The General Intelligence Factor
http://www.psych.utoronto.ca/users/reingold/courses/intelligence/cache/1198gottfred.html
Despite some popular assertions, a single factor for intelligence, called g, can be measured with IQ tests and does predict success in life
No subject in psychology has provoked more intense
public controversy than the study of human intelligence. From its beginning, research on how and why people differ in overall
mental ability has fallen prey to political and social agendas that obscure or
distort even the most well-established scientific findings. Journalists, too,
often present a view of intelligence research that is exactly the opposite of
what most intelligence experts believe. For these and other reasons, public
understanding of intelligence falls far short of public concern about it. The
IQ experts discussing their work in the public arena can feel as though they
have fallen down the rabbit hole into Alice's Wonderland.
The debate over intelligence and intelligence testing focuses on the question of whether it is useful or meaningful to
evaluate people according to a single major dimension of cognitive competence.
Is there indeed a general mental ability we commonly call
"intelligence," and is it important in the practical affairs of life?
The answer, based on decades of intelligence research, is an unequivocal yes. No matter their form or content,
tests of mental skills invariably point to the existence of a global factor
that permeates all aspects of cognition. And this factor seems to have
considerable influence on a person's practical quality of life. Intelligence as
measured by IQ tests is the single most effective predictor known of individual
performance at school and on the job. It also predicts many other aspects of
well-being, including a person's chances of divorcing, dropping out of high
school, being unemployed or having illegitimate children [see illustration].
By now the vast majority of intelligence researchers
take these findings for granted. Yet in the press and in public debate, the
facts are typically dismissed, downplayed or ignored. This misrepresentation
reflects a clash between a deeply felt ideal and a stubborn reality. The ideal,
implicit in many popular critiques of intelligence research, is that all people
are born equally able and that social inequality results only from the exercise
of unjust privilege. The reality is that Mother Nature is no egalitarian.
People are in fact unequal in intellectual potential--and they are born that
way, just as they are born with different potentials for height, physical
attractiveness, artistic flair, athletic prowess and other traits. Although
subsequent experience shapes this potential, no amount of social engineering
can make individuals with widely divergent mental aptitudes into intellectual
equals.
Of course, there are many kinds of talent, many kinds
of mental ability and many other aspects of personality and character that
influence a person's chances of happiness and success. The functional importance
of general mental ability in everyday life, however, means that without onerous
restrictions on individual liberty, differences in mental competence are likely
to result in social inequality. This gulf between equal opportunity and equal outcomes is perhaps what
pains Americans most about the subject of intelligence. The public intuitively
knows what is at stake: when asked to rank personal qualities in order of
desirability, people put intelligence second only to good health. But with a
more realistic approach to the intellectual differences between people, society
could better accommodate these differences and minimize the inequalities they
create.
Extracting g
Early in the century-old study of intelligence, researchers discovered that all tests of mental ability ranked
individuals in about the same way. Although mental tests are often designed to measure
specific domains of cognition--verbal fluency, say, or mathematical skill,
spatial visualization or memory--people who do well on one kind of test tend to
do well on the others, and people who do poorly generally do so across the
board. This overlap, or intercorrelation, suggests that all such tests measure
some global element of intellectual ability as well as specific cognitive
skills. In recent decades, psychologists have devoted much effort to isolating
that general factor, which is abbreviated g, from the other aspects of
cognitive ability gauged in mental tests.
The statistical extraction of g is performed by
a technique called factor analysis. Introduced at the turn of the century by
British psychologist Charles Spearman, factor analysis determines the minimum number of underlying dimensions
necessary to explain a pattern of correlations among measurements. A general
factor suffusing all tests is not, as is sometimes argued, a necessary outcome
of factor analysis. No general factor has been found in the analysis of
personality tests, for example; instead the method usually yields at least five
dimensions (neuroticism, extraversion, conscientiousness, agreeableness and
openness to ideas), each relating to different subsets of tests. But, as
Spearman observed, a general factor does emerge from analysis of mental ability
tests, and leading psychologists, such as Arthur R. Jensen of the University
of California at Berkeley and John B.
Carroll of the University
of North Carolina at Chapel Hill, have
confirmed his findings in the decades since. Partly because of this research,
most intelligence experts now use g as the working definition of
intelligence.
The general factor explains most differences among
individuals in performance on diverse mental tests. This is true regardless of
what specific ability a test is meant to assess, regardless of the test's
manifest content (whether words, numbers or figures) and regardless of the way
the test is administered (in written or oral form, to an individual or to a
group). Tests of specific mental abilities do measure those abilities, but they
all reflect g to varying degrees as well. Hence, the g factor can
be extracted from scores on any diverse battery of tests.
