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 The
modern measurement of human intelligence began in England in the latter
half of the 19th century when Sir Francis Galton, the half-cousin of Charles
Darwin, developed a test from a simplistic theory. People take in information
through their senses; therefore, the most intelligent people must have
the best developed senses. Galton developed a test composed of sensory,
motor, and reaction-time tasks, all of which produced reliable, consistent
results. Ultimately, only Galton’s emphasis on the genetics of intelligence
proved sound; around the turn of the 20th century, his influential tests
were shown to be invalid measures of the complex construct.
Binet
seized the opening and published the first “real” intelligence
test in 1905. Binet’s intelligence test, based on “g” or
general intelligence theory, comprised many brief tasks of memory, judgment,
and reasoning. He focused on language, rather than the nonverbal abilities
emphasized by Galton; introduced the mental age concept; and revolutionized
testing by insisting that one must be willing to accept measurement error
in order to measure complex human intelligence. Lewis Terman of Stanford
University, also a believer in “g,” translated, adapted, and
standardized the Binet-Simon scales in the United States. When he published
the Stanford-Binet in 1916, it became the undisputed king of intelligence
tests and retained that title for nearly a half-century.
A
second great influence on the development of IQ tests in the United States
was America’s entry into World War I in 1917. The necessity of testing
large numbers of recruits quickly led to the development of group-administered
intelligence tests: the Army Alpha (a multiple-choice Binet-like test
of language abilities) and the Army Beta (composed of nonverbal tasks),
to assess the mental abilities of immigrants who spoke English poorly.
Ultimately, the individually administered Army Performance Scale Examination
was developed for suspected malingerers and for others who could not be
tested validly in a group format. The practical outcomes of this war effort
were many, including the notion that IQ tests were useful for adults (not
just children), they were valid (data from almost 2 million soldiers were
analyzed), and they were controversial (thanks to irresponsible misinterpretations
of the data by some prejudiced wartime researchers that led to cries of
racism and inferiority).
The
link between the practical innovations of Binet, Terman, and wartime psychologists
and current clinical assessment practices as we approach the 21st century
is one man: David Wechsler. A clinical examiner during World War I, Wechsler
became aware of the need for fair assessment of people who spoke English
poorly. His Wechsler series of scales continues to reign as the worldwide
king of IQ tests.
Wechsler
blended tasks from the Stanford-Binet and Army Alpha to develop his Verbal
scale and from the Army Beta and Army Performance Scale Examination to
create his Performance scale. His creativity came from his insistence
that everyone be evaluated on both verbal and nonverbal tasks and that
multiscore test profiles are more valuable for interpreting human intelligence
than is a single global IQ. Conventional wisdom encouraged the administration
of relatively brief verbal tasks instead of time-consuming nonverbal items
to anyone who spoke English. (Fig. 1)
Though
Wechsler was a firm believer in “g” theory, he was first and
foremost a clinician who considered his IQ tests to measure an aspect
of personality and to be clinical instruments more than psychometric devices.
Although Wechsler developed his tests from practical and clinical considerations,
his tests have had important theory-based and neurologically relevant
implications. His distinction between Verbal IQ (V-IQ) and Performance
IQ (P-IQ) was subsequently related to neuropsychological theory in the
1950s by Ralph Reitan, and to cerebral specialization theory in the 1960s
by Roger Sperry. Deficits in V-IQ related to damage to the left hemisphere
and deficits in P-IQ were associated with damage to the right hemisphere.
No
test is able to measure all abilities that lie within the complex domain
of intelligence. Tests measure only a segment of the diverse abilities
that define Howard Gardner’s theory of intelligence and assess only
one dimension of Robert Sternberg’s triarchic theory, namely analytic
abilities but not creative or practical abilities. Indeed, they measure
only a small portion of skills that might rightfully define the domain
of intelligent human behavior. Yet they do measure abilities that are
able to be assessed objectively and that are stable over time; that are
either theory-based or intimately related to well-respected neuropsychological
and empirical theories of intelligence; that have demonstrated validity
for predicting school achievement; that have diagnostic utility for clinical
populations such as those with learning disabilities or Alzheimer disease;
that are clinical tools, as Wechsler initially visualized them, to evaluate
aspects of one’s personality; and that offer insight into diverse
real-life issues such as IQ and aging and genetics versus environment.
