doi:10.1093/brain/awr186
Brain 2011: 134; 2787–2790
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BRAIN
A JOURNAL OF NEUROLOGY
BOOK REVIEW
A frontal approach to intelligence
Human intelligence is something of a minefield. Here, I mean ‘intelligence’ in the sense of intellectual ability, and not in the sense
of secret information collected by spies—although that too is a
minefield. The very concept of intelligence, as a dimension that
differentiates between people, has long been used as a basis for
discrimination, whether in education or employment, and more
broadly for asserting differences between races. Since we
humans possess brains that are some three times as large as
those of our closest living relatives, the great apes, it has also
widely been held that brain size itself must be an index of intellectual capacity. As long ago as 1836, the German anatomist
Frederick Tiedmann wrote that there exists ‘an indisputable connection between the size of the brain and the mental energy displayed by the individual man’, and in 1839 the American physician
Samuel George Morton wrote a treatise in which he compared the
skulls of various racial groups, with the aim of drawing conclusions
about their intellectual capacities (Morton, 1839). Not surprisingly,
he declared Caucasians to have the largest brains and superior
intellect, followed in turn by Asians, Native Americans and
‘Negroes’. Morton’s views were widely used to justify slavery, at
least until it was abolished in the USA in 1865, although racial
segregation was advocated well into the 20th century.
Such claims were roundly debunked by Stephen Jay Gould in his
1981 book The Mismeasure of Man, but this has not prevented
claims of racial difference from rippling on. An extreme example is
that of the Canadian psychologist J. Philippe Rushton, whose 1995
book Race, Evolution and Behavior purports to show systematic
racial differences—although for Rushton it is East Asians, not
Caucasians, who score highest, both in terms of intelligence and
average brain size. Africans are again placed at the bottom of the
ranking, notwithstanding the fact that all modern humans derive
from African ancestry. The American psychologist Arthur Jensen
wrote an influential article in the Harvard Educational Review in
1969 declaring black Americans to have lower general intelligence
than white Americans (like himself), and has continued to pursue
this theme, more recently in collaboration with Rushton. The supposed intellectual superiority of white over black Americans was
also reinforced in a high-profile book, The Bell Curve, by the
American psychologists Richard Herrnstein and Charles Murray
(1994), which was attacked in a second edition of Gould’s The
Mismeasure of Man in 1996. Claims of racial differences in intelligence, at least with the implication that they are genetically
HOW INTELLIGENCE
HAPPENS
By John Duncan 2010.
New Haven and London:
Yale University Press
Price £20.00/$28.00
ISBN: 978-0-300-15411-5
based, have been broadly condemned, with critics outnumbering
proponents, and have led to some doubts as to whether intelligence itself is a valid concept.
Measures of intelligence have also widely been used to segregate children in schools, and even to stream children into different
schools. The most infamous example was based on the claim of
the educational psychologist, Sir Cyril Burt, that intelligence was
highly heritable. This provided the basis for the 11-plus exam,
administered in the final year of primary school, to determine
whether students should proceed to grammar school, secondary
modern school or technical school. Burt’s claim was based on a
purported study of identical twins raised apart, but subsequent
futile attempts to locate the twins, or even the assistants who
worked on this research, led to suggestions that Burt had fabricated the data. The so-called ‘Burt affair’ is still cloaked in uncertainty and controversy, although an appraisal by his one-time
friend and biographer Leslie Hearnshaw (1979) concluded that a
good deal of the data he reported was indeed largely fraudulent.
Burt died after a long career in 1971, several years before the
controversy erupted, and of course there have been those who
have defended him. It has also been suggested that even if the
data were fraudulent, his estimate of heritability was probably not
too wide of the mark. Nevertheless, when the Burt affair was set
alongside claims of racial differences, the use of intelligence testing
in schools became much curtailed.
In the book under review, John Duncan skips lightly over the
political and societal controversies surrounding intelligence and its
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| Brain 2011: 134; 2787–2790
measurement, and admonishes that the science and politics of
intelligence should be kept apart. He does not mention the Burt
affair, except indirectly through a fleeting reference to Leon
Kamin’s (1974) searing account of its political implications and
lack of scientific credibility. He suggests that racial differences,
if true, are in any case of little importance, on the grounds that
intelligence is determined by multiple factors, including education,
availability of resources and the influence of family and peers. He
might also have noted the so-called ‘Flynn effect’—the discovery
by the New Zealand political scientist James R. Flynn that measured that intelligence has risen regularly and substantially over
the past century, indicating that factors other than genetic endowment have a strong influence, at least on the way intelligence is
measured.
Duncan’s main concerns are not with racial or even genetic
influences, but are rather with more technical questions concerning the nature and measurement of intelligence, and how it might
relate to brain function. His hero is the British psychologist and
statistician Charles Spearman, who concluded early in the 20th
century from the positive correlations between different measures
of intellectual ability that there must be a general factor, which he
called ‘g’, underlying all these abilities. Each such test was therefore in part a measure of ‘g’, and in part a measure of an ability
specific to that test. Through a technique known as factor analysis,
itself credited largely to Spearman, it is possible to separate out ‘g’
from the specific components, and it is ‘g’ that underlies much of
the discussion about intelligence, including that of authors such as
Rushton and Jensen. General intelligence is measured in terms of
the intelligence quotient, or IQ, originally defined as the ratio of
mental age to chronological age, multiplied by 100. Thus, a
10-year-old who scored at the level of the average 12-year-old
would be deemed to have an IQ of 120. This interpretation is no
longer considered valid or useful, and IQ tests are usually simply
standardized so that for any age group the mean is 100 and the
SD 15.
Not all are agreed, though, that intelligence is unitary. Duncan
contrasts Spearman’s approach with that of his great rival, Sir
Godfrey Thomson, who argued that intelligence was made up of
a large number of separate functions. A more telling contrast
might have been with the widely recognized American psychometrician Louis Leon Thurstone, whose 1947 book Multiple Factor
Analysis showed how the correlations between different tests of
abilities could be factored into seven different, albeit correlated
abilities, which he called the ‘primary mental abilities’; these are
word fluency, verbal comprehension, spatial visualization, number
facility, associative memory, reasoning and perceptual speed. The
question of whether the manifold of correlations between tests
should be parsed into a single general factor plus factors specific
to each test, or into a set of different but correlated factors, is to
some extent a matter of taste. Time-honoured tests of intelligence
are something of a compromise. The much-used Wechsler Adult
Intelligence Scale (WAIS), for example, provides an overall measure of IQ, a subdivision into verbal and performance IQ, and then
subscales within these reflecting more specific abilities.
Duncan recognizes that his discussion of the nature of intelligence is dated—the Spearman–Thomson debates were played out
for the most part in the first two decades of the 20th century. It is
Book Review
nevertheless probably fair to conclude that Spearman’s concept of
‘g’ has largely stood the test of time, although Duncan indirectly
alludes to recent authors, such as the American psychologist
Robert J. Sternberg (e.g. 2007), who have developed more comprehensive models, separating out such aspects of human functioning as creativity, social skills and practical aptitudes. One of the
main criticisms of intelligence, and ‘g’ in particular, is the lack of
any theoretical underpinning—it is a concept based mainly on
correlation, quantitatively compelling enough but theoretically
vacuous. Duncan’s main aim is to rectify this by relating intelligence to measurable aspects of brain function. Given developments in brain imaging, such an attempt is timely, and if
successful should provide a firmer understanding of the very
nature of intelligence, and perhaps provide new evidence as to
whether it is indeed a unitary capacity. Further, functional brain
imaging allows us to move away from the crude measures of size
and focus on the brain’s internal workings.
The question of how mental function is mapped onto the brain
echoes the question of whether intelligence itself is unitary or
made up of distinct components. The 19th century discoveries
by Paul Broca and Carl Wernicke of specific language areas in
the brain, in most cases in the left cerebral hemisphere, led
to the general idea that specific mental capacities are localized
in the brain. This view was amplified in the 20th century by the
American neurologist Norman Geschwind, sometimes known as
the father of behavioural neurology. The most extreme claim in
this tradition is Fodor’s (1983) suggestion that the brain is made
up of distinct, encapsulated ‘modules,’ each dedicated to a highly
specific function, and there can scarcely be a more dramatic example of localization than a recent article showing individual neurons in the inferior temporal lobe to be tuned to highly specific
visual input. One such neuron in one patient responded only to
pictures of the American actress Jennifer Aniston—although it
knowingly refrained when she was paired with her one-time partner Brad Pitt (Quiroga et al., 2005).
In contrast to the localizationists, others have adopted the view
that the brain operates in a holistic fashion. These include the
German neurologist Kurt Goldstein and the gestalt psychologists,
and more recently the connectionists, who model brain function
from diffuse networks without specific modular components. Of
course there are some respects in which the brain is both modular
and holistic. Roger W. Sperry received the Nobel Prize for his
famous ‘split brain’ experiments showing that different functions
are localized in the left and right hemispheres, yet Sperry himself
insisted that the brain operates as a whole to create conscious
experience (e.g. Sperry, 1970). It is extremely unlikely that a
single neuron was uniquely responsible for the presence of
Jennifer Aniston in the admiring patient’s brain. The recording
probe had no doubt tapped into a larger circuit of unknown
jurisdiction.
