Differences in Intelligence

In this module, we will consider the extremes of intelligence -- from the very bright to the very dull. We will also look at a few of the many studies that have been done with IQ scores, correlating many things from sense of humor (Brumbaugh, 1939) to racial background. A few of these correlation studies and their results will be described in this module.

As you read the text, try to answer the following questions.


Measured human intelligence follows a normal distribution in the total population. Scores on intelligence tests range from near 0 to around 200, but the mean is 100. Figure 13 shows an adult IQ distribution developed by Wechsler.
Above130 Very superior 2.2
120-129 Superior 6.7
110-119 Bright normal 16.1
90-109 Average 50.0
80-89 Dull normal 16.1
70-79 Borderline 6.7
Below 70 Defective 2.2

Figure 13. Distribution of intelligence scores on the Intelligence Scale

The Gifted

People who score above 130 on intelligence tests are usually considered gifted. An important study of gifted children was undertaken by who devised the Stanford-Binet test. In the course of developing that test, Terman and his associates tested many thousands of children, and then did further research on those who had lO's of 140 or more. This group of more than 1500 children was in the top 1% of the population. Terman followed most of these children from 1921 until his death in 1956. Some of his associates are still continuing this detailed longitudinal study.

The study revealed an important fact related to the home environments of the children. Approximately one-third were children of professional people and about half were children of persons in business occupations. Only seven percent came from "working classes." These percentages do not agree at ail with the distribution of social class membership in the general population. They indicate that relatively more gifted children come from the higher socioeconomic classes, although they do not tell us why.

Another discovery in Terman's study was that gifted children are more successful in later life. Twenty-five years after the study began, about 700 of the original group were still in contact with Terman. Of these, 150 were considered very successful. This distinction was based on such criteria as being listed in Who's Who or having received recognition for outstanding intellectual or professional achievement. Most of the others were much more successful than people of average intelligence. On the other hand, some persons in the gifted group had committed crimes, dropped out of school early, or been unsuccessful at a number of jobs. Those least successful were more poorly adjusted emotionally and less motivated than the others. On the whole, though, the most striking fact revealed in the study was that gifted children made generally outstanding contributions to society in the form of intellectual achievements.

In their early years, Terman's gifted children were above average in height, weight, and physical development, and they were also better adjusted. It had ben generally believed that very gifted children were likely to be maladjusted and socially backward. Terman's study definitely disproved this notion.

The gifted child is liable to encounter serious difficulties in school. He may be bored by children of his own age and his knowledge may even exceed that of his teacher. Teachers may consider him rude or a show-off. He may become a real problem simply because he is bored and loses interest in a class designed for "average" children. Some schools today are facing this problem by planning appropriate programs for gifted children.

The Retarded

At the other extreme of the intelligence scale is another problem area -- the mentally retarded who have IQ's below 70. These persons have traditionally been classified as morons (IQ 50-70), imbeciles (IQ 20-50), and idiots (IQ below 20). A new classification by degree of retardation was recently drawn up by the President's Panel on Mental Retardation. This classification divides retardation into mild, moderate, severe, and profound. These categories, along with specific characteristics for each category, are shown below.

MILD RETARDATION (IQ 50-70) Often not noticed as retarded by casual observer. Can acquire practical skills and useful reading and arithmetic to sixth-grade level. Can usually achieve social and vocational skills necessary for self maintenance but is slower to walk, feed self, and talk than most children . Can be guided toward social adjustment. May need occasional guidance and support when under unusual social or economic stress.

MODERATE (IQ 36-50) Noticeable delay in motor especially in speech; responds to training in various self-help activities. development,Can learn simple communication, Can perform simple tasks under elementary health and safety habits, and sheltered conditions; participates in simple manual skills; does not progress simple recreation; travels alone in familiar places; usually incapable of self-maintenance.

