic and environmental factors
that affect intelligence test
performance
Cultural differences
EVOLUTIONARY FACTORS IN THE DEVELOPMENT OF HUMAN INTELLIGENCE
Ecological Selection Theory
Intelligence developed in response to environmental demands. Foraging for food,
hunting, and tool use require intelligence. Obtaining food presents a cognitive
challenge as the location and variations in sources of food must be remembered.
Food supplies are often unpredictable and the capture of food may require complex
hunting strategies.
RESEARCH EVIDENCE FOR ECOLOGICAL THEORY
 According to the dietary hypothesis, primate species whose food supply is
patchily distributed need larger brains than primate species eating food that
is more widely available, due to the greater demands on their memory
systems when trying to locate food. Fruits are usually more patchily
distributed than leaves and so frugivores (fruit-eating primates and other
species) should have larger brains than folivores (leaf-eating primates and
other species), and Clutton-Brock and Harvey (1980, see A2 Level Psychology
page 307) found this to be the case.
 According to the mental maps hypothesis, species that cover a wide area in
their search for food (large range size) and/or spend a large proportion of
the day in foraging will have larger brains than those species that cover only
a small area because they will need larger-scale mental maps. Research
evidence on foraging suggests that mammals do have cognitive maps of food
sources. This supports a link between intelligence and food acquisition.
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According to the extractive foraging hypothesis, species that have difficulty in
extracting food (e.g. they have to use tools to gain access to it) should have
larger brains than those having immediate access to food. Tool use is very
limited; only chimps, orang-utans, and humans routinely use tools, which can
be linked to their higher intelligence. This supports the ecological theory as it
suggests that intelligence is related to more complex foraging strategies.
Humans’ diet is more varied and complex than the herbivorous gorillas and
orang-utans and consequently the digestive systems differ. In humans the
small intestine takes up the most space, whereas in gorillas and orang-utans
the colon does. This shows that the food supply exerted selective pressures
on the digestive system and supports the theory that it may have done the
same to the brain.
RESEARCH EVIDENCE AGAINST THE ECOLOGICAL THEORY
 Dunbar (1998, see A2 Level Psychology page 308) reported across 20 primate
species that there was essentially no correlation between range of foraging
and the size of the neocortex and so contradicts the mental maps hypothesis.
He also did not find a relationship between neocortex size and percentage of
fruit in the diet and so contradicts the dietary hypothesis. He also found that
there was no relationship between complexity of extractive foraging and
relative neocortex size and so contradicts the extractive foraging hypothesis.
 The social theory may provide a more convincing explanation of the
development of human intelligence as dealing with social problems probably
provides more of a cognitive challenge than finding food.
 There is not a clear association between tool use and intelligence. For
example, 150,000 to 300,000 years ago was a time of rapid brain growth in
humans, however, tool manufacture did not show a parallel growth. Between
300,000 years ago and today human brain growth has been slight but the
advancement of tools/technology has been enormous. Consequently, tool use
did not exert a direct selection pressure on intelligence.
 A key weakness of the foraging theory, as a basis on which to judge
intelligence, is that animals with brains much smaller than humans’
successfully use cognitive maps and hunt.
Social complexity
According to the social brain hypothesis, intelligence and brain size will tend to be
greater in those species having complex social structures. Intelligence developed in
response to the demands of group living. The social environment presents a
cognitive challenge as a Theory of Mind is needed. That is, the individual must have
self-awareness and an understanding that others’ intentions, viewpoint, thoughts,
and emotions are different from one’s own in order to predict the behaviour of
others. They will also have an evolutionary advantage if they are able to use tactical
deception and detect cheating in others (Machiavellian intelligence). Social
complexity includes the need for deception, the formation of coalitions, co-operative
strategies, and mating strategies.
