Intelligence
Chapter 9
Prepared by
J. W. Taylor V
A Bit of History…
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First attempts to develop intelligence tests
took place in late 19th century England and
in early 20th century France
Embedded in the nature-nurture controversy
Francis Galton
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Sir Francis Galton was trying to
develop an intelligence test for the
purpose of eugenics, selective
reproduction to enhance the capacities of the human race.
Believed in the genetic determination of intelligence and
thought he could measure intelligence by measuring
various aspects of the human brain and nervous system (a
strong nature emphasis)
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Developed tests of sensory abilities and reaction time and tested
thousands of people (found, however, that these were not good
predictors of intelligence)
Nevertheless, invented the basic mathematics behind
correlational statistics
Binet & Simon
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In France in the early part of the
20th century, Binet and Simon
were working on the problem of
mental retardation when France
switched to mass public
education
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Developed a test to diagnose
children who were subnormal
Published in 1905, this test was the
first accepted test of intelligence
Binet & Simon
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Based on the concept of mental
age – the age typically
associated with a child’s level of
performance
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If a child’s mental age was less than
their chronological/actual age, they
would need remedial work
Demonstrates a nurture emphasis
on intelligence
Terman
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Lewis Terman at Stanford University used Binet and
Simon’s test, after revising it for American school children
In 1916, Terman’s revision became known as the StanfordBinet, and Terman used the classic intelligence quotient
formula by William Stern, a German psychologist
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IQ = (mental age/chronological age) X 100
Consequently, when a child’s mental age as
assessed by the test was greater than the child’s
chronological age, the child’s IQ was greater
than 100
When a child’s mental age as assessed by the
test was less than the child’s chronological age,
the child’s IQ was less than 100
Note that the IQ formula itself is no longer used
Weschler
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David Wechsler was Chief
Psychologist at Bellevue Hospital in
New York City in the 1930s and
was in charge of adult patients of diverse backgrounds
l The Stanford-Binet was not designed to assess adult
intelligence, and the IQ was particularly problematic for
adults because at some point the mental age levels off but
the chronological age keeps increasing (so a person’s IQ
declines simply because of natural aging)
Developed his own tests, the Wechsler Bellevue Scale, in 1939
(later called the Wechsler Adult Intelligence Scale – WAIS)
Provides test scores for a battery of both verbal tests (such as
vocabulary and comprehension) and performance (nonverbal) tests (such as block design and picture arrangement)
Deviation IQ Scores
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To calculate a person’s deviation IQ, Wechsler
compared how far the person’s raw score was
from the mean raw score in terms of standard
deviation units from the mean
To make the deviation scores resemble the IQ
formula, he set the mean to 100 and the
standard deviation to 15
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Deviation IQ score = 100 plus or minus (15x the
number of standard deviation units a person’s raw test
score is from the mean for the relevant age group
norms)
Deviation IQ Scores on the WAIS
Controversies About Intelligence
General
vs.
Specific
Nature
vs.
Nurture
Theories of Intelligence
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Charles Spearman argued that intelligence test
performance is a function of two types of factors
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A g factor (general intelligence)
Some s factor (specific intellectual abilities such as
reasoning)
Believed that the g factor was more important
because people who did well on one subtest
usually did well on most of the subtests, and
people who did poorly on one subtest usually
did poorly on most of the subtests
Theories of Intelligence
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L. L. Thurstone argued for the importance of
several mental abilities –
verbal comprehension,
number facility, spatial
relations, perceptual
speed, word fluency,
associative memory, and reasoning
Identified these abilities via factor analysis,
which is a statistical technique that identifies
cluster of test item that measure the same
ability (factor)
Theories of Intelligence
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Cattell and Horn proposed two types of
intelligence, which have been of interest to
researchers in aging
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Fluid intelligence refers to abstract reasoning,
memory, and the speed of information
processing
Crystallized intelligence refers to accumulated
knowledge and verbal and numerical skills
Theories of Intelligence
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Howard Gardner’s theory of multiple intelligences
includes 8 independent types of intelligence
Linguistic
Language ability (e.g., reading, writing, speaking)
Logical-Mathematical Mathematical problem solving & scientific analysis
Spatial
Reasoning about visual spatial relationships
Musical
Musical skills (e.g., the ability to compose and
understand music)
Bodily-Kinesthetic
Skill in body movement and handling objects
Intrapersonal
Understanding oneself
Interpersonal
Understand other people
Naturalist
Ability to discern patterns in nature
Theories of Intelligence
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Robert Sternberg’s triarchic theory of
intelligence proposes three types of
intelligence
1. Analytical intelligence is essentially what is
measured by standard intelligence tests, the
necessary skills for good academic performance
2. Practical intelligence could be equated with
good common sense or “street smarts”
3. Creative intelligence is concerned with the
ability to solve novel problems and deal with
unusual situations
Nature vs. Nurture
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Most contemporary psychologists believe
that both heredity (nature) and
environmental experiences (nurture) are
important in determining intelligence
The disagreement is over the relative
contribution of each part to intelligence
The Case for Nature
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Genetic similarity studies are important in
determining the relative contribution of nature
and nurture to intelligence
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Identical twins have 100% genetic similarity
Fraternal twins and siblings have 50% similarity
Two unrelated people have 0% similarity
If intelligence were due to heredity, the
average correlations between intelligence
scores should decrease as genetic similarity
decreases, and researchers have found this to
be the case
The Case for Nurture
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However, there are also results that support
environmental influences on intelligence
For example, if identical twins are raised
together, the correlation between their
intelligence test scores is +0.86, but if the
identical twins are raised apart, the
correlation falls to +0.72
Both Nature and Nurture
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The average correlation between fraternal
twins raised together (+0.60) is less than that
for identical twins reared apart (+0.72),
indicating the influence of heredity
The average correlation is greater than that for
ordinary siblings reared together (+0.47),
indicating environmental influences because
the environment influences of fraternal twins is
more similar than for ordinary siblings at
different ages
Both Nature and Nurture
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There is a modest correlation between the
intelligence test scores of adopted children with
their parents, and this correlation disappears as
the children age
The correlation between the scores for adopted
children and their biological parents, however,
increases as the children age
This stronger relationship between a person’s
intelligence and that of their biological parents
means that nature plays a larger role in
determining a person’s intelligence than
environmental experiences
Heritability
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An index of the degree of variation of a trait within a
given population that is due to heredity
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For intelligence, most research suggests 50% to 70% of
the variation in intelligence test scores is estimated to be
due to heredity
Because it is not 100%, this means that heredity and
environment interact to determine intelligence
In essence, heredity determines a reaction range,
genetically determined limits for an individual’s
intelligence, but the quality of the person’s
environmental experiences determine where the
individual falls within this range
Caveats
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Heritability is a group statistic and not
relevant to individual people
Heritability has nothing to do with the
differences that have been observed
between populations, such as the difference
in scores for Asian versus American
schoolchildren
The Flynn Effect
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Refers to the fact that in the United States
and other Western industrialized nations,
average intelligence scores have improved
steadily over the past century
Proposed explanations involve many
environmental factors such as better
nutrition and more education