Conversely, because every mental test is
"contaminated" by the effects of specific mental skills, no single
test measures only g. Even the scores from IQ tests--which usually
combine about a dozen subtests of specific cognitive skills--contain some "impurities" that
reflect those narrower skills. For most purposes, these impurities make no
practical difference, and g and IQ can be used interchangeably. But if
they need to, intelligence researchers can statistically separate the g
component of IQ. The ability to isolate g has revolutionized research on
general intelligence, because it has allowed investigators to show that the
predictive value of mental tests derives almost entirely from this global
factor rather than from the more specific aptitudes measured by intelligence
tests.
In addition to quantifying individual differences,
tests of mental abilities have also offered insight into the meaning of
intelligence in everyday life. Some tests and test items are known to correlate
better with g than others do. In these items the "active
ingredient" that demands the exercise of g seems to be complexity.
More complex tasks require more mental manipulation, and this manipulation of
information--discerning similarities and inconsistencies, drawing inferences,
grasping new concepts and so on--constitutes intelligence in action. Indeed,
intelligence can best be described as the ability to deal with cognitive
complexity.
This description coincides well with lay perceptions
of intelligence. The g factor is especially important in just the kind
of behaviors that people usually associate with "smarts": reasoning,
problem solving, abstract thinking, quick learning. And whereas g itself
describes mental aptitude rather than accumulated knowledge, a person's store
of knowledge tends to correspond with his or her g level, probably
because that accumulation represents a previous adeptness in learning and in
understanding new information. The g factor is also the one attribute
that best distinguishes among persons considered gifted, average or retarded.
Several decades of factor-analytic research on mental
tests have confirmed a hierarchical model of mental abilities. The evidence,
summarized most effectively in Carroll's 1993 book, Human Cognitive
Abilities, puts g at the apex in this model, with more specific
aptitudes arrayed at successively lower levels: the so-called group factors,
such as verbal ability, mathematical reasoning, spatial visualization and memory, are just below g, and below these are skills that
are more dependent on knowledge or experience, such as the principles and
practices of a particular job or profession.
Some researchers use the term "multiple intelligences" to label these sets
of narrow capabilities and achievements. Psychologist Howard Gardner of Harvard University, for example, has
postulated that eight relatively autonomous "intelligences" are
exhibited in different domains of achievement. He does not dispute the
existence of g but treats it as a specific factor relevant chiefly to
academic achievement and to situations that resemble those of school. Gardner
does not believe that tests can fruitfully measure his proposed intelligences;
without tests, no one can at present determine whether the intelligences are
indeed independent of g (or each other). Furthermore, it is not clear to
what extent Gardner's intelligences tap personality traits or motor skills
rather than mental aptitudes.
Other forms of intelligence have been proposed; among
them, emotional intelligence and practical intelligence are perhaps the best known. They are
probably amalgams either of intellect and personality or of intellect and
informal experience in specific job or life settings, respectively. Practical
intelligence like "street smarts," for example, seems to consist of
the localized knowledge and know-how developed with untutored experience in
particular everyday settings and activities--the so-called school of hard
knocks. In contrast, general intelligence is not a form of achievement, whether
local or renowned. Instead the g factor regulates the rate of learning:
it greatly affects the rate of return in knowledge to instruction and
experience but cannot substitute for either.
The Biology of g
Some critics of intelligence research maintain that
the notion of general intelligence is illusory: that no such global mental
capacity exists and that apparent "intelligence" is really just a
by-product of one's opportunities to learn skills and information valued in a
particular cultural context. True, the concept of intelligence and the way in
which individuals are ranked according to this criterion could be social
artifacts. But the fact that g is not specific to any particular domain
of knowledge or mental skill suggests that g is independent of cultural
content, including beliefs about what intelligence is. And tests of different
social groups reveal the same continuum of general intelligence. This
observation suggests either that cultures do not construct g or that
they construct the same g. Both conclusions undercut the social artifact
theory of intelligence.
