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role to both innate ability and a host of supportive
environmental factors. In sum, recent research has
clearly demonstrated that quality training, monumental effort, and perseverance are crucial factors in
greatness, but many experts on giftedness maintain
that extraordinary achievement also requires rare,
innate talent.
Young Nirav Gathani made
exam history in England
when he became the youngest student to pass the General Certificate of Secondary
Education at age 7. As amazing as this feat was, it is hard
to say whether Nirav will
go on to achieve eminence,
which typically requires a
combination of exceptional
intelligence, motivation,
and creativity.
REVIEW OF KEY POINTS
© Topham/The Image Works
IQ scores below 70–75 are usually diagnostic of mental
retardation, but these diagnoses should not be based solely
on test results, as adaptive behavior should also be evaluated carefully. Four levels of retardation have been distinguished. The vast majority of retarded people are mildly
retarded.
nate talent, who are likely to find their efforts more
rewarding than others. In other words, innate ability
may be the key factor fostering the single-minded
commitment that seems to be crucial to greatness.
Simonton (1999b) has devised an elaborate theory
of talent development that allocates a significant
Although over 350 biological conditions can cause retardation, biological causes can be pinpointed in only a minority
of cases. Research suggests that cases of unknown origin
are mostly caused by unfavorable environmental factors,
such as poverty, neglect, and poor nutrition.
Children who obtain IQ scores above 130 may be viewed
as gifted, but cutoffs for accelerated programs vary, and
schools rely too much on IQ scores. Research by Terman
showed that gifted children tend to be socially mature
and well adjusted. However, Winner has expressed some
concerns about the adjustment of profoundly gifted
individuals.
Gifted youngsters typically go on to be very successful in
life. However, most do not make genius-level contributions
because such achievements depend on a combination of
high intelligence, creativity, and motivation. Research suggests that intensive training and hard work are crucial to
achieving eminence, but many theorists are reluctant to
dismiss the importance of innate talent.
Heredity and Environment as Determinants of Intelligence
What types of evidence
suggest that intelligence is inherited?
What is heritability,
and what are some
limitations of heritability estimates?
How has research demonstrated that environment influences IQ?
How is the concept
of reaction range used
to explain the interaction of heredity and
environment?
What are some explanations for cultural
differences in average
IQ scores?
352
Most early pioneers of intelligence testing maintained
that intelligence is inherited (Cravens, 1992). Small
wonder, then, that this view lingers on among many
people. Gradually, however, it has become clear that
both heredity and environment influence intelligence
(Bartels et al., 2002; Plomin, 2003; Scarr, 1997). Does
this mean that the nature versus nurture debate has
been settled with respect to intelligence? Absolutely
not. Theorists and researchers continue to argue vigorously about which of the two is more important, in
part because the issue has such far-reaching sociopolitical implications.
Theorists who believe that intelligence is largely
inherited downplay the value of special educational
programs for underprivileged groups (Herrnstein &
Murray, 1994; Rushton & Jensen, 2005). They assert
that a child’s intelligence cannot be increased noticeably, because a child’s genetic destiny cannot be altered. Other theorists take issue with this argument,
pointing out that traits with a strong genetic component are not necessarily unchangeable (Sternberg,
Grigorenko, & Kidd, 2005; Wahlsten, 1997). The people in this camp tend to maintain that even more
funds should be allocated for remedial education
programs, improved schooling in lower-class neighborhoods, and college financial aid for the underprivileged. Because the debate over the role of heredity in intelligence has direct relevance to important
social issues and political decisions, we’ll take a detailed look at this complex controversy.
Evidence for
Hereditary Influence
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PREVIEW QUESTIONS
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Galton’s observation that intelligence runs in families was quite accurate. However, family studies can
determine only whether genetic influence on a trait is
CHAPTER 9
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tend to be quite similar in intelligence. The average
correlation for fraternal twins (.60) is significantly
lower. This correlation indicates that fraternal twins
also tend to be similar in intelligence, but noticeably
less so than identical twins. These results support the
notion that IQ is inherited to a considerable degree
(Bouchard, 1998; Plomin & Spinath, 2004).
Of course, critics have tried to poke holes in this
line of reasoning. They argue that identical twins are
more alike in IQ because parents and others treat
them more similarly than they treat fraternal twins.
This environmental explanation of the findings has
some merit. After all, identical twins are always the
same sex, and gender influences how a child is raised.
However, this explanation seems unlikely in light of
the evidence on identical twins reared apart because
of family breakups or adoption (Bouchard, 1997;
Bouchard et al., 1990). Although reared in different environments, these identical twins still display greater similarity in IQ (average correlation: .72) than fraternal
twins reared together (average correlation: .60). Moreover, the gap in IQ similarity between identical twins
and fraternal twins appears to widen in adulthood,
suggesting paradoxically that the influence of heredity increases with age (Plomin & Spinath, 2004).
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plausible, not whether it is certain (see Chapter 3).
Family members share not just genes, but similar environments. If high intelligence appears in a family
over generations, this consistency could reflect the influence of either shared genes or shared environment.
Because of this problem, researchers must turn to twin
studies and adoption studies to obtain more definitive
evidence on whether heredity affects intelligence.