SEVERE RETARDATION (IQ 20-35) Marked delay in motor development; little or no communication skill; may respond to training in elementary self-help -- for example, self-feeding. Usually walks, barring specific disability;has some understanding of speech and ' some response; can profit from systematic habit training Can conform to daily routines and repetitive activities; needs continuing direction and supervision in protective environment.-

PROFOUND RETARDATION (IQ below 20) Gross retardation; minimal capacity for functioning in sensorimotor areas; needs May walk, Obvious delays in all areas of development; shows basic emotional responses; may respond to skillful training in use of legs, hands, and jaws; needs close supervision May walk, need nursing care, have primitive speech, usually benefits from regular physical activity. ; incapable of self-maintenance

Mental retardation has many causes. Some are genetic accidents, such as Mongolism or Down's syndrome, which are caused by an extra chromosome. Some are caused by physiological problems, such as cretinism, which results from thyroid deficiency. Some are due to physical factors such as not receiving enough oxygen before birth. And many are simply a combination of hereditary and environmental factors; the parents were retarded and did not provide a stimulating home environment for the children.


Gender differences In these days of an increased sexual equality across many activities, the question arises: "Are women as intelligent as men?" The answer is unclear. The sexes differ in specific abilities. Females are typically superior in verbal abilities and in items concerned with social relations. They also perceive details more quickly and accurately and do better on tests of memory. Males, on the other hand, surpass women on items that involve spatial, numerical, and mechanical performance. These differences, however, are averages. The variance within each sex is much greater than the difference in results between the sexes. In scores of overall intelligence, the two sexes seem to be about equal. All differences are restricted to specific abilities, not to the composite that we call intelligence. It is debatable, however, whether the differences in specific abilities are due to biological or cultural factors.

A recent study by Stumpf and Stanley (1999) investigating gender differences in scholastic achievement, suggests that the historic differences between the sexes may not be decreasing as much as earlier suspected. They examined the performance of over eight hundred thousand test takers on the College Board Advanced Placement (AP) tests and the College Board Scholastic Assessment Test (SAT). They found that males showed advantages mainly in Physics, Computer Science, Chemistry and Calculus; but also in Economics, Government, U.S. History and Biology. Females showed advantages in Spanish and French Literature, English Literature, and Art History. This pattern remained relatively stable over the period of the study, from 1984 to 1996, even though the subjects changed. There has been a decline, however, in the advantages which males show in Computer Science. The AP tests were half multiple-choice and half open-ended items, compared to the SAT which was all multiple choice. Gender differences favoring males appears larger on those tests which are multiple-choice, which has been observed in earlier studies.

Socioeconomic classIntelligence differences between socioeconomic classes are well documented. In fact, one researcher (Tyler, 1956) concluded that the relationship of IQ scores to socioeconomic level was one of the best documented facts in mental test history. This was indicated earlier in Terman's study of gifted children, and more evidence will be presented in the next module. On the average, higher IQ's are found in families of the upper socioeconomic levels than in those of the lower levels.

IQ scores reflect educational opportunities. This is indicated in the data on socioeconomic levels. Children from disadvantaged groups frequently have inferior educational facilities, and their attitudes toward education are less favorable. The educational opportunities among various racial groups vary markedly. Therefore, one would expect that differences in average IQ would be apparent between various racial groups. Usually it is not really possible to determine whether the differences are due to racial or socioeconomic factors. Tulkin and Nembrough (1968) used statistical procedures to control for differences in such variables as broken homes, maternal employment, crowded living quarters, and so on. Under these controls their studies with upper social class grade school children indicated no difference between races.

Race differences. Many comparisons have been made between black and white children. Three conclusions have been found repeatedly. Black children, on the average, achieve lower IQ scores than white children. The second conclusion is that scores for black and white children overlap a great deal. Both of these results are shown in Figure 15. The third consistent finding is that the IQ scores of black children rise when their educational opportunities are improved. This fact, plus the knowledge that an enriched environment causes an increase in IQ, suggests that some, if not all, of the differences in measured IQ between black and white children may be explained by environmental factors.

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Figure 15. Distribution of IQ scores for 386 white children and 103 black children of Kent County, Ontario, Canada

We know a great deal about who does well or poorly on IQ tests

A serious criticism of the data shown in Figure 15 can and should be made. It involves the specific content of items on IQ tests. Items were developed and standardized with white children whose backgrounds included similar experiences. Even rural white children tend to score below the norm. The techniques used in developing the tests, then, may explain many of the discrepancies between black and white children, and also between the different socioeconomic classes.

We are still trying to find out why

In an effort to deal with the fact that all people do not have the same background as white middle-class children, culture-fair tests have been suggested. The basic principle in these tests is to avoid items in which experiences in different cultural groups will affect the total score. There are two ways of approaching this problem. One is to use objects and events in the test items that almost everyone has had the same amount of experience with. The other approach is to balance items so that each group's experience is about equally sampled. Regardless of approach, however, efforts to construct culture-fair intelligence tests have not been too successful.

Psychologists clearly do not agree on the heredity vs. environment issue. There is even argument that Asian IQ's are higher than whites (Lynn, 1993). Following the The Bell shaped Curve book by Herrnstein and Murray (1994), there was considerable national commentary on the role of inheritance and cultural factors influencing IQ scores. As a result, the American Psychological Association asked a committee to review the literature and hopefully provide some clarification to this controversy. Neisser et al. (1996) published such a report which was generally positively received; though it did serve to generate further comment and controvery, published in the January 1997 issue of the American Psychologist.


Now test yourself without looking back.

1. Which of the following were Terman's findings as a result of his study of gifted children?
a. More gifted children come from the upper socioeconomic levels.
b. Fewer gifted children are emotionally well-adjusted.
c. Gifted children are above average in height, weight, and physical development.
d. Gifted children are less successful in later life.

2. A person is considered retarded if his IQ is below___________________________

3. In intelligence testing: a. men score higher than women in overall intelligence.
b. both sexes score about the same in overall intelligence.
c. women score higher in verbal areas.
d. (none of these)

4. A child is considered gifted if his IQ is over_____________________________

5. Families in the higher socioeconomic levels produce:
a. fewer gifted children.
b. better-educated children.
c. (neither)

6. Give two possible causes other than heredity for the differences on IQ test results for different racial groups.________________________________________.



IQ Description Percent of Adults
Above 130
120- 129
110- 119
90- 109
80 - 89
70 - 79
Below 70
Bright normal
Dull normal

Distribution of intelligence scores on the Wechsler Adult Intelligence Scale

Terman's study included 1500 children whose lO's were 140 or above. According to the chart above, they would all be classified as_____________________________________6

Persons with lO's between 36 and 52 are moderately mentally retarded. According to the chart above, they would be classified as_______________________________________2

Write the description of each of the following IQ's.

a. 140____________________________

b. 25_____________________________

d. 81_____________________________

e. 72_____________________________

In his classic study of gifted children, Terman studied more than 700 children over a period of 30 years. What kind of study was it?______________________________________________3

Terman found that gifted children, on the average:

a. were from (high/low) socioeconomic levels.

b. had (less success/more success) in later life.

c. were (above average/below average) in height, weight, and physical development.

d. were (better adjusted/less well adjusted) emotionally.


Gifted children may have trouble in school because:
a. they have difficulty with intellectual tasks.
.b. they may become bored or disinterested.
c. classes are designed for average students.
d. (none of these)

Which of the following might be causes of mental retardation?
a. Hereditary factors
b. Over stimulating home environment
c. No opportunity for college
d. Physiological factors


1 a, d

5 a. high
b. more success
c. above average
d. better adjusted

6 gifted

2 retarded

3 longitudinal

7. a. Gifted
b. Retarded
c. Average
d. Dull Normal
e. Borderline

8 b.c

IQ scores reflect educational opportunities. Children of families in high socioeconomic levels tend to have more and better educational opportunities. How would the IQ's of children from higher socioeconomic levels compare with those of children from lower levels?


Write the more modern descriptions of mental retardation for the following traditional terms.

a. moron (IQ 50-70)___________________

b. imbecile (IQ 20-50)__________________

c. idiot (IQ below 20)______________________________________5


1 lower lQ's

2 Opportunities

3 The children from higher socioeconomic levels would have higher IQ scores

4 retardation

5 a.mild
b.moderate, severe
c. profound



1. Give two important findings of Terman's study of gifted children________________.

2. Label each of these lO's as either gifted, retarded or normal (within the normal range).

IQ 1--137__________________________

2-- 100 ________________________-
3-- 115__________________________

4-- 86___________________________

5--69 ___________________________


3. How do men and women compare on intelligence tests?________________________

4. Racial differences in measured IQ might occur because of heredity or_______________


Unit 7 Table of Contents

Psych 200 Home Page

March 8, 2002