Moreover, research on the physiology and genetics of g
has uncovered biological correlates of this psychological phenomenon. In the past decade, studies by teams
of researchers in North America and Europe have linked several attributes of
the brain to general intelligence. After taking into account gender and
physical stature, brain size as determined by magnetic resonance imaging is
moderately correlated with IQ (about 0.4 on a scale of 0 to 1). So is the speed
of nerve conduction. The brains of bright people also use less energy during problem
solving than do those of their less able peers. And various qualities of brain
waves correlate strongly (about 0.5 to 0.7) with IQ: the brain waves of
individuals with higher IQs, for example, respond more promptly and
consistently to simple sensory stimuli such as audible clicks. These
observations have led some investigators to posit that differences in g
result from differences in the speed and efficiency of neural processing. If
this theory is true, environmental conditions could influence g by modifying
brain physiology in some manner.
Studies of so-called elementary cognitive tasks (ECTs), conducted by Jensen and others, are bridging the gap between
the psychological and the physiological aspects of g. These mental tasks
have no obvious intellectual content and are so simple that adults and most
children can do them accurately in less than a second. In the most basic
reaction-time tests, for example, the subject must react when a light goes on
by lifting her index finger off a home button and immediately depressing a
response button. Two measurements are taken: the number of milliseconds between
the illumination of the light and the subject's release of the home button,
which is called decision time, and the number of milliseconds between the
subject's release of the home button and pressing of the response button, which
is called movement time.
In this task, movement time seems independent of
intelligence, but the decision times of higher-IQ subjects are slightly faster
than those of people with lower IQs. As the tasks are made more complex,
correlations between average decision times and IQ increase. These results
further support the notion that intelligence equips individuals to deal with
complexity and that its influence is greater in complex tasks than in simple
ones.
The ECT-IQ correlations are comparable for all IQ
levels, ages, genders and racial-ethnic groups tested. Moreover, studies by
Philip A. Vernon of the University of Western Ontario and others have shown that the ECT-IQ overlap results almost entirely
from the common g factor in both measures. Reaction times do not reflect
differences in motivation or strategy or the tendency of some individuals to
rush through tests and daily tasks--that penchant is a personality trait. They
actually seem to measure the speed with which the brain apprehends, integrates
and evaluates information. Research on ECTs and brain physiology has not yet
identified the biological determinants of this processing speed. These studies
do suggest, however, that g is as reliable and global a phenomenon at
the neural level as it is at the level of the complex information processing
required by IQ tests and everyday life.
The existence of biological correlates of intelligence
does not necessarily mean that intelligence is dictated by genes. Decades of
genetics research have shown, however, that people are born with different
hereditary potentials for intelligence and that these genetic endowments are
responsible for much of the variation in mental ability among individuals. Last
spring an international team of scientists headed by Robert Plomin of the Institute of Psychiatry in London announced
the discovery of the first gene linked to intelligence. Of course, genes have
their effects only in interaction with environments, partly by enhancing an
individual's exposure or sensitivity to formative experiences. Differences in
general intelligence, whether measured as IQ or, more accurately, as g
are both genetic and environmental in origin--just as are all other
psychological traits and attitudes studied so far, including personality,
vocational interests and societal attitudes. This is old news among the
experts. The experts have, however, been startled by more recent discoveries.
One is that the heritability of IQ rises with
age--that is to say, the extent to which genetics accounts for differences in
IQ among individuals increases as people get older. Studies comparing identical
and fraternal twins, published in the past decade by a group led by Thomas J. Bouchard, Jr., of the University
of Minnesota and other scholars, show that about 40 percent of IQ
differences among preschoolers stems from genetic differences but that
heritability rises to 60 percent by adolescence and to 80 percent by late
adulthood. With age, differences among individuals in their developed
intelligence come to mirror more closely their genetic differences. It appears
that the effects of environment on intelligence fade rather than grow with
time. In hindsight, perhaps this should have come as no surprise. Young
children have the circumstances of their lives imposed on them by parents,
schools and other agents of society, but as people get older they become more
independent and tend to seek out the life niches that are most congenial to
their genetic proclivities.
A second big surprise for intelligence experts was the
discovery that environments shared by siblings have little to do with IQ. Many
people still mistakenly believe that social, psychological and economic
differences among families create lasting and marked differences in IQ.
Behavioral geneticists refer to such environmental effects as
"shared" because they are common to siblings who grow up together.
Research has shown that although shared environments do have a modest influence
on IQ in childhood, their effects dissipate by adolescence. The IQs of adopted
children, for example, lose all resemblance to those of their adoptive family
members and become more like the IQs of the biological parents they have never
known. Such findings suggest that siblings either do not share influential
aspects of the rearing environment or do not experience them in the same way.