Twin Studies
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Genetic overlap
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The best evidence regarding the role of genetic factors
in intelligence comes from studies that compare identical and fraternal twins. The rationale for twin studies is that both identical and fraternal twins normally
develop under similar environmental conditions.
However, identical twins share more genetic kinship
than fraternal twins. Hence, if pairs of identical twins
are more similar in intelligence than pairs of fraternal twins, it’s presumably because of their greater genetic similarity. (See Chapter 3 for a more detailed
explanation of the logic underlying twin studies.)
What are the findings of twin studies regarding intelligence? The data from over 100 studies of intellectual similarity for various kinds of kinship relations
and child-rearing arrangements are summarized in
Figure 9.13. This figure plots the average correlation
observed for various types of relationships. As you can
see, the average correlation reported for identical
twins (.86) is very high, indicating that identical twins
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Adoption Studies
Research on adopted children also provides evidence
about the effects of heredity (and of environment, as
Figure 9.13
Relationship
Studies of IQ similarity.
100%
Identical twins reared together
100%
Identical twins reared apart
50%
Fraternal twins reared together
50%
Siblings reared together
50%
Siblings reared apart
50%
Biological parent and child, lived together
50%
Biological parent and child, lived apart
0%
Adoptive parent and child, lived together
0%
Adoptive siblings, reared together
12.5%
The graph shows the mean
correlations of IQ scores for
people of various types of
relationships, as obtained in
studies of IQ similarity. Higher
correlations indicate greater
similarity. The results show
that greater genetic similarity
is associated with greater
similarity in IQ, suggesting
that intelligence is partly inherited (compare, for example, the correlations for identical and fraternal twins).
However, the results also
show that living together is
associated with greater IQ
similarity, suggesting that
intelligence is partly governed
by environment (compare,
for example, the scores of
siblings reared together and
reared apart). (Data from
McGue et al., 1993; Plomin
& Spinath, 2004)
Cousins reared apart
0
.10
.20
.30
.40
.50 .60
.70
.80
.90
1.00
Mean correlation in intelligence
Intelligence and Psychological Testing
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Experts have sifted through mountains of correlational evidence to estimate the heritability of intelligence. A heritability ratio is an estimate of the proportion of trait variability in a population that is
determined by variations in genetic inheritance.
Heritability can be estimated for any trait. For example, the heritability of height is estimated to be around
90% (Plomin, 1994). Heritability can be estimated in
a variety of ways that appear logically and mathematically defensible (Grigerenko, 2000; Loehlin, 1994).
Given the variety of methods available and the strong
views that experts bring to the IQ debate, it should
come as no surprise that heritability estimates for intelligence vary considerably (see Figure 9.14).
At the high end, some theorists estimate that the
heritability of IQ ranges as high as 80% (Bouchard,
2004; Jensen, 1980, 1998). That is, they believe that
only about 20% of the variation in intelligence is attributable to environmental factors. Estimates at the
low end of the spectrum suggest that the heritability
of intelligence is around 40% (Plomin, 2003). In recent years, the consensus estimates of the experts tend to
hover around 50% (Petrill, 2005; Plomin & Spinath,
2004).
Figure 9.14
The concept of heritability. A heritability ratio is an estimate of the portion of trait variation
in a population determined by heredity—with the remainder presumably determined by environment—as these pie charts illustrate. Typical heritability estimates for intelligence range between
a high of 80% and a low of 40%. In recent years, the consensus of the experts seems to hover
around 50%. Bear in mind that heritability ratios are estimates and have certain limitations that
are discussed in the text.
Heritability estimates for intelligence
“High” estimate
“Low” estimate
20%
of variation
in intelligence
determined by
environment
80%
of variation
in intelligence
determined
by heredity
40%
of variation
in intelligence
determined
by heredity
60%
of variation
in intelligence
determined
by environment
Evidence for
Environmental Influence
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However, it’s important to understand that heritability estimates have certain limitations (Ceci et al.,
1997; Grigorenko, 2000; Reeve & Hakel, 2002). First,
a heritability estimate is a group statistic based on
studies of trait variability within a specific group. A
heritability estimate cannot be applied meaningfully
to individuals. In other words, even if the heritability
of intelligence were 70%, it would not mean that
each individual’s intelligence was 70% inherited.
Second, a specific trait’s heritability may vary from one
group to another depending on a variety of factors. For
instance, in a group with a given gene pool, heritability will decrease if a shift occurs toward rearing
youngsters in more diverse circumstances. Why? Because environmental variability will be increased.
Third, it is crucial to understand that “there really is
no single fixed value that represents any true, constant value for the heritability of IQ or anything else”
(Sternberg et al., 2005, p. 53). Heritability ratios are
merely sample-specific estimates.
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Heredity unquestionably influences intelligence, but
a great deal of evidence indicates that upbringing
also affects mental ability. In this section, we’ll examine various approaches to research that show how life
experiences shape intelligence.
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we shall see). If adopted children resemble their biological parents in intelligence even though they were
not reared by these parents, this finding supports the
genetic hypothesis. The relevant studies indicate
that there is indeed more than chance similarity between adopted children and their biological parents
(Plomin et al., 2001; refer again to Figure 9.13).
Adoption Studies
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Research with adopted children provides useful evidence about the impact of experience as well as heredity (Dickens & Flynn, 2001; Locurto, 1990; Loehlin,
Horn, & Willerman, 1997). Many of the correlations
in Figure 9.13 reflect the influence of the environment. For example, adopted children show some resemblance to their foster parents in IQ. This similarity is usually attributed to the fact that their foster
parents shape their environment. Adoption studies
also indicate that siblings reared together are more
similar in IQ than siblings reared apart. This is true
even for identical twins who have the same genetic
endowment. Moreover, entirely unrelated children
who are raised in the same home also show a significant resemblance in IQ. All of these findings indicate
that environment influences intelligence.
Environmental Deprivation
and Enrichment
If environment affects intelligence, children who are
raised in substandard circumstances should experience a gradual decline in IQ as they grow older (since
other children will be progressing more rapidly).
354
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Generational Changes: The Flynn Effect
The most interesting, albeit perplexing, evidence
showcasing the importance of the environment is
the finding that performance on IQ tests has steadily
increased over generations. This trend was not widely
appreciated until recently because the tests are renormed periodically with new standardization groups,
so that the mean IQ always remains at 100. However,
in a study of the IQ tests used by the U.S. military,
James Flynn noticed that the level of performance
required to earn a score of 100 jumped upward every
time the tests were renormed. Curious about this unexpected finding, he eventually gathered extensive
data from 20 nations and demonstrated that IQ performance has been rising steadily all over the industrialized world since the 1930s (Flynn, 1987, 1994,
1999, 2003). Thus, the performance that today would
earn you an average score of 100 would have earned
you an IQ score of about 120 back in the 1930s (see
Figure 9.15). Researchers who study intelligence are
now scrambling to explain this trend, which has been
dubbed the “Flynn effect.” About the only thing they
mostly agree on is that the Flynn effect has to be attributed to environmental factors, as the modern
world’s gene pool could not have changed overnight
(in evolutionary terms, 70 years is more like a fraction
of a second) (Dickens & Flynn, 2001; Neisser, 1998;
Sternberg et al., 2005).
At this point, the proposed explanations for the
Flynn effect are conjectural, but it is worth reviewing
some of them, as they highlight the diversity of environmental factors that may shape IQ performance.
Some theorists attribute generational gains in IQ test
performance to reductions in the prevalence of severe
130
100
125
95
120
90
115
85
110
80
105
75
100
70
Average IQ using 1918 norms
105
95
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
Year
Figure 9.15
Generational increases in measured IQ. IQ tests are renormed periodically so that the mean
score remains at 100. However, research by James Flynn (1998) has demonstrated that performance
on IQ tests around the world has been increasing throughout most of the century. This graph traces
the estimated increases in IQ in the United States from 1918 to 1995. In relation to the axis on the
right, the graph shows how average IQ would have increased if IQ tests continued to use 1918 norms.
In relation to the axis on the left, the graph shows how much lower the average IQ score would have
been in earlier years if 1995 norms were used. The causes of the “Flynn effect” are unknown, but
they have to involve environmental factors.
Source: Adapted from Horgan, J. (1995). Get smart, take a test. Scientific American, 273 (5), p. 14. Copyright © 1995 by
Scientific American, Inc. Adapted by permission of the publisher and author.
malnutrition among children (Colom, Lluis-Font, &
Andres-Pueyo, 2005; Sigman & Whaley, 1998). Others attribute the Flynn effect to increased access to
schooling and more demanding curricula in schools
over the course of the last century (Blair et al., 2005).
Patricia Greenfield (1998) argues that advances in
technology, including much maligned media such
as television and video games, have enhanced visuospatial skills and other specific cognitive skills that
contribute to performance on IQ tests. Wendy Williams (1998) discusses the importance of a constellation of factors, including improved schools, smaller
families, better-educated parents, and higher-quality
parenting. All of these speculations have some plausibility and are not mutually exclusive. Thus, the causes
of the Flynn effect remain under investigation.
The Interaction of Heredity
and Environment
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This cumulative deprivation hypothesis was tested decades ago. Researchers studied children consigned to
understaffed orphanages and children raised in the
poverty and isolation of the back hills of Appalachia
(Sherman & Key, 1932; Stoddard, 1943). Generally,
investigators did find that environmental deprivation led to the predicted erosion in IQ scores.
Conversely, children who are removed from a deprived environment and placed in circumstances
more conducive to learning should benefit from their
environmental enrichment. Their IQ scores should
gradually increase. This hypothesis has been tested
by studying children who have been moved from
disadvantaged homes into middle- and upper-class
adoptive homes (Scarr & Weinberg, 1977, 1983; Schiff
& Lewontin, 1986; Skodak & Skeels, 1947). Although
there are limits on the improvements seen, the IQs of
these children tend to increase noticeably (typically
10–12 points). These findings also show that environment influences IQ.
IQ scores (1995)
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Clearly, heredity and environment both influence
intelligence to a significant degree. And their effects
involve intricate, dynamic, reciprocal interactions
(Dickens & Flynn, 2001; Grigerenko, 2000; Petrill,
2005). Genetic endowments influence the experiences
that people are exposed to, and environments influence the degree to which genetic predispositions are
realized. Indeed, many theorists now assert that the
question of whether heredity or environment is more
important ought to take a back seat to the question of
how they interact to govern IQ.
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One prominent model of this interaction, perhaps championed most prominently by Sandra Scarr
(1991), is that heredity may set certain limits on intelligence and that environmental factors determine
where individuals fall within these limits (Bouchard,
1997; Weinberg, 1989). According to this idea, genetic
makeup places an upper limit on a person’s IQ that
can’t be exceeded even when environment is ideal.
Heredity is also thought to place a lower limit on an
individual’s IQ, although extreme circumstances (for
example, being locked in an attic until age 10) could
drag a person’s IQ beneath this boundary. Theorists
use the term reaction range to refer to these genetically determined limits on IQ (or other traits).
According to the reaction-range model, children
reared in high-quality environments that promote
the development of intelligence should score near
the top of their potential IQ range (see Figure 9.16).
Children reared under less ideal circumstances should
score lower in their reaction range. The concept of a
reaction range can explain why high-IQ children
sometimes come from poor environments. It can also
explain why low-IQ children sometimes come from
very good environments. Moreover, it can explain
these apparent paradoxes without discounting the
role that environment undeniably plays.
Scientists hope to achieve a more precise understanding of how heredity and environment interactively govern intelligence by identifying the specific
genes that influence general mental ability. Advances
in molecular genetics, including the mapping of the
human genome, are allowing researchers to search
for individual genes that are associated with measures of intelligence (Plomin, 2003). This new line of
research is both exciting and promising, although
progress has been slower than expected. The prob-
Image Not Available
SANDRA SCARR
“My research has been
aimed at asking in what
kind of environments
genetic differences shine
through and when do
they remain hidden.”
lem is that intelligence may be influenced by several
hundred specific genes, each of which may have a
small effect that is extremely difficult to detect with
current technologies (Petrill, 2005). However, researchers in this area hope to achieve breakthroughs
as the technology of molecular genetics gradually becomes more powerful (Butcher et al., 2005).
Cultural Differences
in IQ Scores
The age-old nature versus nurture debate lies at the
core of the current controversy about ethnic differences in average IQ. Although the full range of IQ
scores is seen in all ethnic groups, the average IQ for
many of the larger minority groups in the United
States (such as African Americans, Native Americans,
and Hispanics) is somewhat lower than the average
for whites. The disparity ranges from 3 to 15 points,
depending on the group tested and the IQ scale used
(Loehlin, 2000; Nisbett, 2005; Perlman & Kaufman,
1990; Suzuki & Vraniak, 1994). There is little argument about the existence of these group differences,
variously referred to as racial, ethnic, or cultural differences in intelligence. The controversy concerns
why the differences are found. A vigorous debate continues as to whether cultural differences in intelligence
are mainly attributable to the influence of heredity
or of environment.
Heritability as an Explanation
In 1969 Arthur Jensen sparked a heated war of words
by arguing that racial differences in average IQ are
largely the result of heredity. The cornerstone for Jensen’s argument was his analysis suggesting that the
Figure 9.16
84
Reaction range. The concept of reaction
117
Tom
Enriched
Kimberly
127
Ted
Quality of
environment
(for realizing
intellectual
potential)
103
122
Jerome
Average
84
Chris
Susan
125
97
Deprived
66
Alice
Jill
Jack
55
70
85
100
IQ scores
Inherited reaction range
Measured IQ, as shaped by interaction of heredity and environment
356
115
130
145
range posits that heredity sets limits on one’s
intellectual potential (represented by the horizontal bars), while the quality of one’s environment influences where one scores within
this range (represented by the dots on the
bars). People raised in enriched environments
should score near the top of their reaction
range, whereas people raised in poor-quality
environments should score near the bottom
of their range. Genetic limits on IQ can be inferred only indirectly, so theorists aren’t sure
whether reaction ranges are narrow (like Ted’s)
or wide (like Chris’s). The concept of reaction
range can explain how two people with similar genetic potential can be quite different in
intelligence (compare Tom and Jack) and how
two people reared in environments of similar
quality can score quite differently (compare
Alice and Jack).
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c o n c e p t c h e c k 9.2
Understanding Correlational Evidence
on the Heredity-Environment Question
Check your understanding of how correlational findings relate to the nature versus nurture issue by indicating
how you would interpret the meaning of each “piece” of evidence described below. The numbers inside the
parentheses are the mean IQ correlations observed for the relationships described (based on McGue et al.,
1993), which are shown in Figure 9.13 (on page 353). In the spaces on the left, enter the letter H if the findings suggest that intelligence is shaped by heredity, enter the letter E if the findings suggest that intelligence is
shaped by the environment, and enter the letter B if the findings suggest that intelligence is shaped by both (or
either) heredity and environment. The answers can be found in Appendix A.
_______ 1. Identical twins reared apart are more similar (.72) than fraternal twins reared together (.60).
_______ 2. Identical twins reared together are more similar (.86) than identical twins reared apart (.72).
_______ 3. Siblings reared together are more similar (.47) than siblings reared apart (.24).
_______ 4. Biological parents and the children they rear are more similar (.42) than unrelated persons who
are reared apart (no correlation if sampled randomly).
_______ 5. Adopted children show similarity to their biological parents (.24) and to their adoptive parents (.24).
heritability of intelligence is about 80%. Essentially,
he asserted that (1) intelligence is largely genetic in
origin, and (2) therefore, genetic factors are “strongly
implicated” as the cause of ethnic differences in intelligence. Jensen’s article triggered outrage and bitter criticism in many quarters, as well as a great deal
of additional research on the determinants of intelligence. Twenty-five years later, Richard Herrnstein
and Charles Murray (1994) reignited the same controversy with the publication of their widely discussed
book The Bell Curve. They argued that ethnic differences in average intelligence are substantial, not easily reduced, and at least partly genetic in origin. The
implicit message throughout The Bell Curve was that
disadvantaged groups cannot avoid their fate because
it is their genetic destiny. And as recently as 2005,
based on an extensive review of statistical evidence,
J. Phillipe Rushton and Arthur Jensen argued that genetic factors account for about half of the gap between
races in average IQ, a conclusion that was echoed by
Linda Gottfredson (2005).
As you might guess, these analyses and conclusions have elicited many lengthy and elaborate rebuttals. Critics argue that heritability explanations for
ethnic differences in IQ have a variety of flaws and
weaknesses (Devlin et al., 2002; Horn, 2002; Brody,
2003; Nisbett, 2005; Sternberg, 2003b, 2005). For example, recent research suggests that the heritability of
intelligence may be notably lower in samples drawn
from the lower socioeconomic classes as opposed to
higher socioeconomic classes (Turkheimer et al.,
2003). However, heritability estimates for intelligence
have largely been based on samples drawn from white,
middle-class, North American and European popula-
tions (Grigerenko, 2000). Hence, there is doubt about
the validity of applying these heritability estimates to
other cultural groups.
Moreover, even if one accepts the assumption that
the heritability of IQ is very high, it does not follow
logically that differences between groups must be due
largely to heredity. Leon Kamin has presented a compelling analogy that highlights the logical fallacy in
this reasoning (see Figure 9.17):
We fill a white sack and a black sack with a mixture of
different genetic varieties of corn seed. We make certain
that the proportions of each variety of seed are identical
in each sack. We then plant the seed from the white sack
in fertile Field A, while that from the black sack is planted
in barren Field B. We will observe that within Field A, as
within Field B, there is considerable variation in the
Individual variation in corn plant heights within
each group (cause: genetic variation in the seeds)
Image Not Available
ARTHUR JENSEN
“Despite more than half a
century of repeated efforts
by psychologists to improve the intelligence of
children, particularly those
in the lower quarter of the
IQ distribution relative to
those in the upper half of
the distribution, strong
evidence is still lacking as
to whether or not it can
be done.”
Figure 9.17
SEED
Field B:
Field A:
Less fertile soil
More fertile soil
Differences in average corn plant height between groups
(cause: the soils in which the plants were grown)
Genetics and betweengroup differences on a
trait. Leon Kamin’s analogy
(see text) shows how between-group differences on
a trait (the average height of
corn plants) could be due to
environment, even if the trait
is largely inherited. The same
reasoning can be applied to
ethnic group differences in
average intelligence.
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Image Not Available
“I don't know anything about the bell curve,
but I say heredity is everything.”
height of individual corn plants. This variation will be
due largely to genetic factors (seed differences). We will
also observe, however, that the average height of plants
in Field A is greater than that in Field B. That difference
will be entirely due to environmental factors (the soil).
The same is true of IQs: differences in the average IQ of
various human populations could be entirely due to environmental differences, even if within each population all
variation were due to genetic differences! (Eysenck &
Kamin, 1981, p. 97)
Web Link 9.5
Upstream-Issues:
The Bell Curve
The editors of Upstream,
champions of “politically
incorrect” conversation,
have assembled perhaps
the broadest collection of
commentaries on the Net
regarding Herrnstein and
Murray’s The Bell Curve.
Despite the marked political conservatism of this
site, it contains a full range
of opinion and analyses of
the book.
358
This analogy shows that even if within-group differences in IQ are highly heritable, between-groups differences in average IQ could still be caused entirely by environmental factors (Block, 2002). For decades, critics
of Jensen’s thesis have relied on this analogy rather
than actual data to make the point that betweengroups differences in IQ do not necessarily reflect genetic differences. They depended on the analogy because no relevant data were available. However, the
recent discovery of the Flynn effect has provided compelling new data that are directly relevant (Dickens
& Flynn, 2001; Flynn, 2003). Generational gains in IQ
scores show that a between-groups disparity in average IQ (in this case the gap is between generations
rather than ethnic groups) can be environmental in
origin, even though intelligence is highly heritable.
Another problem raised by many theorists is that
the concept of race is much fuzzier than generally
believed (Helms, Jernigan, & Mascher, 2005; Smedley & Smedley, 2005). The notion that individuals
can easily be sorted into a handful of discrete racial
categories with distinct ancestries and gene pools
makes intuitive sense to most people. However, scientific research has demonstrated that the boundaries
between racial groupings are extremely porous and
characterized by huge genetic overlap (Cooper, 2005).
For example, Puerto Ricans, on average, have an ancestry that is 37% African, 45% European, and 18%
Native American, whereas Mexican Americans’ average ancestry is 8% African, 61% European, and 18%
Native American (Shields et al., 2005). Thus, theorists
argue that race is a social concept based on perceived
differences in appearance rather than a biological
concept based on clear disparities in genetic makeup
(Sternberg et al., 2005). This analysis does not mean
that the concept of race is meaningless, but it does
pose problems for heritability explanations of ethnic
differences in IQ.
The available evidence certainly does not allow
us to rule out the possibility that ethnic and cultural
disparities in average intelligence are partly genetic.
And the hypothesis should not be dismissed without
study simply because many people find it offensive
or distasteful. However, there are several alternative
explanations for the culture gap in intelligence that
seem more plausible. Let’s look at them.
Socioeconomic Disadvantage
as an Explanation
Some theorists have approached the issue by trying to
show that socioeconomic disadvantages are the main
cause of ethnic differences in average IQ. Many social scientists argue that minority students’ IQ scores
are depressed because these children tend to grow up
in deprived environments that create a disadvantage—both in school and on IQ tests. Obviously, living circumstances vary greatly within ethnic groups,
but there is no question that, on the average, whites
and minorities tend to be raised in different circumstances. Most minority groups have endured a long
history of economic discrimination and are greatly
overrepresented in the lower social classes. A lowerclass upbringing tends to carry a number of disadvantages that work against the development of a
youngster’s full intellectual potential (Evans, 2004;
Lareau, 2003; Lott, 2002; McLoyd, 1998; Seifer, 2001).
In comparison to the middle and upper classes, lowerclass children are more likely to come from large families and from single-parent homes, factors that may
often limit the parental attention they receive. Lowerclass children also tend to be exposed to fewer books,
to have fewer learning supplies, to have less privacy
for concentrated study, and to get less parental assistance in learning. Typically, they also have poorer
role models for language development, experience
less pressure to work hard on intellectual pursuits,
and attend poorer-quality schools that are underfunded and understaffed. Many of these children
grow up in crime-, drug-, and gang-infested neighborhoods where it is far more important to develop
street intelligence than school intelligence. Some theorists also argue that children in the lower classes are
more likely to suffer from malnutrition or to be exposed to environmental toxins (Brody, 1992). Either
of these circumstances could interfere with young-
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sters’ intellectual development (Bellinger & Adams,
2001; Grantham-McGregor, Ani, & Fernald, 2001).
In light of these disadvantages, it’s not surprising
that average IQ scores among children from lower
social classes tend to run about 15 points below the
average scores obtained by children from middleand upper-class homes (Seifer, 2001; Williams & Ceci,
1997). This is the case even if race is factored out of
the picture by studying whites exclusively. Admittedly, there is room for argument about the direction
of the causal relationships underlying this association
between social class and intelligence (Turkheimer,
1994). Nonetheless, given the overrepresentation of
minorities in the lower classes, many researchers argue
that ethnic differences in intelligence are really social
class differences in disguise.
Stereotype Vulnerability
as an Explanation
Socioeconomic disadvantages probably are a major
factor in various minority groups’ poor performance
on IQ tests, but some theorists maintain that other
factors and processes are also at work. For example,
Claude Steele (1992, 1997), a social psychologist at
Stanford University, has argued that derogatory stereotypes of stigmatized groups’ intellectual capabilities
create unique feelings of vulnerability in the educational arena. These feelings of stereotype vulnerability
can undermine group members’ performance on tests,
as well as other measures of academic achievement.
Steele points out that demeaning stereotypes of
stigmatized groups are widely disseminated, creating
a subtle climate of prejudice, even in the absence of
overt discrimination. He further notes that members
of minority groups are keenly aware of any negative
stereotypes that exist regarding their intellect. Hence,
when an African American or Hispanic American does
poorly on a test, he or she must confront a disturbing possibility: that others will attribute the failure to
racial inferiority. Steele maintains that females face the
same problem when they venture into academic domains where stereotypes suggest that they are inferior to males, such as mathematics, engineering, and
the physical sciences. That is, they worry about people
blaming their failures on their sex. According to Steele,
minorities and women in male-dominated fields are
in a no-win situation. When they do well and contradict stereotypes, people tend to view their success with
suspicion, but when they do poorly, people readily
view their failure as vindication of the stereotypes.
Steele maintains that stigmatized groups’ apprehension about “confirming” people’s negative stereotypes can contribute to academic underachievement
in at least two ways. First, it can undermine their emotional investment in academic work. As Steele notes,
“Doing well in school requires a belief that school
achievement can be a promising basis of self-esteem,
and that belief needs constant reaffirmation even
for advantaged students” (1992, p. 72). When this belief is relentlessly undercut instead of frequently reaffirmed, students tend to “disidentify” with school
and write off academic pursuits as a source of selfworth. Their academic motivation declines and their
performance suffers as a result. Second, standardized
tests such as IQ tests may be especially anxiety arousing for members of stigmatized groups because the
importance attributed to the tests makes one’s stereotype vulnerability particularly salient. This anxiety
may impair students’ test performance by temporarily disrupting their cognitive functioning. How Steele
tested his theory is the topic of our Featured Study.
Image Not Available
CLAUDE STEELE
“I believe that in significant part the crisis in black
Americans’ education
stems from the power of
this vulnerability to undercut identification with
schooling.”