RESEARCH EVIDENCE FOR SOCIAL COMPLEXITY
 Research on self-recognition supports the Theory of Mind. The mirror test
involves applying a red mark to an animal’s forehead. Animals with a selfconcept should touch this mark when they look in the mirror. Chimps and
orang-utans reliably demonstrate self-recognition, whereas lower primates
and non-primates do not. Theory of mind develops in humans during infancy.
 Another aspect of the Theory of Mind is the ability to deceive and recognise
deception in others. This is called Machiavellian intelligence and is adaptive
as the individual has much to gain from being able to deceive and cheat
others without raising suspicion. Observational evidence suggests that only
the higher primates show tactical deception. This supports the social theory
that intelligence is a result of social complexity.
 Dunbar (1998, see A2 Level Psychology page 309) correlated both
environmental and social complexity with the size of the neocortex, the area
of the brain associated with higher order thinking. No relationship was found
between neocortex size and environmental complexity, whereas a strong
positive correlation was found between this and group size as an indicator of
social complexity.
 The fossil record provides evidence that the size of the human social group
has increased as we have evolved over time—from Homo sapiens co-habiting
in groups of 150, to the much larger villages and towns of agricultural man.
As the social group increases, so does the need for more complex
interpersonal skills, supporting the theory that intelligence is required for
success and survival within the group.
 Schillaci (2006, see A2 Level Psychology page 309) found among several
primate species that those having the largest relative brain sizes had
monogamous mating systems. Superficially, it looks as if this finding doesn’t
fit the social brain hypothesis. However, it can be argued speculatively that
primate monogamy involves more complex social skills than alternative
mating strategies.
RESEARCH EVIDENCE AGAINST SOCIAL COMPLEXITY
 There is great diversity in the social systems of primates with a Theory of
Mind. Orang-utans have a Theory of Mind and are thought to be intelligent
but do not live in large social groups, which challenges the social complexity
theory.
 Social groups may exist without knowledge of others’ minds (e.g. ants) and
so intelligence is not inextricably linked to social living, as the Theory of Mind
and the Machiavellian hypothesis suggest.
 Given that many apes do live in social groups, much larger brains should be
found in apes and monkeys if intelligence had a social origin. The
encephalisation quotient (EQ) for primates is 2.34 and the EQ for humans is
7; the proportion of the cortex to the rest of the brain is 50% in primates and
80% in humans. Therefore we would expect primates’ brains to be two to
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three times bigger if group living was the main factor in the development of
intelligence.
According to Byrne and Bates (2007, see A2 Level Psychology page 309), we
shouldn’t exaggerate social skills. The enlarged neocortex improves
perception, learning in social contexts, and long-term memory. Thus, these
may contribute more to social complexity than social skills.
EVALUATION OF ECOLOGICAL AND SOCIAL THEORIES
 Face validity. Both ecological theory and social theory make sense as both
food acquisition and living in groups do present cognitive demands and do
enhance survival and reproductive potential. However, Dunbar’s (1998)
research presents strong evidence that social factors drove the evolution of
intelligence. When we consider numerous primate species, several aspects of
social complexity (e.g. group size; deception; mating strategy) predict brain
size (especially relative neocortex size). It is thus unlikely to be a coincidence
that the human species has both relatively the largest neocortex of any
species and is also the most intensely social species.
 Anthropomorphic measures of intelligence. There are great difficulties in
measuring and interpreting animal intelligence. Consider the difficulty we
have in creating a culturally fair test of IQ in humans to appreciate the even
greater difficulty of devising a species-fair test of intelligence across animals
and humans. Thus, the research evidence may lack validity as the measures
of intelligence may lack accuracy, as most research relies on indirect
measures of intelligence such as neocortex size.
 Neocortex size is not synonymous with intelligence. Neocortex size is not
a direct measure of a species’ intelligence, and so we may not be assessing
intelligence very well.
 Difficult to operationalise and so test foraging and social complexity. It
is difficult to compare the cognitive or intellectual demands of finding food
across numerous species leading very different lives in very different kinds
of environment. This means the values assigned to foraging complexity may
not be valid and this may have affected the correlational findings. Similarly, it
is difficult to measure social complexity, group size, and mating strategy may
not be valid measures of this.
