Differences in Intelligence: What Mainstream
public statement, signed by 52 internationally known scholars, was active on
the information highway early in 1995 following several rather heated and negative
responses to Herrnstein & Murray's The Bell Curve. It was first published
in The Wall Street Journal, Tuesday, December 13, 1994. An alphabetical
listing of the scholars and their home institutions are given at the end of
the publication of "The BELL CURVE," many commentators have offered opinions
about human intelligence that misstate current scientific evidence. Some conclusions
dismissed in the media as discredited are actually firmly supported.
statement outlines conclusions regarded as mainstream among researchers on intelligence,
in particular, on the nature, origins, and practical consequences of individual
and group differences in intelligence. Its aim is to promote more reasoned discussion
of the vexing phenomenon that the research has revealed in recent decades. The
following conclusions are fully described in the major textbooks, professional
journals and encyclopedias in intelligence.
and Measurement of Intelligence
is a very general mental capability that, among other things, involves the
ability to reason, plan, solve problems, think abstractly, comprehend complex
ideas, learn quickly and learn from experience. It is not merely book learning,
a narrow academic skill, or test-taking smarts. Rather, it reflects a broader
and deeper capability for comprehending our surroundings -- "catching on,"
"making sense" of things, or "figuring out" what to do.
so defined, can be measured, and intelligence tests measure it well. They
are among the most accurate (in technical terms, reliable and valid) of all
psychological tests and assessments. They do not measure creativity, character,
personality, or other important differences among individuals, nor are they
there are different types of intelligence tests, they all measure the same
intelligence. Some use words or numbers and require specific cultural knowledge
(like vocabulary). Others do not, and instead use shapes or designs and require
knowledge of only simple, universal concepts (many/few, open/closed, up/down).
spread of people along the IQ continuum, from low to high, can be represented
well by the BELL CURVE (in statistical jargon, the "normal CURVE"). Most people
cluster around the average (IQ 100). Few are either very bright or very dull:
About 3% of Americans score above IQ 130 (often considered the threshold for
"giftedness"), with about the same percentage below IQ 70 (IQ 70-75 often
being considered the threshold for mental retardation).
tests are not culturally biased against American blacks or other native-born,
English-speaking peoples in the U.S. Rather, IQ scores predict equally accurately
for all such Americans, regardless of race and social class. Individuals who
do not understand English well can be given either a nonverbal test or one
in their native language.
brain processes underlying intelligence are still little understood. Current
research looks, for example, at speed of neural transmission, glucose (energy)
uptake, and electrical activity of the brain.
of all racial-ethnic groups can be found at every IQ level. The BELL CURVES
of different groups overlap considerably, but groups often differ in where
their members tend to cluster along the IQ line. The BELL CURVES for some
groups (Jews and East Asians) are centered somewhat higher than for whites
in general. Other groups (blacks and Hispanics) are centered somewhat lower
than non-Hispanic whites.
BELL CURVE for whites is centered roughly around IQ 100; the BELL CURVE for
American blacks roughly around 85; and those for different subgroups of Hispanics
roughly midway between those for whites and blacks. The evidence is less definitive
for exactly where above IQ 100 the BELL CURVES for Jews and Asians are centered.
is strongly related, probably more so than any other single measurable human
trait, to many important educational, occupational, economic, and social outcomes.
Its relation to the welfare and performance of individuals is very strong
in some arenas in life (education, military training), moderate but robust
in others (social competence), and modest but consistent in others (law-abidingness).
Whatever IQ tests measure, it is of great practical and social importance.
high IQ is an advantage in life because virtually all activities require some
reasoning and decision-making. Conversely, a low IQ is often a disadvantage,
especially in disorganized environments. Of course, a high IQ no more guarantees
success than a low IQ guarantees failure in life. There are many exceptions,
but the odds for success in our society greatly favor individuals with higher
practical advantages of having a higher IQ increase as life settings become
more complex (novel, ambiguous, changing, unpredictable, or multi-faceted).
