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The General Intelligence Factor
Despite some popular assertions, a single factor for intelligence,
called g, can be measured with IQ tests and does predict success in
life --- Linda S. Gottfredson.
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.
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.
[The above has been labeled behavioral Lamarckism, where maternal
investment in the shaping of behavioral styles and the transmission of
social learning can continue for generations in the absence of
selecting control by any genetic aspect of variation. Those groups who
want to deny their own high genetic intelligence promote social
Lamarckism. They claim that they have high intelligence because of
maternal care and/or just trying harder, while those with low
intelligence suffer from systemic- or institutional-racism. But there
is no scientific basis for such a hypothesis and genetic differences
must remain the most parsimonious factor in group differences--unless
one want to infer that Whites have low IQs in relation to East Asians
and Ashkenazi Jews because we are like Blacks--oppressed in our own
societies. Matt Nuenke]
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. [This means that only about one-half of all Blacks can enter the
military service, which makes the military the only organization in
the U.S. that is allowed to discriminate based on intelligence. All
others are forced to hire in one way or another by quotas rather than
merit. In addition, testing of applicants is banned for all practical
purposes except by the military. Apparently affirmative action is too
important when it comes to national defense. Matt Nuenke]
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.
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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