 Cause and effect. The research evidence on ecological and social factors
identifies associations, which do not indicate cause and effect. Consequently,
conclusions are limited to “links” rather than causes.
 Direction of the effect. It cannot be established which came first: varied diet
or intelligence; large social groups or intelligence. It may be that better diet
and group living were consequences of the development of intelligence. In
which case, what caused intelligence in the first place is not clearly
established.
 A chance mutation. Human intelligence may be the result of a chance
mutation that resulted in bipedalism and so freed our hands to forage and
create tools. Brain size and intelligence would be a consequence of better
diet.
The Relationship Between Brain Size and Intelligence
Humans have brains two to three times bigger than our closest related species and
so finding the reason why the human brain is so large and if size is all important
leads to interesting debate.
RESEARCH EVIDENCE FOR A RELATIONSHIP BETWEEN BRAIN SIZE AND INTELLIGENCE
 Co-evolution. This refers to when two characteristics evolve in tandem.
Analogies have been drawn in which the brain growth is the “hardware” and
the mental abilities the “software”, and consequently they are
interdependent. This suggests that there is a positive correlation between
brain size and intelligence.
 The triune model. According to MacLean (1970, see A2 Level Psychology page
305), the brain consists of three main sections:
o The reptilian core. Inherited from our reptile-like ancestors, all
animals have this primitive core, which is responsible for basic drives
and simple behaviour.
o Old mammalian (limbic system)—the mid-section. This area of the
brain is concerned with fighting, feeding, self-preservation, sociability,
attachments, and parental care. It integrates sensory perception with
bodily functions.
o The neocortex—outer layer. The cortex is found only in mammals and
is responsible for higher order mental processes. The cortex is the
outer layer of the cerebral hemispheres and is known as the grey
matter compared to the white matter of the hemispheres. The cortex
is only 2 millimetres thick and has a bumpy, folded appearance. These
folds mean that the actual area of the cortex is large.
The triune model shows the increasing growth and sophistication of the brain in
animals higher up the phylogenic tree (evolution hierarchy). It provides research
evidence for a relationship between brain size and intelligence.
Measures of Brain Size
The question of whether size is a clear indicator of intelligence has been researched
extensively through comparative studies. Initially, crude measures of gross brain
size were compared. However, measures of brain size are over-simplistic as the
elephant’s brain is four times the size of a human’s and some species of whale have
brains five times human size, which is explained by their greater body mass. The
ratio of brain size to body mass was thought to be a better indicator. However, using
this criterion, the mouse outclasses the human with a relative brain to body size of
3% compared to 2% for humans.
Encephalisation quotient (EQ). Consequently, the encephalisation quotient (EQ)
was introduced. This is the calculation of the brain size relative to body mass
compared to what would be expected for a mammal of similar body size. An
allometric line (in this case, allometry equals the relationship between the size of a
mammal and the size of its brain) can be constructed on a graph where those above
the line have a positive encephalisation showing the brain size is greater than that
expected of an animal of its body mass. Humans lie well above the allometric line as
we have a high EQ—the human brain is twice the size expected for an ape of similar
size. This is show by the amount of energy the human brain consumes. A chimp
devotes only 8% of its basal metabolic rate (i.e. number of calories when resting) to
maintaining the brain, whereas humans devote 22%.
Fossil records. The fossil records suggest that the human brain underwent rapid
expansion about 2 million years ago, as Australopithecines possessed normal size
brains for body mass whereas Homo sapiens possess brains twice the size. Thus, the
Australopithecines’ brain size falls on the allometric line, whereas the Homo sapiens’
lies above it, showing a positive encephalisation of 2.95. Seven million years ago our
ancestors had a brain that was only about 400 cubic centimetres (cc). Brain size
didn’t change much for millions of years after that, but by 2.5 millions years ago
with Homo habilis the brain size had almost doubled to about 700 cc. Homo habilis
then evolved into Homo erectus who had a brain size of about 1000cc. Over the past
500,000 years, human brain size increased again and is now about 1350cc, Stewart
(1997, see A2 Level Psychology page 306).