FEATURED
Racial Stereotypes and Test Performance
In this article, Steele and Aronson report on a series of
four studies that tested various aspects of Steele’s theory about the ramifications of stereotype vulnerability.
We will examine their first study in some detail and then
discuss the remaining studies more briefly. The purpose
of the first study was to test the hypothesis that raising
the threat of stereotype vulnerability would have a negative impact on African American students’ performance
on a mental ability test.
Method
Participants. The participants were 114 black and white
undergraduates attending Stanford University who were
recruited through campus advertisements. As expected,
given Stanford’s highly selective admissions, both groups
of students were well above average in academic ability,
as evidenced by their mean scores on the verbal subtest
of the SAT. The study compared black and white students
with high and roughly equal ability and preparation
(based on their SAT scores) to rule out cultural disadvantage as a factor.
Procedure. The participants were asked to take a challenging, 30-minute test of verbal ability composed of
items from the verbal subtest of the Graduate Record
Exam (GRE). In one condition, the issue of stereotype vulnerability was not made salient, as the test was presented
to subjects as a device to permit the researchers to analyze problem-solving strategies (rather than as a measure
of ability). In another condition, the specter of stereotype vulnerability was raised, as the test was presented
Study
Source: Steele, C. M., &
Aronson, J. (1995). Stereotype threat and the intellectual test performance of
African Americans. Journal
of Personality and Social
Psychology, 69, 797–811.
Intelligence and Psychological Testing
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Figure 9.18
14
Stereotype vulnerability
and test performance.
12
Source: Adapted from Steele, C. M., &
Aronson, J. (1995). Stereotype threat
and the intellectual test performance of
African Americans. Journal of Personality
and Social Psychology, 69, 797–811.
Copyright © 1995 by the American
Psychological Association. Reprinted by
permission of the publisher and author.
White
students
Score on verbal ability test
Steele and Aronson (1995)
compared the performance
of African American and white
students of equal ability on a
30-item verbal ability test constructed from difficult GRE
questions. When the black
students’ stereotype vulnerability was not salient, their
performance did not differ
from that of the white students; but when the specter
of stereotype vulnerability
was raised, the African American students performed significantly worse than the
white students.
Black
students
10
8
6
4
Based on their initial study, the authors inferred that
stereotype vulnerability does appear to impair minority
group members’ test performance. They went on to
replicate their finding in a second study of 40 black and
white female students. In a third study, they demonstrated that their manipulations of stereotype vulnerability were indeed activating thoughts about negative
stereotypes, ability-related self-doubts, and performance
apprehension in their African American participants.
Their fourth study showed that stereotype vulnerability
can be activated even when a test is not explicitly presented as an index of one’s ability.
2
Comment
0
Not salient
Very salient
Stereotype vulnerability
as an excellent index of one’s general verbal ability. The
principal dependent variable was subjects’ performance
on the verbal test.
Results
When the African American students’ stereotype vulnerability was not made salient, the performance of the
black and white students did not differ, as you can see in
Figure 9.18. However, when the same test was presented
in a way that increased blacks’ stereotype vulnerability,
the African American students scored significantly lower
than their white counterparts (see Figure 9.18).
Cultural Bias on IQ Tests
as an Explanation
Some critics of IQ tests have argued that cultural differences in IQ scores are partly due to a cultural bias
built into IQ tests. They argue that because IQ tests
are constructed by white, middle-class psychologists,
they naturally draw on experience and knowledge
typical of white, middle-class lifestyles and use language and vocabulary that reflect the white, middleclass origins of their developers (Cohen, 2002; Fagan
& Holland, 2002; Helms, 1992; Hilliard, 1984). Given
these concerns, many testing experts assert that minority students’ IQ scores should be interpreted with
extra caution (Puente, 1990). However, the balance
of evidence suggests that the cultural slant on IQ
tests is modest to negligible. The charges of bias stimulated a great deal of research on the issue in the
1970s and 1980s. As a whole, the accumulated evidence
suggests that cultural bias produces only weak and inconsistent effects on the IQ scores of minority examinees
(Hunter & Schmidt, 2000; Reynolds, 2000; Reynolds
& Ramsay, 2003). However, Suzuki and Valencia
360
Discussion
The potential negative effects of stereotype vulnerability have been replicated in numerous studies (Aronson
et al., 1999; Croizet et al., 2004; Steele, Spencer, & Aronson, 2002). The concept of stereotype vulnerability has
the potential to clear up some of the confusion surrounding the controversial issue of racial disparities in IQ scores.
It seems likely that socioeconomic disadvantage makes
a substantial contribution to cultural differences in average IQ, but various lines of evidence suggest that this
factor cannot account for the culture gap by itself (Neisser
et al., 1996). For years, many theorists have argued that
test bias accounts for the rest of the culture gap, but as
we will discuss momentarily, recent research suggests
otherwise. Thus, Steele’s groundbreaking research gives
scientists an entirely new explanatory tool for understanding the vexing cultural disparities in average IQ.
(1997) express some caution about this conclusion,
noting that the studies of test bias may use culturally biased criteria of academic success to evaluate
the tests. They also assert that little research has been
done on some widely used tests and with some minority populations.