Much behavioral genetics research currently focuses on the still mysterious
processes by which environments make members of a household less alike.
g on the Job
Although the evidence of genetic and physiological
correlates of g argues powerfully for the existence of global
intelligence, it has not quelled the critics of intelligence testing. These skeptics argue that even if such a global entity exists, it has
no intrinsic functional value and becomes important only to the extent that
people treat it as such: for example, by using IQ scores to sort, label and assign students and employees. Such concerns over the proper use of mental
tests have prompted a great deal of research in recent decades. This research
shows that although IQ tests can indeed be misused, they measure a capability
that does in fact affect many kinds of performance and many life outcomes,
independent of the tests' interpretations or applications. Moreover, the
research shows that intelligence tests measure the capability equally well for
all native-born English-speaking groups in the U.S.
If we consider that intelligence manifests itself in
everyday life as the ability to deal with complexity, then it is easy to see
why it has great functional or practical importance. Children, for example, are
regularly exposed to complex tasks once they begin school. Schooling requires
above all that students learn, solve problems and think abstractly. That IQ is
quite a good predictor of differences in educational achievement is therefore
not surprising. When scores on both IQ and standardized achievement tests in
different subjects are averaged over several years, the two averages correlate
as highly as different IQ tests from the same individual do. High-ability
students also master material at many times the rate of their low-ability
peers. Many investigations have helped quantify this discrepancy. For example,
a 1969 study done for the U.S. Army by the Human Resources Research Office found that enlistees in the bottom
fifth of the ability distribution required two to six times as many teaching
trials and prompts as did their higher-ability peers to attain minimal
proficiency in rifle assembly, monitoring signals, combat plotting and other
basic military tasks. Similarly, in school settings the ratio of learning rates
between "fast" and "slow" students is typically five to
one.
The scholarly content of many IQ tests and their
strong correlations with educational success can give the impression that g
is only a narrow academic ability. But general mental ability also predicts job
performance, and in more complex jobs it does so better than any other single
personal trait, including education and experience. The army's Project A, a
seven-year study conducted in the 1980s to improve the recruitment and training
process, found that general mental ability correlated strongly with both
technical proficiency and soldiering in the nine specialties studied, among
them infantry, military police and medical specialist. Research in the civilian
sector has revealed the same pattern. Furthermore, although the addition of
personality traits such as conscientiousness can help hone the prediction of
job performance, the inclusion of specific mental aptitudes such as verbal
fluency or mathematical skill rarely does. The predictive value of mental tests
in the work arena stems almost entirely from their measurement of g, and
that value rises with the complexity and prestige level of the job.
Half a century of military and civilian research has
converged to draw a portrait of occupational opportunity along the IQ
continuum. Individuals in the top 5 percent of the adult IQ distribution (above
IQ 125) can essentially train themselves, and few occupations are beyond their
reach mentally. Persons of average IQ (between 90 and 110) are not competitive
for most professional and executive-level work but are easily trained for the
bulk of jobs in the American economy. In contrast, adults in the bottom 5
percent of the IQ distribution (below 75) are very difficult to train and are
not competitive for any occupation on the basis of ability. Serious problems in
training low-IQ military recruits during World War II led Congress to ban
enlistment from the lowest 10 percent (below 80) of the population, and no
civilian occupation in modern economies routinely recruits its workers from
that range. Current military enlistment standards exclude any individual whose
IQ is below about 85.
The importance of g in job performance, as in
schooling, is related to complexity. Occupations differ considerably in the
complexity of their demands, and as that complexity rises, higher g
levels become a bigger asset and lower g levels a bigger handicap.
Similarly, everyday tasks and environments also differ significantly in their
cognitive complexity. The degree to which a person's g level will come
to bear on daily life depends on how much novelty and ambiguity that person's
everyday tasks and surroundings present and how much continual learning,
judgment and decision making they require. As gamblers, employers and bankers
know, even marginal differences in rates of return will yield big gains--or
losses--over time. Hence, even small differences in g among people can
exert large, cumulative influences across social and economic life.
In my own work, I have tried to synthesize the many
lines of research that document the influence of IQ on life outcomes. As the illustration
shows, the odds of various kinds of achievement and social pathology
change systematically across the IQ continuum, from borderline mentally
retarded (below 70) to intellectually gifted (above 130). Even in comparisons
of those of somewhat below average (between 76 and 90) and somewhat above
average (between 111 and 125) IQs, the odds for outcomes having social
consequence are stacked against the less able. Young men somewhat below average
in general mental ability, for example, are more likely to be unemployed than
men somewhat above average. The lower-IQ woman is four times more likely to
bear illegitimate children than the higher-IQ woman; among mothers, she is
eight times more likely to become a chronic welfare recipient. People somewhat
below average are 88 times more likely to drop out of high school, seven times
more likely to be jailed and five times more likely as adults to live in
poverty than people of somewhat above-average IQ. Below-average individuals are
50 percent more likely to be divorced than those in the above-average category.