The fact that the human brain has grown to a size that incurs great costs (such as the
dangers of child birth and an extended period of parental care, as the human infant
is now born at an earlier stage in development because the brain is larger) suggests
that large brains must have adaptive value so that the benefits outweigh the costs.
This supports the argument that larger brains are linked to intelligence.
Brain size and IQ. Initial studies measured the head perimeter and correlated this
with IQ score. No correlation was found. However, this is due to the crudeness of the
measure of brain size! Today we can obtain good measures of brain size in living
individuals by using brain-imaging techniques such as magnetic resonance imaging
(MRI). McDaniel (2005, see A2 Level Psychology pages 306–307) reported an
average correlation between brain size and IQ of +.33 across 37 samples.
RESEARCH EVIDENCE AGAINST A RELATIONSHIP BETWEEN BRAIN SIZE AND
INTELLIGENCE
 The size of the human brain is not solely genetic as it is also influenced by
environment, i.e. nurture. A more stimulating environment and even feeding
babies breast milk have been found to increase IQ, showing that brain size is
not just a product of evolution.
 Not all parts of the brain are equal. The triune model shows that some parts
are more sophisticated than others. This applies not just to size but to brain
specialisation and localisation. As the triune model shows, the cortex is a
specialised area of the brain found only in mammals. It is the seat of higher
order mental functions and so is linked to thought, language, perception,
attention, and intelligence. The frontal cortex in humans is significantly larger
than that of other mammals. In mammals the neocortex has six layers
whereas in whales and dolphins it has five. However, the neural density of
cetaceans’ frontal lobes is similar to humans’—therefore quality not just
quantity needs to be considered. This shows that it is not overall brain size
that is significant but particular specialised areas of the brain.
 Gender differences provide further evidence that brain specialisation is as
important as overall size. Males’ brains are bigger than females’ yet women
have the same IQ scores. This may be because male and female brains are
specialised in different ways; males are better at spatial tasks (reading maps,
parking a car, finding the way round a supermarket) whilst females are
specialised for language, and the male specialisms may require extra
capacity. In fact females’ smaller brain size may be compensated by the fact
that they are better organised than males. Females have a larger corpus
callosum (the nerve fibres that connect the two hemispheres), which would
improve communication between the hemispheres—so perhaps in this case
size does matter!
 A post-mortem examination of Albert Einstein’s brain found that the neurons
in the prefrontal cortex were more tightly packed. This supports the idea that
brain localisation (the organisation of the brain whereby different regions
relate to different functions) is more important than size.
EVALUATION OF THE RELATIONSHIP BETWEEN BRAIN SIZE AND INTELLIGENCE
 Anthropomorphism. Comparative studies may be invalidated because
animals are usually tested and judged on tasks that are unnatural because
they test human abilities. It is difficult for researchers to escape their own
anthropomorphic bias.
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Validity of IQ tests. As these are culturally biased their meaningfulness as a
measure of intelligence is reduced, so providing only weak support to the
relationship between brain size and intelligence.
Research evidence. Evolution is accepted by the scientific community as
fact. There is evidence of the gradual change of evolution, such as changes in
anatomy, in particular brain size that necessitated an increase in meat
consumption. However, we cannot be sure of the exact processes of evolution
because the theory is post hoc (made up after the event) and so we have to
rely on fossils and other sources of incomplete evidence. This means we
cannot be entirely sure of the evolution of brain size, and in particular
intelligence, because judgements about intelligence are made based on our
knowledge of our ancestors’ behaviour, which may not be very
comprehensive.
Scientific criticisms. The lack of evidence means that evolutionary
explanations lack scientific validity because they can neither be verified nor
falsified and so do not meet Popper’s criterion of science that the theory can
be tested and rejected.
