The Novartis
Foundation, an international science and education charity located in London,
regularly hosts symposia that deal with cutting edge issues in science;
appropriately, the organization often deals with topics related to evolution
and psychology. “The Nature of Intelligence”
meeting, held in late 1999, convened to address questions at the intersections
of evolutionary psychology and behavioral genetics.The
book by the same name (edited by Gregory Bock, Jamie Goode, and Kate Webb)
consists of the papers read at that conference, as well as transcripts
of the fascinating discussions and conversations that took place between
the participants.

While the topics
covered range widely over issues such as whether or not “general intelligence”
(known as ‘g,’ first postulated by Sir Francis Galton in 1869) exists
to the modularity of mind, the primary question addressed by the authors
revolves around a fundamental tension: one would predict that a trait so
strongly adaptive as g would, owing to intense selection for it,
show only weak individual variation within the population. Evolutionary
psychologists such as John Tooby and Leda Cosmides have argued for this
position. Behavioral geneticists,
on the other hand, note that there is ample empirical evidence that intelligence
is highly variable as well as highly heritable. How
are we to reconcile this tension? Answering
that question requires addressing a host of assumptions and hypotheses,
many of which are surfaced in the book. I’ll
spend most of the remainder of this review discussing each paper in the
volume. Ultimately, they are very
satisfying, but what is most provocative is the inclusion of transcripts
of the discussions that took place; here, the “give and take” of science
can be examined first hand, and the reader gains an appreciation for the
fallibilism and humility that undergirds the pursuit of scientific knowledge. These
interludes punctuate each paper and are a delight to read.

After Michael
Rutter sets the stage for the conference in his introduction, David Lubinski
begins (in “Intelligence: success and fitness”) by defending the scientific
evidence for the existence of g, arguing that it is a very robust
construct. It has “…achieved the
status of differential psychology’s most central dimension” (p. 17) General
intelligence does not merely co-vary with other measures of abstract learning
and work performance; rather, he states, its role is primarily causal.

Arthur Jensen
argues in his essay “The g factor: psychometrics and biology” that
we should not expect to find a cognitive module for “general problem solving”;
rather, g as a construct cuts across the various cognitive functions
of the brain, and is instead highly correlated with very basic brain properties
such as brain size, reaction time, nerve conduction velocity, metabolic
rate, etc. Jensen approvingly quotes
Charles Spearman, a pioneer in the investigation of g, who noted
in 1927 that the final understanding of g “…must come from the most
profound and detailed direct study of the human brain in its purely physical
and chemical aspects” (p. 46).

Next, Ian Deary’s
essay “Psychometric intelligence differences and brain function” makes
the case for a multilevel exploration of g’s instantiation in the
brain, arguing that investigation at the psychometric, cognitive, psychophysical
and physiological levels will prove useful; the real “explanatory toil
must come from linking individual differences at different levels of description”
(p. 71).

Britt Anderson
shifts gears slightly, examining “The g factor in non-human animals.” He
argues that there are direct correlates of g in other animals besides
people; given that we can directly intervene in brain development in these
creatures in a way that we can’t in humans, “genetic contributions to g
can be directly tested by modifying gene expression and determining the…benefits”
(p. 79). We can move beyond the
correlation/causation debate by using animals as test cases for hypotheses
about the heritability, and neural substrates, of g.

Randolph Neese
argues in his essay “Natural selection, mental modules and intelligence”
that the debate regarding whether g is one trait or many has no
commonly agreed-upon answer. In part,
this is because knowing functional demands gives only modest guidance as
to the structures that mediate those functions. Rather
than look for mental modules that will constitute g, we should instead
recognize the mixed modular/holistic nature of the mind, and gain what
explanatory purchase we can upon the problem using concepts from evolutionary
psychology.

Nathan Brody
defends g’s conceptual hegemony in “g and the one-many problem:
is one enough?” Brody thinks one
concept is enough: “g is a heritable component of intelligence
that accounts for approximately 50% of the covariance among diverse measures
of intelligence…the covariances…are, in large measure, attributable to
common genetic influences” (p. 122). He
defends the explanatory necessity of g in the domain of intellect,
rebutting the argument that it does not deserve pride of place in any general
theory of intelligence.

In “General intelligence
and the definition of phenotypes,” Douglas Detterman acknowledges the empirical
support for g, but nonetheless argues that it is not the “perfect
phenotype,” as it is probably a statistical abstraction that correlates
a number of separable basic cognitive processes While
we know g is important, we still don’t know what it is, and
we won’t know what it is until we have a better grasp of its phenotypic
characteristics.

David Houle’s
interesting essay “Is there a g factor for fitness” argues that
fitness components have a variable but positive relationship with overall
fitness (captured by an ‘f factor’), in much the same way that the
factors that constitute g have a variable but positive relationship
with it. His conclusion is that we
should not expect fitness to automatically increase with increases in g;
using the analogy of body size and growth rates, where increases in either
of those factors are not automatically correlated with fitness, we can
expect that in certain environments more g will not lead to more
fitness. Indeed, it seems likely
that current estimates of the relationship between g and fitness
“…are neither likely to illuminate our evolutionary history, nor enable
us to predict our evolutionary future” (p. 157).