Taken as a whole, the various alternative explanations for cultural and ethnic disparities in average IQ
provide serious challenges to genetic explanations,
which appear weak at best—and suspiciously racist
at worst. Unfortunately, since the earliest days of IQ
testing some people have used IQ tests to further elitist goals. The current controversy about ethnic differences in IQ is just another replay of a record that
has been heard before. For instance, beginning in
1913, Henry Goddard tested a great many immigrants
to the United States at Ellis Island in New York. Goddard reported that the vast majority of Italian, Hungarian, and Jewish immigrants tested out as feebleminded (Kamin, 1974). As you can see, claims about
ethnic deficits in intelligence are nothing new—only
the victims have changed.
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REVIEW OF KEY POINTS
identify the specific genes that influence intelligence, but
progress has been slow thus far.
The debate about the influence of heredity and environment on intelligence has important sociopolitical implications. Twin studies show that identical twins, even when
raised apart, are more similar in IQ than fraternal twins,
suggesting that intelligence is inherited. Adoption studies
reveal that people resemble their parents in intelligence
even when not raised by them. Estimates of the heritability
of intelligence range from 40% to 80% with the consensus
estimate hovering around 50%, but heritability ratios have
certain limitations.
Arthur Jensen and the authors of The Bell Curve sparked
great controversy by arguing that cultural differences in
average IQ are partly due to heredity. Genetic explanations
for cultural differences in IQ have been challenged on a
variety of grounds. Even if the heritability of IQ is great,
group differences in average intelligence may not be due
to heredity. Race is a much fuzzier concept than widely
appreciated. Moreover, ethnicity varies with social class,
so socioeconomic disadvantage may contribute to low
IQ scores among minority students.
Many lines of evidence, including adoption studies, studies
of environmental deprivation and enrichment, and research on the Flynn effect indicate that environment is also
an important determinant of intelligence. The concept of
reaction range posits that heredity places limits on one’s
intellectual potential while the environment determines
where one falls within these limits. Scientists are striving to
Claude Steele has collected some thought-provoking data
suggesting that stereotype vulnerability contributes to the
culture gap in average IQ. Cultural bias on IQ tests may also
contribute a little to ethnic differences in IQ, but it does
not appear to be a crucial factor.
New Directions in the Assessment and Study of Intelligence
Intelligence testing has been through a period of turmoil, and changes are on the horizon. In fact, many
changes have occurred already. Let’s discuss some of
the major new trends and projections for the future.
Exploring Biological Indexes
and Correlates of Intelligence
The controversy about cultural disparities in IQ scores
has led to increased interest in biological indexes
and correlates of intelligence. Arthur Jensen (1987,
1993, 1998), Hans Eysenck (1988, 1989), and other
researchers have attempted to find raw physiological
indicators of general intelligence. Their search for a
“culture-free” measure of intelligence has led them
to focus on sensory processes, much as Sir Francis
Galton did over a hundred years ago. Armed with
much more sophisticated equipment, they hope to
succeed where Galton failed.
Jensen’s (1982, 1987, 1992) studies of mental speed
are representative of this line of inquiry. In his studies, Jensen measures reaction time (RT), using a panel
of paired buttons and lights. On each trial, the subject rests a hand on a “home button.” When one of
the lights is activated, the subject is supposed to push
the button for that light as quickly as possible. RT is
typically averaged over a number of trials involving
varied numbers of lights. Modest correlations (.20s
to .30s) have been found between faster RTs and
higher scores on conventional IQ tests (Deary, 2003).
Jensen’s findings suggest an association between
raw mental speed and intelligence, as Galton originally suggested. This correlation is theoretically in-
teresting and, in retrospect, not all that surprising.
Many conventional IQ tests have imposed demanding time limits on examinees, working under the assumption that “fast is smart.” However, the correlation between RT and IQ appears to be too weak to give
RT any practical value as an index of intelligence.
However, another approach to measuring mental
speed may have more practical potential. Measures
of inspection time assess how long it takes participants
to make simple perceptual discriminations that meet
a certain criterion of accuracy (Deary & Stough, 1996).
For example, in a series of trials, participants may be
asked repeatedly to indicate which of two lines is
shorter. The pairs of lines are presented for very brief
exposures and participants are told to concentrate
on making accurate judgments. A person’s inspection
time is the exposure duration required for that person
to achieve a specific level of accuracy, such as 85%
correct judgments (see Figure 9.19 on the next page).
Correlations in the .30s and .40s have been found
between participants’ inspection time scores and
their scores on measures of intelligence (Deary, 2000;
Nettelbeck, 2003). These correlations are closing in
on being high enough to have some practical value,
although a great deal of work remains to be done to
standardize inspection time measures and to figure
out why they are associated with intelligence.
Some researchers have also begun to explore the
relations between brain size and intelligence. The
early studies in this area used various measures of head
size as an indicator of brain size. These studies generally found positive, but very small correlations (average .15) between head size and IQ (Vernon et al.,
PREVIEW QUESTIONS
Do measures of mental
speed correlate with
intelligence?
Does IQ correlate with
brain size or longevity?
What are the key features
of Sternberg’s theory of
successful intelligence?
What is Gardner’s thesis
about the nature of
intelligence?
What is emotional intelligence, and can it be
measured?
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