Reductionism. Intelligence is extremely complex and so there is unlikely to
be a simple relationship between this and the EQ. The triune model is very
oversimplified, given the brain’s overall complexity. The brain can’t really be
neatly subdivided into just three parts. In addition, there are important
differences within the different parts of each layer. For example, within the
neocortex there are areas specialised for visual processing and other areas
specialised for language processing.
Determinism. Evolutionary theories are considered deterministic because
they suggest that genes control behaviour, which ignores the free will of the
individual, i.e. their ability to control their own behaviour. Whilst
evolutionary psychologists do not usually take the view that we cannot
escape our genes, the theories themselves can appear deterministic, and so
this ignores our free will, which many do exercise in the development of their
own intelligence.
Correlational criticisms. The research evidences an association only
between brain size and intelligence. Thus, cause and effect cannot be inferred
and so conclusions are limited to association rather than causation.
Furthermore, the correlation quotient of +.33 is not a strong correlation.
Another weakness is that correlations are reductionist because they only
analyse two factors, when in reality the relationship is likely to be
multifactorial, as supported by the identification of factors such as nurture
and organisation.
Direction of effect. Utilisation versus atrophy. Use it or lose it! The direction
of effect is difficult to establish, e.g. was Einstein’s prefrontal cortex the cause
or effect of his intellectual powers?
Nature/nurture. The evolutionary explanations over-emphasise the role of
nature and ignore nurture. It is not a question of nature or nurture, as
indisputably an interactionist perspective must be taken. However, as the
evolutionary explanations ignore nurture this is a key weakness.
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Multi-perspective. Evolutionary explanations need to be considered in
combination with other explanations. Both biological and psychological
factors interact in the development of intelligence and so a compromise
position such as the diathesis–stress model is needed, which accounts for the
influence of genetic predisposition and environmental experience.
Role of genetic and environmental factors in intelligence test performance
Individual differences in intelligence can be linked to either heredity or
environment. Heredity consists of each person’s genetic inheritance, the
instructions that tell your body to produce hair of a particular colour, etc.
Environment consists of the situations and experiences encountered by people
during their lives.
Many psychologists have studied the relative contributions of genetics versus
environment and this leads to the conclusion that individual differences in
intelligence depend on differences in genetic endowment or differences in the
environment.
Those who believe in the importance of heredity draw a distinction between the
genotype and the phenotype. The genotype is the genetic inheritance whereas the
phenotype consists of an individual’s observable characteristics. So far as
intelligence is concerned, we can’t assess the genotype. All we can do is assess the
phenotype by administering an intelligence test.
However, the reality is we cannot separate out the effects of hereditary and
environment because our genetic makeup influences the types of environmental
experiences we have.
Plomin (1990, see A2 Level Psychology page 312) identified three types of
interdependence between genetic factors and environment:
1. Active covariation: occurs when children of differing genetic ability look for
situations reinforcing their genetic differences (e.g. children of high genetic
ability reading numerous books).
2. Passive covariation: occurs when parents of high genetic ability provide a
more stimulating environment than parents of lower genetic ability.
3. Reactive environment: occurs when an individual’s genetically influenced
behaviour helps to determine how he/she is treated by other people.
Twin studies are a useful way of assessing the relative importance of genetic factors
and environment by comparing monozygotic twins (100% same genes) and
dizygotic (also known as fraternal) twins (approximately 50% of the same genes). If
genetic factors influence individual differences in intelligence, identical twins should
be more alike in intelligence than fraternal twins. In contrast, if environmental
factors are all-important, identical twins should be no more alike than fraternal
twins. The degree of similarity in intelligence shown by pairs of twins is usually
reported in the form of correlations. A correlation of +1.00 would mean that both
twins in a pair have essentially the same IQs, whereas a correlation of 0.00 would
mean there is no relationship between the IQs of twins.
Adoption studies provide another way of assessing the relative importance of
genetic factors and environment in determining individual differences in
intelligence. If genetic factors are more important than environment, adopted
children’s IQs will be more similar to those of their biological parents than their
adoptive parents. The opposite pattern will be found if environment is more
important.