J. Michael Bailey’s
essay “How can psychological adaptations be heritable?” is a review of
the nascent literature linking psychological traits to fitness and heritability. The
most successful examples of such linkages, Bailey argues, are in the field
of sociosexuality, where one finds the “…most successful evolutionary account
to date of human psychological variation.”

In a similar
vein, Andrew Whiten argues in his essay “Social complexity and social intelligence”
that the Social (or “Machiavellian”) Intelligence Hypothesis—that primate
intelligence came to exist because of the demands of primate social life—has
been steadily gaining empirical support over the last 20 years. Whiten’s
thesis is that domain specificity may not be an appropriate characterization
of social intelligence; rather, our social intelligence abilities may be
parasitic upon a cluster of advanced cognitive abilities that all ultimately
rest on our capacity for second-order mental representation. The
fact that we can “think about thinking” is critical—while second-order
representation might subserve social intelligence, it is nonetheless a
general purpose capacity that contributes to other important primate cognitive
skills as well.

In “IQ gains,
WISC [Wechsler Intelligence Scale for Children–a common IQ test] subtests
and fluid g: g theory and the relevance of Spearman’s hypothesis
to race,” James Flynn’s statistical work indicates that human groups can
make “massive fluid g gains in a period too short to accommodate
radical change in the speed and efficiency of neural processes” (p. 202).Flynn
explores the impact this might have on debates about IQ and race, noting
that verification of the Spearman hypothesis (that
black/white IQ differences will tend to be largest on tests with the greatest
g-loading) will not allow us to settle the “race and IQ” debate.

Andrew Pomiankowksi,
in his paper “Mutation, selection and the heritability of complex traits,”
argues that genetic variation in intelligence is probably maintained in
the same way that variation in complex sexual ornaments used in courtship
displays has been maintained. Contrary
to expectation, comparative data demonstrates that sexual ornaments have
higher additive genetic variation than similar non-sexual traits. This
is probably because selection for extreme phenotypes (“my, what lovely
l-o-n-g tailfeathers you have”) results in “condition-dependent expression
of sexual traits.” That is, the number
of genes that contribute to variation between traits is itself subject
to evolutionary change—higher genetic variance is expected in traits affected
by more genetic loci (as intelligence probably is).

In “The quantitative
and molecular genetics of human intelligence,” Peter McGuffin argues that,
whatever it is, g has a genetic component, and that current molecular
genetic research can help us identify genes influencing intelligence. He
concludes that new research techniques, such as detection of single nucleotide
polymorphisms on microarrays, will help us better map quantitative trait
loci on the human genome; “this together with the completion of a draft
version of the human genome sequence which has recently been announced
means that genes contributing to g may be identified (as opposed
to just located) within a matter of years” (p. 253-4).

Finally, in his
essay “Sexual selection for indicators of intelligence,” Geoffrey Miller
argues that sexually selected characteristics share the same general diagnostic
features as human intelligence. This
gives us reason to think that intelligence is sexually selected; Miller
proposes nine “novel, testable predictions about human intelligence derived
from sexual selection theory,” including that the g factor will
prove to be subordinate to general fitness factors (recall the “f
factor” discussed earlier in Houle’s essay), and that mate choice mechanisms
should favor courtship behaviors with high g-loadings.

So where does
this leave us with regards to the question that motivated the conference? The
several papers that touched directly on the issue favor the idea that since
g’s expression will be a function of a multitude of genetic
loci that we should expect such expression to be more highly variable despite
obvious selection pressures; but even this idea received constructive criticism
during the meeting.In the end,
Michael Rutter’s closing remarks are indicative of the range of topics
considered in these essays and the variety of viewpoints represented. As
he notes, “it would be foolhardy…to attempt any kind of ‘bottom line’ message
from the meeting. Even so…we agreed
on the value of both evolutionary psychology and behavioural genetics.Both
must constitute parts of the overall research portfolio to be brought to
bear on the question of the nature of intelligence….” (p. 286). These
remarks are generous and ecumenical; still, the conceptual breakthroughs
that will enable paradigm shifts in the study of intelligence will mostly
likely come from new methods of integrating the inputs from these
two fields of study, and from deciding how they should be supplemented
so as to compensate for their particular strengths and weaknesses.

This volume is an excellent
introduction to the state of play in the relationship between intelligence
and genetics; someone looking for “end-of-science”-style monolithic statements
about the field should look elsewhere, but those looking for interesting
and provocative science and philosophy about both fields should start with
“The Nature of Intelligence.”

©2001 William Casebeer.

William
D. Casebeer is an Assistant Professor of Philosophy at the US Air Force
Academy in Colorado Springs, Colorado. His
research interests include ethics, cognitive philosophy, philosophy of
mind, and philosophy of biology.

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Categories: Philosophical

Tags: Behavioral and Cognitive Sciences, Learning Disorders