testing and Individual Differences ❮ 219As a result of their responses to test items, children were assigned a mental age or mental
level reflecting the age at which typical children give those same responses. Although mental
age differentiates between abilities of children, it can be misleading when a 6-year-old
and an 8-year-old, for example, have mental ages 2 years below their actual (chronologi-
cal) ages. The younger child would be proportionally further behind peers than the older
child. German psychologist William Stern suggested using the ratio of mental age (MA)
to chronological age (CA) to determine the child’s level of intelligence.
Mental Age and the Intelligence Quotient
In adapting Binet’s test for Americans, Lewis Terman developed the Stanford-Binet
Intelligence Scale reporting results as an IQ, intelligence quotient, which is the child’s
mental age divided by his or her chronological age, multiplied by 100; or MA/CA × 100.
A 10-year-old who answers questions typical of most 12-year-olds has an IQ score of 120.
Another 10-year-old who answers questions typical of an 8-year-old scores 80. With the
development of intelligence tests for adults, the ratio IQ becomes meaningless and has
been replaced by the deviation IQ determined as a result of the standardizing process for
a particular test. For the fifth edition of the Stanford-Binet Intelligence Scale for Adults,
the test has been standardized with a representative sample of test takers up to age 90. Fluid
reasoning, visual-spatial processing, working memory, and quantitative reasoning seem to
peak in the 30s, whereas knowledge seems to peak in the 50s.
The newest version assesses each of five ability areas, such as knowledge, fluid reason-
ing, and quantitative reasoning, both nonverbally and verbally. By combining these subtest
scores, one IQ score is determined.
The Wechsler Intelligence Scales
David Wechsler developed another set of age-based intelligence tests: the Wechsler Preschool
and Primary Scale of Intelligence (WPPSI) for preschool children, the Wechsler Intelligence
Scale for Children (WISC) for ages 6 to 16, and the Wechsler Adult Intelligence Scale (WAIS )
for older adolescents and adults. The latest edition, the WAIS-III, has a verbal scale includ-
ing items on comprehension, vocabulary, information, similarities, arithmetic, and digit
span; and a performance scale including items dealing with object assembly, block design,
picture completion, picture arrangement, and digit symbols. Wechsler based his measures
on deviation IQs or how spread out the scores were from the mean of 100 (Figure 15.1).
Since intelligence has a bell curve distribution, 68 percent of the population will have an
IQ between 85 and 115. These test takers are considered to be low normal through high
normal. Test takers who fall two deviations below the mean have a score of 70, typically
considered the borderline for intellectual disability, while test takers two standard devia-
tions above the mean have scores of 130, sometimes considered intellectually gifted, and
those three standard deviations above the mean have scores of 145, sometimes considered
geniuses. The Wechsler tests are judged more helpful for determining the extremes of intel-
ligence at the intellectual disability and the genius level than the Stanford-Binet. They also
help indicate possible learning disabilities when a child’s performance IQ is very different
from his or her verbal score.
Intellectual Disability
Over the past two decades, the term mental retardation has been replaced by intellectual
disability (intellectual developmental disorder). To be considered intellectually disabled,
an individual must earn a score at or below 70 on an IQ test and also show difficulty
adapting in everyday life. Adaptive behavior is expressed in conceptual skills, social skills,
and practical skills. Severity is determined by adaptive functioning rather than IQ score.
Typically individuals with mild intellectual disability (about 85 percent) can care for