Heritability is a population measure that provides an estimate of the importance of
genetic factors in determining individual differences in intelligence. If everyone in a
given population were exposed to precisely the same environmental conditions then
all individual differences in intelligence would be due to genetic factors and so
heritability would be extremely high. Whereas in societies with enormous
environmental differences the role of genetic factors in producing individual
differences in intelligence would be small and so heritability would be low.
Research evidence for genetic factors
 Bouchard and McGue (1981, see A2 Level Psychology page 312) reviewed 111
studies, and reported that the mean correlation for identical twins was +.86
compared to +.60 for fraternal twins.
 McCartney, Harris, and Bernieri (1990, see A2 Level Psychology page 312)
reported similar findings from a later analysis of numerous studies: the mean
correlation for identical twins was +.81 compared to +.59 for fraternal twins.
 Bouchard and McGue (1981, see A2 Level Psychology page 313) found that
the mean correlation coefficient for identical twins brought up apart was
+.72. Identical twins brought up apart should be very similar to each other in
IQ if genetic factors are very important. Thus, the +.72 seems to provide fairly
convincing evidence for the importance of both genetic and environmental
factors. The finding that the correlation is higher than that for fraternal twins
brought up together suggests the importance of genetic factors. The finding
that the correlation (+.72) is lower than that for identical twins (+.86)
suggests the importance of environmental factors.
 Bouchard et al. (1990, see A2 Level Psychology page 313) found similar
findings to the above study as they studied more than 40 adult identical twin
pairs separated at a mean age of 5 months, and found their IQs correlated
+.75. The similarity of the correlations supports the reliability and validity of
the genetic basis of intelligence.
 Mackintosh (1998, see A2 Level Psychology page 314) reviewed the evidence
based on heritability measures. He concluded that between 30% and 75% of
individual differences in intelligence in modern industrialised societies are
due to genetic factors.
 Brace (1996, see A2 Level Psychology page 314) found that the heritability of
intelligence was much higher among people living in affluent white American
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suburbs than among people living in American urban ghettoes. This is
because the favourable environment experienced by those in the suburbs
meant individual differences in intelligence depended mainly on genetic
factors.
Horn (1983, see A2 Level Psychology page 315) reported findings from the
Texas Adoption Project, which involved almost 500 adopted children. The
correlation between the adopted children and their biological mothers was
+.28, and between the adopted children and their adoptive mothers was even
lower at +.15. Both of these correlations are very low but they do suggest a
greater role for heredity as the correlation between biological relatives was
higher than between adopted relatives.
Loehlin, Horn, and Willerman (1989, see A2 Level Psychology page 315)
found there were some differences in the findings when the adopted children
were tested again 10 years later. Now the children showed an increased
correlation with their biological mothers, but a reduced one with their
adoptive mothers. Shared family environment between the adopted children
and their adoptive mothers was reduced in importance. In contrast, genetic
factors had a greater influence on the adopted children’s intelligence than 10
years earlier.
Research evidence against genetic factors and so for environmental factors
 Loehlin and Nichols (1976, see A2 Level Psychology page 314) point out that
the differences between MZ and DZ twins may not be solely due to genetic
factors because identical twins are treated in a more similar fashion than
fraternal twins. This can include parental treatment, playing together,
spending time together, dressing in a similar style, and being taught by the
same teachers. Thus, the differences in intelligence may be due to
environmental, rather than genetic, factors.
 The prenatal environment may also explain the differences in intelligence
between MZ and DZ. Two-thirds of identical twins (MZ) share a placenta
whereas fraternal twins (DZ) have separate placentas. This means the
prenatal environment of most identical twins is more similar than that of
fraternal twins and so environmental factors could explain the intelligence
correlations. Identical twins sharing a single placenta are more similar in
intelligence than those having separate placentas (Phelps, Davis, & Schwartz,
1997, see A2 Level Psychology page 313).
