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IntelligenceRobert J. Sternberg
Background and context
Intelligence is one’s ability to learn from experience and to adapt to, shape, and select
environments. The modern study of intelligence is often dated back to the work of Charles
Spearman, who scientifically studied intelligence and proposed that it could be understood in
terms of a general ability that pervaded all intellectual tasks, and specific abilities that were
unique to each particular intellectual task. Modern testing of intelligence, however, dates
back to the work of Alfred Binet and Theodore Simon, who proposed the forerunner of the
modern Stanford-Binet Intelligence Scales. Another critical figure in the early testing of
intelligence was David Wechsler, whose Wechsler Scales of Intelligence are today the most
widely used in the world. Wechsler’s original scale differed from Binet’s in that, in addition to
an overall intelligence quotient (IQ), it also yielded separate scores for verbal and
performance measures of intelligence. Binet and Wechsler succeeded in their measure-
ments because they viewed intelligence as based in judgement and good sense.
Although some researchers believe intelligence to be highly stable, IQ, at the least, can be
quite variable. For example, it can vary both across the lifespan and across generations.
This article will discuss aspects of intelligence of contemporary importance: theories of
intelligence, biological bases of intelligence, heritability of intelligence, and race differences
in intelligence.
Theories of intelligence
Theories of intelligence have been of several kinds. The most visible theories have been
psychometric theories, which conceptualize intelligence in terms of a sort of ‘’map’’ of the
mind. Such theories specify the underlying structures posited to be fundamental to
intelligence, based upon analyses of individual differences in subjects’ performance on
psychometric tests. These theories have been the basis for most conventional tests of
intelligence. A more recent type of theory is the systems theory, which attempts to
characterize the system of structures and mechanisms of mind that comprise intelligence. A
third kind of theory is the biologically based theory, which attempts to account for intelligence
in terms of brain-based mechanisms.
CHC theory
The most widely accepted theory is a synthesis sometimes referred to as CHC theory,
named after Cattell, Horn, and Carroll, the authors of the original theories that have been
synthesized. The theory is based largely upon psychometric evidence - that is, factor-
analytic studies that have sought to uncover sources of individual differences in performance
on standardized tests of intelligence. The basic idea of CHC theory is that there are three
strata of intelligence that are hierarchically related to each other. Stratum I includes narrow
abilities, Stratum II, broad abilities, and Stratum III, general ability. The most important
abilities are general ability (Stratum III), also referred to as g, and fluid and crystallized ability
(Stratum II), also referred to as g-f and g-c. General ability is an overarching ability that is
theorized to be relevant to, and involved in, a very wide variety of cognitive tasks. Fluid
ability is one’s ability to cope with novelty and to think rapidly and flexibly. Crystallized ability
is one’s general store of knowledge relevant to adaptation in one’s life, including vocabulary
and general information.
, Gardner’s theory of multiple intelligences
Gardner has argued that intelligence is not unitary - that there is no ‘’general intelligence’’
broadly construed - but rather that it is multiple. These multiple intelligences include
linguistic, mathematical, spatial, musical, bodily-kinesthetic, naturalist, interpersonal and
intrapersonal intelligence. Gardner’s theory is based upon a variety of sources of evidence,
among them neuropsychological as well as psychometric evidence.
Sternberg’s triarchic theory
Sternberg has proposed what he refers to as a ‘’triarchic theory’’ of human intelligence. The
original version of the theory is triarchic in that it argues that intelligence comprises three
sets of skills: creative, analytical, and practical. In its augmented version, it specifies the
importance of wisdom-based skills as well. According to this theory, the essential skills for
people to be intelligent are: (i) creative skills to generate novel ideas; (ii) analytical skills in
order to assure that the ideas are good ones; (iii) practical skills in order to implement their
ideas and persuade others of their value; and (iv) wisdom-based skills in order to ensure that
the ideas help to achieve a common good over the long as well as the short term through the
infusion of positive ethical values. When the various aspects of the theory are measured,
they improve prediction of both academic and nonacademic performance in university
settings and reduce ethnic-group differences.
Biological bases of intelligence
Biological approaches to intelligence directly examine the brain and its functioning.