These odds diverge even more sharply for people with
bigger gaps in IQ, and the mechanisms by which IQ creates this divergence are
not yet clearly understood. But no other single trait or circumstance yet
studied is so deeply implicated in the nexus of bad social outcomes--poverty,
welfare, illegitimacy and educational failure--that entraps many low-IQ
individuals and families. Even the effects of family background pale in
comparison with the influence of IQ. As shown most recently by Charles Murray of the American
Enterprise Institute in Washington, D.C., the
divergence in many outcomes associated with IQ level is almost as wide among
siblings from the same household as it is for strangers of comparable IQ
levels. And siblings differ a lot in IQ--on average, by 12 points, compared
with 17 for random strangers.
An IQ of 75 is perhaps the most important threshold in
modern life. At that level, a person's chances of mastering the elementary
school curriculum are only 50-50, and he or she will have a hard time
functioning independently without considerable social support. Individuals and
families who are only somewhat below average in IQ face risks of social
pathology that, while lower, are still significant enough to jeopardize their
well-being. High-IQ individuals may lack the resolve, character or good fortune
to capitalize on their intellectual capabilities, but socioeconomic success in
the postindustrial information age is theirs to lose.
What Is versus What Could
Be
The foregoing findings on g's effects have been
drawn from studies conducted under a limited range of circumstances--namely,
the social, economic and political conditions prevailing now and in recent
decades in developed countries that allow considerable personal freedom. It is
not clear whether these findings apply to populations around the world, to the
extremely advantaged and disadvantaged in the developing world or, for that
matter, to people living under restrictive political regimes. No one knows what
research under different circumstances, in different eras or with different
populations might reveal.
But we do know that, wherever freedom and technology
advance, life is an uphill battle for people who are below average in
proficiency at learning, solving problems and mastering complexity. We also
know that the trajectories of mental development are not easily deflected.
Individual IQ levels tend to remain unchanged from adolescence onward, and
despite strenuous efforts over the past half a century, attempts to raise g
permanently through adoption or educational means have failed. If there is a
reliable, ethical way to raise or equalize levels of g, no one has found
it.
Some investigators have suggested that biological
interventions, such as dietary supplements of vitamins, may be more effective
than educational ones in raising g levels. This approach is based in
part on the assumption that improved nutrition has caused the puzzling rise in
average levels of both IQ and height in the developed world during this
century. Scientists are still hotly debating whether the gains in IQ actually
reflect a rise in g or are caused instead by changes in less critical,
specific mental skills. Whatever the truth may be, the differences in mental
ability among individuals remain, and the conflict between equal opportunity
and equal outcome persists. Only by accepting these hard truths about
intelligence will society find humane solutions to the problems posed by the
variations in general mental ability.
Related Links
IQ Tests on the WWW: Web Directory
Intelligence and Personality
Assessment: A Study Guide by Jon Potter
IQ: A Structure for Understanding by Timothy Bates, Macquarie University Sydney
Great Ideas in Personality --
Intelligence by G. Scott Acton, Northwestern University
Intelligence and IQ: Book reviews, commentaries and links to other Net resources.
The Author
LINDA S. GOTTFREDSON is professor of educational
studies at the University
of Delaware, where she has been since 1986, and co-directs the
Delaware-Johns Hopkins Project for the Study of Intelligence and Society. She
trained as a sociologist, and her earliest work focused on career development.
"I wasn't interested in intelligence per se," Gottfredson says.
"But it suffused everything I was studying in my attempts to understand who
was getting ahead." This "discovery of the obvious," as she puts
it, became the focus of her research. In the mid-1980s, while at Johns
Hopkins University, she published several
influential articles describing how intelligence shapes vocational choice and
self-perception. Gottfredson also organized the 1994 treatise "Mainstream
Science on Intelligence," an editorial with more than 50 signatories
that first appeared in the Wall Street Journal in response to the controversy surrounding publication of The Bell Curve. Gottfredson is the mother of identical twins--a "mere
coincidence," she says, "that's always made me think more about the
nature and nurture of intelligence." The girls, now 16, follow
Gottfredson's Peace Corps experience of the 1970s by joining her each summer
for volunteer construction work in the villages of Nicaragua.
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