 The correlation of +.28 between adopted children and their biological
mothers found by Horn (1983) is much less than the correlation of +.42
between parents and children when children aren’t adopted (Bouchard et al.,
1981, see A2 Level Psychology page 315) and so this difference must be due
to environmental factors.
 Bouchard and McGue (1981, see A2 Level Psychology page 316) found that
the correlation for identical twins brought up together was +.86 compared to
+.72 for identical twins brought up apart. Thus, whilst twin studies are
usually used as evidence for genetic factors this difference supports
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environmental factors because it is due to the fact that identical twins
brought up together have more similar environments than those brought up
apart.
The Flynn effect shows the environmental factors can have a substantial
effect on intelligence because Flynn (1987, see A2 Level Psychology page 316)
found a rapid rise in average IQ in many Western countries in recent
decades. Such large and rapid increases in IQ are due mainly to
environmental factors, such as longer time spent in education and greater
access to information.
Further evidence for environmental factors is provided by Sameroff et al.
(1987, see A2 Level Psychology page 317) who identified 10 family risk
factors related to lower IQ, which included: mother didn’t go to high school;
father had a semi-skilled job. They found that at the age of 4, high-risk
children were 24 times more likely to have IQs below 85 than low-risk
children. On average, each risk factor reduced the child’s IQ score by 4 points.
EVALUATION OF THE ROLE OF GENETIC AND ENVIRONMENTAL FACTORS IN
INTELLIGENCE TEST PERFORMANCE
 Sample bias. Identical twins are relatively rare, and identical twins brought
up in separate families are obviously even rarer.
 Adoption studies do not isolate genetic factors. Many identical twins
brought up apart were brought up in separate branches of the same family,
and so their environments may have been fairly similar. Other identical twins
were brought up together for several years before being separated. Note
Bouchard’s later study (Bouchard et al., 1990, see A2 Level Psychology page
313) addressed this, as it only involved twins separated before 5 months of
age.
 Difficult to interpret the findings from adoption studies. It is very hard to
interpret the findings of many adoption studies because of selective
placement, which is when children are placed in homes similar to those of
their biological parents’ in educational and social backgrounds. Thus, the
correlation between adopted children and their biological parents may be
due to selective placement rather than to genetic factors.
 Impossible to establish that environmental risk factors cause lower
intelligence. Sameroff et al.’s (1987) findings don’t show that the
environmental risk factors they identified were actually responsible for low
IQs. It is also likely that the parents of the high-risk children were less
intelligent than those of the low-risk children and so there are differences in
genetic potential between the low-risk and high-risk groups of children. It
seems likely that the adverse environmental factors have some negative
effects on children’s intelligence but it is, as always, impossible to separate
out the influence of genes versus environment.
 Strong empirical support. Twin studies provide convincing evidence
because they allow us to observe the effects of varying degrees of genetic
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similarity on intelligence, and so provide strong support for both the
influence of genetic and environmental factors.
Genetic and environmental factors are positively correlated. Individuals
with the greatest genetic potential for intelligence tend to find themselves in
environments favourable for the development of intelligence (e.g. staying at
school until the age of 18, going to university). This makes it hard to
disentangle the effects of genetic and environmental factors.
Correlational evidence. Research into environmental factors is
correlational because the environment cannot be manipulated. This means
cause and effect cannot be established and so we cannot conclude that
environmental factors cause changes in intelligence.
Validity of IQ tests. Intelligence tests are not necessarily a valid measure of
intelligence. They are culture biased and narrow in scope because they fail to
assess social or emotional intelligence. Consequently, the evidence is on
differences in intelligence as assessed by intelligence tests, which is not
necessarily a valid measure of intelligence.
Culture
Culture has an effect on intelligence test performance because the cognitive skills
that are important vary from one culture to another. Thus, for example, language
skills including reading and writing are very important within most Western
cultures, but more practical skills are emphasised in other cultures.