For example, a high IQ is generally necessary to perform well in highly complex
or fluid jobs (the professions, management); it is a considerable advantage
in moderately complex jobs (crafts, clerical and police work); but it provides
less advantage in settings that require only routine decision making or simple
problem solving (unskilled work).
in intelligence certainly are not the only factor affecting performance in
education, training, and highly complex jobs (no one claims they are), but
intelligence is often the most important. When individuals have already been
selected for high (or low) intelligence and so do not differ as much in IQ,
as in graduate school (or special education), other influences on performance
loom larger in comparison.
personality traits, special talents, aptitudes, physical capabilities, experience,
and the like are important (sometimes essential) for successful performance
in many jobs, but they have narrower (or unknown) applicability or "transferability"
across tasks and settings compared with general intelligence. Some scholars
choose to refer to these other human traits as other "intelligences."
and Stability of Within-Group Differences
differ in intelligence due to differences in both their environments and genetic
heritage. Heritability estimates range from 0.4 to 0.8 (on a scale from 0
to 1), most thereby indicating that genetics plays a bigger role than does
environment in creating IQ differences among individuals. (Heritability is
the squared correlation of phenotype with genotype.) If all environments were
to become equal for everyone, heritability would rise to 100% because all
remaining differences in IQ would necessarily be genetic in origin.
of the same family also tend to differ substantially in intelligence (by an
average of about 12 IQ points) for both genetic and environmental reasons.
They differ genetically because biological brothers and sisters share exactly
half their genes with each parent and, on the average, only half with each
other. They also differ in IQ because they experience different environments
within the same family.
IQ may be highly heritable does not mean that it is not affected by the environment.
Individuals are not born with fixed, unchangeable levels of intelligence (no
one claims they are). IQs do gradually stabilize during childhood, however,
and generally change little thereafter.
the environment is important in creating IQ differences, we do not know yet
how to manipulate it to raise low IQs permanently. Whether recent attempts
show promise is still a matter of considerable scientific debate.
caused differences are not necessarily irremediable (consider diabetes, poor
vision, and phenal ketonuria), nor are environmentally caused ones necessarily
remediable (consider injuries, poisons, severe neglect, and some diseases).
Both may be preventable to some extent.
and Stability of Between-Group Differences
is no persuasive evidence that the IQ BELL CURVES for different racial-ethnic
groups are converging. Surveys in some years show that gaps in academic achievement
have narrowed a bit for some races, ages, school subjects and skill levels,
but this picture seems too mixed to reflect a general shift in IQ levels themselves.
differences in IQ BELL CURVES are essentially the same when youngsters leave
high school as when they enter first grade. However, because bright youngsters
learn faster than slow learners, these same IQ differences lead to growing
disparities in amount learnedas youngsters progress from grades one to 12.
As large national surveyscontinue to show, black 17-year-olds perform, on
the average, more likewhite 13-year-olds in reading, math, and science, with
reasons that blacks differ among themselves in intelligenceappear to be basically
the same as those for why whites (or Asians orHispanics) differ among themselves.
Both environment and geneticheredity are involved.
is no definitive answer to why IQ bell curves differ acrossracial-ethnic groups.
The reasons for these IQ differences betweengroups may be markedly different
from the reasons for why individualsdiffer among themselves within any particular
group (whites or blacks orAsians). In fact, it is wrong to assume, as many
do, that the reason whysome individuals in a population have high IQs but
others have low IQs must be the same reason why some populations contain more
such high (or low) IQ individuals than others. Most experts believe that environment
is important in pushing the bell curves apart, but that genetics could be
differences are somewhat smaller but still substantial for individuals from
the same socioeconomic backgrounds. To illustrate, black students from prosperous
families tend to score higher in IQ than blacks from poor families, but they
score no higher, on average, than whites from poor families.
all Americans who identify themselves as black have white ancestors -- the
white admixture is about 20%, on average -- and many self-designated whites,
Hispanics, and others likewise have mixed ancestry. Because research on intelligence
relies on self-classification into distinct racial categories, as does most
other social-science research, its findings likewise relate to some unclear
mixture of social and biological distinctions among groups (no one claims
for Social Policy
research findings neither dictate nor preclude any particular social policy,
because they can never determine our goals. They can, however, help us estimate
the likely success and side-effects of pursuing those goals via different
professors — all experts in intelligence and allied fields — have
signed this statement: