Evolution of intelligence

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To read up on the evolution of intelligence, refer to pages 305–322 of Eysenck’s A2 Level Psychology.


  • 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?




  • Ecological and social factors

  • The relationship between brain size and intelligence

  • Measures of brain size


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.

  • 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.

  • 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.


  • 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 on self-recognition supports the Theory of Mind. The mirror test involves applying a red mark to an animal’s forehead. Animals with a self-concept 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.


  • 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 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.


  • 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.

  • 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:

    • The reptilian core. Inherited from our reptile-like ancestors, all animals have this primitive core, which is responsible for basic drives and simple behaviour.

    • 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.

    • 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.

  • 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.


  • 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.

  • 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.

  • 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 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 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.


  • 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 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 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.


  • 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.


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.


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)

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