Nature vs Nurture
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.
In looking for the causes of individual differences in intelligence, a major issue is the relative contribution of genetics and environment.
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.
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.
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.
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".
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
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!
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
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:
And less evidence for genetic influence on:
There is, however, relatively little research along these lines.
"Give me a dozen healthy infants & my own specific world to bring
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:
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:
"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."
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 :
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
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
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:
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.
"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.
Control group received:
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.
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.
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 size...no other variable, either of ability or personality, can approach this figure."
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
An underresearched area, while the nature vs. nurture debate has raged,
is the contribution of interactions between genetics and environment on
So, what can we say about nature vs. nurture as causal determinants of
"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."
Anastasi, A., & Urbina, S. (1997). Psychological Testing (7th ed.).
Upper Saddle River, NJ: Prentice-Hall.
Manolakes, L. A. (1997). Cognitive Ability, Environmental Factors, and
Crime: Predicting Frequent Criminal Activity. In B. Devlin, S.E.
Fienberg., & K. Roeder (pp. 235-255). Intelligence, Genes, and Success:
Scientists respond to The Bell Curve. New York: Springer.