Culture is an issue in terms of IQ tests because the tests have been devised by
psychologists working in the United States or in Europe. It has sometimes been
claimed that American and/or Europeans are more intelligent than people from
most other parts of the world. However, this is simply not true. The problem is that
the IQ tests are ethnocentric, i.e. biased to favour the culture in which they were
devised, and so they are not a valid measure of intelligence in other cultures.
Research evidence of cultural differences
 Okagaki and Sternberg (1993, see A2 Level Psychology page 319) studied
ethnic groups in San Jose, California and found concepts of intelligence varied
within a culture. Asian parents emphasised the importance of cognitive skills
in their conception of intelligence. In contrast, Latino parents argued that
social-competence skills are of particular importance in their conception of
intelligence.
 Grigorenko et al. (2004, see A2 Level Psychology page 320) studied different
aspects of intelligence in Yup’ik Eskimo children living in southwest Alaska.
Some of these children lived in the towns and others lived out in the country.
The study tested children’s practical intelligence (e.g. knowledge of how to
travel in the virtual absence of landmarks) and also used traditional
intelligence tests. The urban children performed better than the rural
children on traditional intelligence tests, whereas the rural children
outperformed the urban ones on the test of practical intelligence. These

findings clearly represent the skills most relevant to the children in their
everyday lives.
Sternberg et al. (2002, see A2 Level Psychology page 320) argued that
children in many cultures perform poorly on conventional intelligence tests
because they have little experience of this form of assessment. He introduced,
where the individuals are tested on two separate occasions with training in
the skills assessed by the tests being provided between tests. Children in
Tanzania showed substantial improvements between the first and second
test suggesting that they had abilities and an ability to learn not revealed on
the first testing occasion. Thus, dynamic testing in developing countries can
provide a better assessment of intelligence than traditional single testing.
EVALUATION OF THE RESEARCH INTO CULTURAL DIFFERENCES
 Validity of intelligence tests. If we want to obtain a valid assessment of
intelligence in any given culture we must always consider the cultural
context. Research such as Grigorenko et al.’s, which has done this with the
assessment of practical knowledge, provides a much more valid measure of
intelligence.
 Dynamic testing is less biased. Dynamic testing is potentially a very useful
way of assessing intelligence. It assesses an individual’s speed of learning,
which is of great importance to intelligence.
SO WHAT DOES THIS MEAN?
The factors in the development of human intelligence thus far offer no conclusive
explanation for why human brains grew so large. A combination of ecological and
social factors seems the most comprehensive account and so a multi-perspective
needs to be taken. Particular weight needs to be given to social factors, given
Dunbar’s (1998) research. However, these factors do not effectively pinpoint the
origin of intelligence.
The mutant gene that led to bipedalism may be the origin, and so the development
of intelligence needs to be traced back to this. The co-evolution of brain size and
mental abilities may be in part due to a chance mutation. There is evidence to
support co-evolution as brain size and intelligence are related. However, other
factors are involved in the association. The human brain is highly complex and more
organised than any other animal’s and this may be equally, if not more, relevant
than total brain size.
It seems likely that environmental and then social complexity are the earliest origins
of human intelligence, however, these do not account for why the human brain is
more advanced, at least according to EQ measures! Thus, social and ecological
factors must interact with later factors, more specific to humans, such as sexual
selection, language, bipedalism, and the ability to control fire.
The environmental factors that influence intelligence testing show that intelligence
is not solely an evolved mechanism. Research evidence from twin, adoption, and
family studies does show the role of genetic factors, however these studies can also
be turned around to show the influence of environmental factors, for example the
fact that intelligence correlations are lower for identical twins raised apart than
those together is due to environmental factors. The influence of culture on
intelligence shows quite how pronounced the effect of the environment can be as
the very concept of what intelligence is depends on the cultural context.
OVER TO YOU
1. Critically consider evolutionary factors in the development of human intelligence.
(25 marks)
2. Critically consider the relationship between brain size and human intelligence.
(25 marks)