It is to the latter topic that I now turn.
The
main way that scientists have studied the role of genetics and environment
in determining IQ and other variables has been to investigate individuals
who differ in their degree of blood relationship. For example, if genetics
plays a role in IQ, then identical twins should have IQs that correlate
more highly than the IQs of fraternal twins; siblings’ IQs should
correlate more highly than those of cousins; and so forth. The results
of this aggregation of data can be used either by genetics-oriented or
environment-oriented individuals to support their position.
The importance of genetics in influencing one’s IQ
is supported by these results:
1. Identical (monozygotic or MZ) twins’ IQs
are more similar than those of fraternal (dizygotic or DZ) twins (0.86
versus 0.60).
2. IQs of siblings correlate more highly than IQs of half-siblings
(0.47 versus 0.31), which, in turn, correlate higher than IQs of cousins
(0.15).
3. Correlations between a biological parent and child living together
are higher than correlations between an adoptive parent and child living
together (0.42 versus 0.19).
In contrast, the following results support the role of environment in
determining one’s IQ:
1. IQs of DZ twins correlate more highly than IQs of siblings of
different ages (0.60 versus 0.47) despite the same degree of genetic similarity.
2. Unrelated siblings reared together (adoptive/natural or adoptive/adoptive)
have IQs that are more similar than do biological siblings reared apart
(0.32 versus 0.24).
3. Correlations between IQs of an adoptive parent and a child living
together are similar to correlations of a biological parent and a child
living apart (0.19 versus 0.22).
4. Siblings reared together have IQs that are more similar than siblings
reared apart (0.47 versus 0.24). The same finding holds for parent and
child, when they are living together (0.42) or apart (0.22).
The
results of data from thousands of subjects indicate that heredity is important
in determining a person’s IQ, but that environment is also a vital
factor. Based on many twin studies, the heritability percentage for IQ
is approximately 50, not as high as the value of 80 for height, but comparable
in magnitude with the value for weight. The weight comparison is good.
In general, overweight people have a genetic predisposition for a large
frame and a metabolism that promotes gaining weight, whereas extremely
thin people have the opposite predisposition. But for any given individual,
lifestyle (eating habits, exercising) has a substantial effect on weight.
The analogy to IQ development is self-evident. And, as with weight, genetics
and environment interact in determining IQ. People with genetic overlaps
(parents, siblings) usually share common environments.
One
of fascinating aspect of genetic research provides a caveat regarding
all previous data and conclusions discussed. Heritability estimates are
based to a large extent on correlations involving MZ versus DZ twins.
However, one potentially crucial variable has been ignored in most genetic
research, namely a placentation (chorion) effect. Did the MZ twins share
a single placenta (monochorionic) or did they have separate placentas
(dichorionic)? When the zygote divides within 72 hours after fertilization,
the MZ twins are dichorionic but when the division occurs from day 4 to
7, they share one placenta. Data indicate that MZ twins who differ on
the chorion effect also differ in birth weight, cord blood cholesterol
level, adult personality, and cognition. About two thirds of MZ twins
are monochorionic.
Regarding
cognitive differences, correlations between IQ tasks may be a function
of the chorion effect. Table 1 demonstrates this finding for 2 WAIS subtests,
Vocabulary (a prototypical measure of Gc) and Block Design (a measure
of Gf). As shown, adult MZ twins scored almost identically on Vocabulary
regardless of chorion classification, but on Block Design the close similarity
held only for monochorionic twins. Dichorionic twins were no more similar
in their test performance than DZ twins. This finding was upheld for 8-
to 12-year-old MZ twins in France.
These
cross-validated findings from Canada and France (hospitals in the United
States do not typically record chorion category) indicate that an infant’s
earliest environment may influence later IQ. Those adult MZ twins whose
intrauterine environment was different (dichorionic) performed more differently
on Block Design than those with virtually the same environment. Ironically,
the greatest effect seems to be on a nonverbal measure of an ability believed
to be closely aligned with neurological development, rather than with
the verbal, education-dependent Gc subtest. The results require additional
replication and generalization to be accepted as scientific findings;
furthermore, some significant findings in the chorion studies indicate
greater similarities among dichorionic than monochorionic MZ twins. The
findings are, however, sufficiently provocative to challenge all known
heritability estimates pertaining to intelligence and personality because
pertinent studies failed to control for the chorion effect.
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