Individual Differences


Nature vs Nurture
in Intelligence

Last updated:
10 Apr 2005


The issue of what causes individual differences in intelligence goes beyond psychology, and involves moral, political, ethical, educational, social, physiological and statistical issues to name just a few.  The issue of how differences in intelligence come about between individuals and groups is a topic of much fascination and controversy - it can arouse strong reactions and elicit personal beliefs and biases.

This page outlines some of the main psychological concepts and evidence that relates to explaining individual differences in intelligence.

As a student, it is your responsibility to develop a familiarity with the basic arguments, strengths, and weaknesses for and against the causal influences and correlates of intelligence.

Nature vs Nurture - What do you think?

In looking for the causes of individual differences in intelligence, a major issue is the relative contribution of genetics and environment.




100% Genetics

90% Gen.

80% Gen.

70% Gen.

60% Gen.


60% Env.

70% Env.

80% Env.

90% Env.

100% Environ.


Rate the extent to which you believe nature and nature influence (cause) human intelligence.

Warning!  As you learn more about the theory and research on genetic and environmental influences on human intelligence, you may find that you change some of your beliefs and assumptions.

Pendulum of opinion on Nature vs. Nurture through history

The zeitgeist (the intellectual and culture "flavor" of a time and place) has swung back and forth over time with regard to the amount of influence that nature vs. nature has on human intelligence.

  • For example, in the late 1800's in the UK, as Darwinism took off, the role of genetically determined capability was considered very important. 

  • This was in constrast, for example, to the 1960's in the USA, when views were more in favor a "tabula rasa" (blank state) view of human intelligence - in other words, all people are capable of much more, if given conducive environmental conditions in which to reach their potential

  • Currently the Zeitgeist is the Western psychological world is somewhere inbetween - both genetics and environment are seen as playing important roles.  To be more precise, the modern view about nature vs nurture in intelligence is "interactionist".  This view is well expressed by Ridley (1999, p.77):

"Mother Nature has plainly not entrusted the determination of our intellectual capacities to the blind fate of a gene or genes; she gave us parents, learning, language, culture and education to program ourselves with."

Historical trends in the nature-nurture debate

Late 19th century - early 20th century (Nature)

From the mid to late 1800's through to the early 1900's opinions rested in the nature camp. This was consistent with the scientific discoveries of the role of inheritance and natural selection by Mendel and Darwin.

The major contributor to the psychological argument was Francis Galton in his book "Hereditary Genius: Its Laws and Consequences” (1869).

Galton had observed that the gifted individuals tended to come from families which had other gifted individuals. He went on to analyze biographical dictionaries and encyclopedias, and became convinced that talent in science, the professions, and the arts, ran in families.

Galton took this observation one step further, to argue that it would be "quite practicable to produce a high gifted race of men by judicious marriages during several consecutive generations".

This suggestion became know as eugenics, "the study of the agencies under social control that may improve or repair the racial qualities of future generations, either physically or mentally."  Galton wanted to speed up the process of natural selection, stating that: "What Nature does blindly, slowly, and ruthlessly, man may do providently, quickly, and kindly”.

Galton was convinced that "intelligence must be bred, not trained". Such arguments have had massive social consequences and have been used to support apartheid policies, sterilization programs, and other acts of withholding basic human rights from minority groups.

Post WWI: 1920’s-1930’s

After World War I, careful reanalysis of the mass of intelligence test data took place. This began to challenge the commonly held view that intelligence was directly, genetically linked to racial differences:

  • e.g. blacks from Illinois had higher IQ scores than whites from 9 southern states - a finding difficult to reconcile with the simple idea that whites are intellectually superior to blacks.

Evidence now seemed to support a closer link between social class and intelligence, rather than race and intelligence. As a result, a number of psychologists in the 1920s and 1930s shifted their position towards the environmental camp.

The shift against 'nature' views was given momentum by the backlash against the social consequences of government policies:

  • e.g. sterilization laws had been passed in 24 US States, resulting in 20, 000 people being sterilized against their will. 320, 000 people suffered the same fate in Germany.


The backlash faded, and the pendulum swung back towards the middle. From the early 1940's, it seemed there was a rejection of simplistic nature or nurture views, with more common recognition of their complex interplay. Nevertheless, social prejudices and inequalities were still evident and growing.

Thus, in the 1960's, the focus of the problem was shifted away from the individual as the cause of the problem, and centered on social determinants. Thhe pendulum swung towards the nurture/environmental end and away from the nature/genetic end. Efforts were made to arrest poor educational achievement through special schooling, and to alleviate poor living conditions through welfare.

It became politically correct to minimize talk and discussion of the role of 'nature' in contributing to any individual differences, let alone intelligence. The evidence of differences in intelligence between socioeconomic groups and racial groups, however, did not go away.

Recent trends – "The Bell Curve" controversy

From time to time, there have been inflammatory articles which present and interpret evidence of IQ differences between groups (in particular Jensen, 1969). The most recent, and most major of these publications was Herrnstein and Murray's (1994) "The Bell Curve". This book provided momentum to swing the pendulum in the direction of 'nature', at least in the public's eye, but even more so, it generated massive debate and controversy in psychology, sociology, education, and politics, not to mention the media and household. The 800+ page book, written for laypersons, hit the best-seller lists in the U.S.

"The work's main thesis is that an individual's intelligence - no less than 40% and no more than 80% of which is inherited genetically from his or her parents - has more effect than socioeconomic background on future life experiences."
Manolakes (1997), p.235

In addition to the premise that measured intelligence (IQ) is largely genetically inherited, a second important premise was that IQ is correlated positively with a variety of measures of socioeconomic success in society, such as a prestigious job, high annual income, and high educational attainment; and is inversely correlated with criminality and other measures of social failure. It was suggested that SES successes (and failures) are largely genetically caused.

Some sample controversial quotes from "The Bell Curve"

  • “IQ has more effect on future life experiences than SES”

  • “intervention efforts are largely a waste of time and money”

  • “increasing population of 'lower caste' intelligences, lessening the
    nation's 'genetic capital”

Reactions to The Bell Curve:

  • The Bell Curve" re-ignited the nature-nurture debate.

  • The public debate was (and is) divided.

    • The politically left saw the authors as "un-American”;- "pseudo-scientific racists”;- and the book as "alien and repellent"

    • The politically right saw the authors as:
      - "brave and respectable scholars,”;- whose book was "lucid" and "powerfully written"

  • The part of The Bell Curve that captured public attention was on the differences in IQ between African and Caucasian Americans. Further to this were the suggestions made by Herrnstein and Murray about the implications of a predominantly genetically-inherited intelligence for public and social policy. Since IQ was largely seen as genetically determined, the authors expressed resistance to educational and environmental interventions. They argued that money spent in this way is wasted. The authors also argued that America is becoming a society of 'cognitive castes', with the lower caste including a large proportion of African-Americans. Hence their statement that the 'genetic capital' of society is being eroded because the less intelligence, lower class is reproducing at a greater rate than high IQ classes.

Evidence in favour of “nature”

In the heyday of eugenic IQ testing in the 1920s there was no evidence for the heritability of IQ. It was just an assumption of the practitioners. Today that is no longer the case. The heritability of IQ (whatever IQ is!) is now an hypothesis that has been tested - on twins and adoptees. The results really are quite startling. No study of the causes of intelligence has failed to find a certain and often substantial heritability. What varies from study to study is the amount that can be attributed to heritability.

Concordance rates of IQ scores

  • Evidence from family studies provides the main supporting evidence from which arguments about the relative roles of genetics and environment are constructed.

    A large number of the study of twins reared apart was undertaken by Thomas Bouchard of the University of Minnesota starting in 1979. He “collected” pairs of separated twins from all over the world and reunited them while testing their personalities and IQs. Other studies at the same time concentrated on comparing the IQs of adopted people with those of their adopted parents and their biological parents or their siblings. Put all these studies together, which include the IQ tests of tens of thousands of individuals, and the table looks like this:

    Same person tested twice 87%

  • Identical twins reared together 86%

  • Identical twins reared apart 76%

  • Fraternal twins reared together 55%

  • Biological siblings reared together 47% (studies show that reared apart about 24%)

  • Parents and children living together 40%

  • Parents and children living apart 31%

  • Adopted children living together 0%

  • Unrelated people living apart 0%

    Ridley, 1999, p.83 [The number is a percentage correlation . Attach section from Ridley’s book.]

Meta-analytic estimates of the heritability of intelligence

A meta-analysis of 9 family studies was conducted by Daniels, Devlin and Roeder (1997): it included 212 correlations and produced very similar results to those quoted by Matt Ridley. These authors conclude that heritability can account for 48% of the variation in IQ. The highest estimates have come from reviews of research by Herrnstein & Murray, 1994 (74%) and Eysenck (80%). A safer bet is probably to sit on the fence - 50:50!

Heritability indices, however, are not pure measures of genetic inheritance - they included prenatal environmental influences, (e.g. whether the mother smokes, what she eats, etc.) and the postnatal material environment. Thus these heritability indices are likely to overestimate the role of genetics.

Correlation of child-parent verbal ability scores

This graph shows correlations between children and their parents and adopted children and their biological and adoptive parents on verbal ability scores.

Heritability & intelligence

It must be noted, however, that heritabilty is not pure genetic influence as the pre and postnatal environments must be taken into account. Heritability estimates based on comparing correlations between IQs of monozygotic (identical) twins reared together with IQs of dizygotic (fraternal) twins and siblings are likely to overestimate the genetic component because monozygotic twins share more similar environments - both in the womb and out

  • twins reared apart are not assigned at random to foster or adoptive parents - since homes are selected purposely to with regard to characteristics of the child and characteristics of the family. This would partially account for the IQ correlations attributed to inheritance

  • twin studies may not be generalizable to the population at large as twins are more susceptible to prenatal trauma leading to retardation. The inclusion of retarded cases may increase the twin correlation in intellligence test scores.
    heritability indexes refer to the population on which they were found at the time and is not applicable to an analysis of test performance between two population groups e.g. ethnic groups.

  • heritability does not indicate the degree to which a trait can be modified e.g. even if the heritabilty of a trait, like intelligence were found to be 100% it wouldn’t mean it couldn't be modified. (Anastasi & Urbina, 1997).

In the discussion to date, we have focused on the heritability (or otherwise) of general intelligence. What about the subcomponents of intelligence? There is, indeed, evidence of a greater genetic link for:

1. Spatial ability
2. Reasoning

And less evidence for genetic influence on:

1. Divergent thinking
2. Verbal fluency

There is, however, relatively little research along these lines.

Evidence in favour of “nurture”

"Give me a dozen healthy infants & my own specific world to bring
them up in, & I'll guarantee to take any one at random & train him to
become any type of specialist I might select - doctor, lawyer, artist,
merchant, chef & yes, even beggar & thief, regardless of his talents,
penchants, tendencies, abilities, vocations, and race of his ancestors."
- John B. Watson, 1924

This was a famous quote in the heyday of behaviorism, when the child was considered to be a 'tabula rasa' (blank slate) onto which anything could be sculpted through environmental experience. This would be a 100% environmental view, but virtually no psychologists would accept such an extreme position today.

The Flynn effect: Are we getting smarter?

In the 1980s, a NZ-based political scientist, James Flynn, noticed that IQ was increasing in all countries all the time, at an average rate of about 3 IQ points per decade i.e. the average IQ across the world has risen over 1 standard deviation (i.e. 15 points) since WWII - predominantly due to environmental effects. As a result, new norms continue to be used to rescale IQ tests to '100'.

Could this be due to diet? Possibly but IQ scores are still rising just as rapidly in well-nourished western countries. Could it be schooling? Interruptions to schooling only have temporary effects on IQ. Importantly, it is those test s that test abstract reasoning ability that show the steepest improvements. One researcher, Ulric Neisser suggests that the Flynn effect is due to the way we are being saturated with sophisticated visual images: ads, posters, videogame and TV graphics etc - rather than written messages. He suggests that children experience a much richer visual environment than in the past and that this assists them with visual puzzles of the kind that dominate IQ tests.

The evidence for the rise in IQ comes from:
• Adoption studies
• Nutrition studies
• Educational intervention studies

Intelligence varies with at least 21 factors

Some of the other circumstances and attributes that have been found to vary to a greater or lesser (but always significant) extent in relation with IQ (Bouchard & Segal, 1985; Liungman, 1975) - note that not all of these relationships support an environmental view.

Intelligence varies with:

  • Infant malnutrition (-ve)

  • Birth weight

  • Birth order

  • Height

  • Number of siblings (-ve)

  • Number of years in school

  • Social group of parental home

  • Father's profession

  • Father's economic status

  • Degree of parental rigidity (-ve)

  • Parental ambition

  • Mother's education

  • Average TV viewing (-ve)

  • Average book-reading

  • Self-confidence according to attitude scale measurement

  • Age (negative relationship, applies only in adulthood)

  • Degree of authority in parental home (-ve)

  • Criminality (-ve)

  • Alcoholism (-ve)

  • Mental disease (-ve)

  • Emotional adaptation

"No single environmental factor seems to have a large influence on IQ. Variables widely believed to be important are usually weak....Even though many studies fail to find strong environmental effects....most of the factors studied do influence IQ in the direction predicted by the investigator....environmental effects are multifactorial and largely unrelated to each other."
- Bouchard & Segal (1985), p.452

So, it would appear that there are many psychological and biological factors each contributing a small a small fraction to the variance in IQ scores.

Intelligence & race

Let's focus on some of the correlates of intelligence examining in particular the interplay between race, environment and intelligence; between SES, environment and intelligence; between education, environment and intelligence; and between occupation, environment and intelligence.

Herrnstein and Murray (1994) in The Bell Curve state that :
• Asians and Asian-Americans have a (.32 standard deviation) higher average IQ than white Americans, and that
• white Americans have a (1.58 standard deviation) higher mean IQ than black Americans.

Further, they claim that this difference is not a function of cultural testing bias.

Herrnstein and Murray (1994) acknowledge that the causes of these differences could be environmental, however the differences in IQ appear to be too large to be accounted for by environmental influences alone. They provide much qualification, cautioning, and warnings about how their evidence should be interpreted and used. In particular, they remind the reader that
• IQ is not strongly linked to many so-called 'desirable' human qualities; and
• The fallacy of drawing conclusions about individual on the basis of group findings.

It would be incorrect to characterize "The Bell Curve" as out-and-out a racist, eugenicist, etc. book. Even detractors acknowledge the importance of its contribution to psychological and social debate.  But the book does, in general, support a view that  intelligence is largely heritable.

Note that there have also been investigations into racial differences on subcomponents of intelligence.  Herrstein and Murray (1994) report that:
• East Asian scores are typically the same or slightly lower than White American scores on verbal IQ, but much higher on visuo-spatial IQ
• Black Americans tend to score higher than whites on subtests involving arithmetic and immediate memory, whereas whites typically score high than blacks on subtests of spatial-perceptual ability

Differences in IQ scores between races does not necessarily imply genetic causes. Most psychologists accept that there are group mean differences in IQ scores for non-culturally-loaded tests. However, these differences are confounded with the effects of SES, e.g.

1. Children with black fathers, brought up in white family - no evidence of lower IQs
2. Adoption studies - e.g. black children brought up by white families only slightly lower IQs than white adopted children (Howe, 1997)

Some theorists suggest that socioeconomic disadvantages are the main causes of ethnic differences in IQ. 

Even if the variation within a group reflects genetic differences, the average differences between the groups could be wholly due to environmental factors.  Imagine, two pots, with randomly allocated seeds from the same batch.  The two plots have equivalent genetic potential.  One plot received fertilizer (an environmental condition), the other pot receives no fertilizer.  The average height (i.e. intelligence) differences between each pot will be due to environmental differences (fertilizer), however the height differences amongst individual plants within a pot are due to genetic differences (assuming similar conditions throughout the pot).

There is debate about whether heritability estimates even matter, since they can't be applied to an individual or be used to help people:

Wahlsten (1997, p. 84) states that:

"It does not matter whether the field of human behavior genetics finally decides that the heritability of IQ in the United States is 25%, 40%, 50%, or 70%. Any such estimate will be utterly useless to anyone seeking better ways to improve the intelligence of the nation through health care and education."

Intelligence & socioeconomic status

Herrstein and Murray (1994) argue that low intelligence causes low SES, rather than the other way around. So, according to these authors, while SES is correlated with IQ, it should be considered a consequence rather than a cause.

However, adoption studies seem to indicate that SES has a strong, causal effect on intelligence, e.g.:

"Well-controlled adoption studies done in France have found that transferring an infant from a family having low socioeconomic status (SES) to a home where parents have high SES improves childhood IQ scores by 12 to 16 points or about one standard deviation, which is considered a large effect size in psychological research." Wahlsten (1997, p. 76).

Several recent US studies have demonstrated improvements in children's IQ's by improving the lives of infants in disadvantaged circumstances.
These studies employed random assignment of children and families to treatment and control conditions.

These studies selected families with:
• low parental IQ
• low parental education
• minimal financial resources

Experimental group received:
• enriched, educational day care outside the home every weekday from 3 months to start of schooling

Control group received:
• nutritional supplements and pediatric medical care or crisis intervention but no educational day care

Even though the children returned to their home environment every day and spent holidays and weekends with their families (mostly unemployed, single mothers) in poverty-stricken neighbourhoods, there were large gains in IQ; almost as much as in the French studies previously mentioned.

Furthermore, the mean IQ of the enriched groups appeared to be quite typical of healthy American children. These children continued to show higher IQ scores than controls at age 12 (Wahlsten, 1997). Of course, in these American studies, SES and education were being manipulated. There is of course a strong correlation between SES and education in both directions.

Intelligence & education

A number of studies have shown that schooling and intelligence influence each other. Higher intelligence tends to lead to prolonged schooling and longer schooling leads to higher IQ.

Intelligence at age 5 predicts better than any other variable a child's future educational progress and attainment (Kline, 1991).

Wahlsten (1997):
• delays in schooling cause IQ to 'drop' 5 points per year
• temporary drop in IQ during school vacations

Winship & Korenman (1997):
• 2.7 IQ point advantage for each year of schooling
• thus to predict later IQ, two estimates are useful: early IQ estimates and number of years of schooling

A study by Cahan and Cohen, found that older children in a grade tended to score slightly higher than their younger classmates but importantly they found that children who are in a higher grade but are virtually the same age as children in the grade lower have higher IQ scores. It is postulated this is due to the extra year of schooling.

However a number of authorities believe that enriched or increased schooling has little effect on intelligence and have refute the suggestion that intelligence can be modified.

"Compensatory education has been tried and it apparently has failed."
- Jensen, 1969

"the story of attempts to raise intelligence is one of high hopes, flamboyant claims, and disappointing results."
- Herrstein & Murray (1994), p.389

One justifiable criticism levelled at educational enrichment studies conclusions about increased IQ is that what is being modified is performance on a test rather than an actual modification in intelligence. Children in enrichment programmes often receive extensive instruction and practice in test-taking. “What has been temporarily modified in the early stages of early intervention programmes is performance on a test, not the child’s general intelligence” (Herman Spitz, 1999, p. 289).

This would account for the well-known fade-out effect, whereby initial, often very high gains in IQ scores, in experimental “enriched schooling” groups, return to the level of the control group a few years after the “experiment”.

In the US a large nationwide programme, the Head Start programme, aims to enrich the schooling of disadvantaged children. On the whole, the results have been mixed. Head Start and programmes like it have been criticized for not living up to expectations in changing IQ. The main defense is that the primary aim is not to improve IQ, but to accelerate academic development - IQ change is a bonus, academic development is more important.

Head Start type programmes have also been criticized for not effecting last changes. This is not surprising if children return to poor, unsupportive, deprived environments. And, in fact, such a finding supports the idea that IQ is malleable - in both directions.

New and better ways of educating, improving, and maximizing individual potentials in intelligence are likely to be developed. As this happens, more and more of the environmentally-influenced variation in IQ is likely to come under control and estimates of the 'environment' proportion could increase.

Intelligence & occupation

"In more than 10,000 studies the average correlation of IQ with occupational success was 0.3...this correlation is certainly a low estimate of its true other variable, either of ability or personality, can approach this figure."
- Kline (1991), p. 139

Herrstein and Murray estimate the relationship between IQ and occupation to be between .2 and .6 (i.e. that IQ explains between 4% and 36% of the variations in occupation). These correlations are slightly higher for skilled, professional jobs, and slightly lower for jobs that require less skill. Whilst this might be useful in describing groups, it means there is questionable value in administering an IQ test to an individual in an attempt to help determine their occupational options. It may be a useful approach, however, to help select the best 100 employees from a 1000 applicants (Howe,1997, p.97).

Comments on the scope & quality of intelligence research

All this research uses psychometric, quantitative tests of intelligence, which we know from previous lectures corresponds somewhat (but not entirely) with our conceptions of what intelligence is.

The research is very focused on North American populations.

Multiple intelligences or subcomponents of intelligence are barely considered in much of this research. Neither are alternative measures of intelligence, such as speed of processing, evoked potentials, or practical intelligence, etc.

Future research on brain-behaviour connections will probably help to more accurately isolate the functions of 'intelligence' (Weinberg, 1989).

In addition, as we come to understand and develop more effective environmental interventions to maximise individuals' IQs, this will possibly expand the relative important of the 'nurture' component.

Nevertheless, there remains the spectre of eugenics - those who would argue for selective breeding on the basis of intellectual ability. This issue is likely to rear its head again in the future and with new genetic technologies could appear in a more dynamic form.

The role of “interaction"

An underresearched area, while the nature vs. nurture debate has raged, is the contribution of interactions between genetics and environment on IQ variance.
In the overfocus on nature vs. nurture issues, the attempts to estimate the relative contribution rests on the somewhat naive notion that there is a constant, true value. In reality, "gene expression is environment dependent" and it impossible to obtain pure estimates of genetic vs. environmental contribution - one could not exist without the other.
The environment a child experiences is partly a consequence of the child’s genes as well as external factors. To some extent a person seeks out and creates his or her environment. If she is of a mechanical bent she practices mechanical skills; if a bookworm, she seeks out books. Thus genes may create an appetite rather than an aptitude. Remember that the high heritability of short-sightedness is accounted for not just by the heritability of a gene for short sightedness but by the heritability of literate habits.
Thus, a future area for research which blends those in the nature camps with those in the nurture camps would be examine which environmental components allow people to optimally realise their genetic potentials for a variety of areas of cognitive performance (e.g. see Feldman, 1986).

What have we learnt about intelligence?

So, what can we say about nature vs. nurture as causal determinants of intelligence?
A conservative, seemly safe position is that:
"In the field of intelligence, there are three facts about the transmission of intelligence that virtually everyone seems to accept:

1. Both heredity and environment contribute to intelligence.
2. Heredity and environment interact in various ways.
3. Extremely poor as well as highly enriched environments can interfere with the realization of a person's intelligence, regardless of the person's heredity” (Sternberg & Grigorenko, 1997, p.xi).
4. Although most would accept a causal role of genetics, the exact genetic link and how it operates is very far from being understood - another point that most psychologists would agree on. It is certainly not a single gene, but a complex combination of smaller genetic markers.
5. But likewise, it is difficult to pin-down single, identifiable elements of the environment which directly influence IQ scores. Several environmental factors influence intelligence.

So what have we learned about intelligence: that it’s difficult to define but that there is SOMETHING we call intelligence that appears to relate to ability to reason abstractly, to learn and to adapt. That we can measure some part of it, although poorly; that it’s
partially caused by genetics, partially be environment; that the real causes are the complex, not well understood interplay between genetics and environment; that it is somewhat though not greatly modifiable; that sometimes what we learn from tests is used inappropriately but that IQ tests can be useful in helping children attain their potential.

Final quotes

"Measures of intelligence have reliable statistical relationships with important social phenomena, but they are a limited tool for deciding what to make of any given individual. Repeat it we must, for one of the problems of writing about intelligence is how to remind readers often enough how little an IQ score tells you about whether the human being next to you is someone whom you will admire or cherish." Herrnstein and Murray (1994, p. 21)
"Mother Nature has plainly not entrusted the determination of our intellectual capacities to the blind fate of a gene or genes; she gave us parents, learning, language, culture and education to program ourselves with."
(Ridley, 1999, p. 77)


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Winship, C., & Korenman, S. (1997). Does staying in school make you smarter? The effect of education on IQ in The Bell Curve. In B. Devlin, S.E. Fienberg., & K. Roeder (pp. 215-234). Intelligence, Genes, and Success: Scientists respond to The Bell Curve. New York: Springer.