Intelligence as measured by IQ tests appears to be localized, in part in the prefrontal cortex
(PFC) and across the neocortex. People with higher IQs show higher levels of functioning in
the superior parietal, temporal, and occipital cortexes as well as in the subcortical regions of
the brain, especially the striatum. Integration of functioning in the parietal and frontal lobes
appears to be especially important. Several different biological approaches have been used,
most comparing biologically based measures to IQ.
Neural efficiency
Complex patterns of electrical activity in the brain as promoted by specific stimuli correlate
with scores on IQ tests. In particular, speed of conduction of neural impulses may correlate
with intelligence as measured by IQ tests. This supports a view that intelligence is based on
neural efficiency. Additional support for neural efficiency as a measure of intelligence can be
found from studies of how the brain metabolizes glucose during mental activities. Haier and
his colleagues have found that as a result of practice, individuals with higher IQ demonstrate
lower cerebral glucose metabolism overall, but they also show more specifically localized
metabolism of glucose. Thus, people with higher IQ may have learned how to use their
brains more efficiently.
Genetic and heritability studies of intelligence
Although numerous attempts have been made to identify genes that are critical to
intelligence, no single gene has been conclusively identified, and it looks as though there will
be no ‘’gene’’ for intelligence to be found. So far, investigators have conducted at least six
genome-wide scans for genes contributing to intelligence and other aspects of cognition.
The data from these scans vary, but there are definitely some partial overlaps. In particular,
the data suggest genes related to intelligence in regions on chromosomes 2q, 6p, and 14q.
The overlap in investigations in identifying these regions suggest the existence of genes that
might account for at least some of the variation in IQ.
, Most attempts to investigate genes underlying intelligence have been indirect, through
studies of heritability, but heritability itself is a troubled concept. It is the ratio of genetic
variation to total variation in an attribute within a given population. As a result, the coefficient
of heritability says nothing with regard to sources of between-population variation. The
coefficient of heritability further does not tell us the proportion of a trait that is genetic in
absolute terms, but rather the proportion of variation in a trait that is due to genetic variation
within a specific population.
Heritability is typically evaluated on a 0 to 1 scale, with a value of 0 signifying no heritability
at all and a value of 1 indicating complete heritability. Heritability and environmentality add
up to 1. Thus, if IQ has a heritability of .50 within a certain population, then 50% of the
variation in scores on the attribute within that population is due to genetic influences. This
statement is completely different from the statement that 50% of the attribute is inherited.
Thus, heritability is not tantamount to genetic influence. A trait could be highly influenced by
genes and yet have low heritability, or none at all. This is because heritability depends on
the existence of individual differences.
Heritability has no fixed value for a given attribute such as intelligence. Although we may
read about ‘’the heritability of IQ’’, there is no single fixed value of heritability that represents
some true, constant value for the heritability of IQ or anything else. Heritability is dependent
on numerous factors, but the most important single factor is the range of environments.
Because heritability represents a proportion of variation, its value will depend on the amount
of variation. Thus, in speaking of heritability, we must remember that genes always operate
within environment contexts. As a result, the environment will likely have differential effects
on the same genetic structure. Furthermore, if different genotypes respond differently to
environmental variation, heritability will differ depending on the mean and variance of
relevant factors in the environment. Thus the statistic is not a fixed, constant value.
The heritability of intelligence is typically estimated as between .4 en .8. The value typically
depends on the method used to estimate heritability, such as studies of degrees of
relatedness or identical twins reared apart. Most of the Iq heritability studies up to today
have been carried out in nations within the developed world and relatively little information
exists regarding the heritability of IQ in the developing world, although what evidence there
is suggests moderate heritability in these nations as well. Heritability also varies as a
function of socioeconomic status (SES). Researchers found that heritability is very
substantially higher in higher SES families than in lower SES families. In particular, at the
lowest levels of SES, shared environment accounted for almost all of the variation in IQs,
whereas at the highest levels shared environment accounted for practically no variation. In
sum, heritability estimates do not explain in any meaningful sense genetic regulation of
human behavior. Furthermore, they do not provide accurate estimates of the strength of the
genetic regulation. Rather, genes act within the context of environments and their effects
must be understood within these contexts.
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