Duncan is an experimental psychologist turned cognitive neuroscientist, with a major interest in the frontal lobes. It is, therefore,
not surprising that he should turn to the frontal lobes, and more
specifically the prefrontal cortex, for an understanding of how
intelligence might work. There is of course some precedence for
this. Humans have high foreheads relative to other apes, and even
relative to reconstructions of the Neanderthal skull, despite the
Book Review
fact that the overall volume of the Neanderthal skull was on average slightly greater than that of our own. The 19th century
pseudoscientific movement known as phrenology interpreted psychological characteristics in terms of the shapes of the skull, and
regarded a high forehead as indicative of superior intellect. In
popular culture, this gave rise to the term ‘highbrow’ dating
from the late 19th century in the USA. The term ‘lowbrow’
emerged later, followed by ‘middlebrow’, to the relief of most
of us. The purported role of the prefrontal cortex has also been
employed to sustain claims of racial superiority. The American
psychologist Henry Edward Garrett, who was president of the
American Psychological Association in 1946 and died in 1973,
spent the last two decades of his life supporting racial segregation,
claiming that a normal African is equivalent to a European who
has undergone prefrontal lobotomy.
Deficits resulting from frontal lobe damage are often diffuse and
difficult to define. In some respects, frontal lobe function seems to
underlie the highest level of human deportment, including ability
to maintain an orderly life, and even such elusive concepts as free
will. The most famous case is that of Phineas Gage, the railroad
worker who was the victim of an accident in 1848 that drove a
tamping rod through his brain, destroying much of his left frontal
lobe. Intellectually, he made a remarkable recovery after the accident but underwent a profound change in personality and general deportment. He became erratic and flamboyant, carrying the
offending tamping rod with him when he made public appearances. Other such cases have proven similarly difficult to interpret.
In the monkey brain, the same cells in the frontal lobes seem to
respond to different inputs or tasks. Such complexity differs from
the more orderly arrangements evident elsewhere in the brain, as
in the occipital and temporal lobes.
Duncan suggests that the essence of intelligence, in which the
prefrontal cortex plays a critical role, resides in what he calls the
‘multiple-demand system’. This is activated when people try to
solve problems requiring structured plans—and especially tasks in
which testees must suppress familiar responses to meet specific
demands. Such tests tend to conform to what the American
psychologist Raymond Cattell called fluid intelligence, which is
the ability to solve novel problems, as distinct from crystallized
intelligence, which is the compendium of what a person knows.
One test that seems to conform to multiple demands is the Stroop
test, which requires the subject to name the colours in which
colour words are printed (e.g. the word RED might be printed in
green, the word BLUE in red, and so on). Brain imaging suggests
that tests of fluid intelligence involve a circuit linking medial and
lateral frontal regions with parietal cortex. Duncan does offer
some evidence that performance on tests like the Stroop test do
distinguish those who score well from those who score poorly on
standard tests of intelligence, but the hope is for a neurologically
based test of intelligence. It remains more of a hope than a reality,
and Duncan resorts to a thought experiment, involving identification of brain areas activated in diverse tasks with high multiple
demands, which might distinguish the notion of general intelligence from that of multiple intelligences.
Other studies suggest that there may be more differentiation
between circuits that link prefrontal with parietal regions of the
brain than Duncan’s analysis implies. The most famous, of course,
Brain 2011: 134; 2787–2790
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is the language circuit, involving Broca’s and Wernicke’s areas, and
usually largely restricted to the left side of the brain. The work of
Giacomo Rizzolatti and Corrado Sinigaglia (2010) suggests a frontoparietal ‘mirror’ system in the primate brain, which responds
both when the animal performs a goal-directed action and when
it observes another individual making the same action. They refer
to this as a system that allows the individual to gain a first-person
understanding of the actions and intentions of others. There may
be good reasons to suppose that this system provided a template
for the subsequent evolution of language (Corballis, 2009).
A frontoparietal network has also been implicated in episodic
memory and mental time travel (Schacter et al., 2008). The question is whether these systems will parse into a general multipurpose system or into discrete networks. The Pearson–Thomson
debate may well play out again in relation to brain circuits
involved in higher level cognition.
Given the past controversies over the nature of intelligence and
its relation to brain function, Duncan may indeed have entered
mine-infested waters. Nevertheless, the tone of the book is gentle
and ruminative, even apologetic, and seems unlikely to attract
screaming headlines from the tabloid press, or even to provoke
disapproval from academic colleagues. The book has interesting
discursions into philosophical issues, such as free will, and autobiographical asides. It is perhaps better suited to the intelligent lay
reader than to practicing scientists, since the coverage of research
into frontal lobe function is light and discussions of the nature of
intelligence dated. Nevertheless, I think the book is valuable in
reviving age-old questions about intelligence in the light of
modern neuroscience.
Finally, one might ask what consequences might ensue if neuroscientists were indeed to discover an objective way to measure
intelligence through brain recording, and so predict with some
accuracy how well individuals might perform in academic or vocational settings. Such a discovery would almost certainly be met
with denial, even outrage. As Duncan himself notes, ‘g implies a
fate hanging over us no matter what efforts we put in’, and goes
on to suggest that ‘the idea of “g” threatens a basic belief of
Americans in particular’ (p. 50). It is perhaps as well that this
book has not yet achieved the goal of discovering a firm basis
for ‘g’ in the brain, although it has indicated how it might be
approached.
Michael C. Corballis
University of Auckland, Auckland, New Zealand
E-mail: [email protected]
Advance Access publication August 9, 2011
References
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Book Review
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