The distribution of IQ scores

Intelligence test scores follow an approximately normal distribution, meaning that most people score near the middle of the distribution of scores and that scores drop off fairly rapidly in frequency as one moves in either direction from the centre. For example, on the IQ scale, about two out of three scores fall between IQs of 85 and 115, and about 19 out of 20 scores fall between 70 and 130. Put another way, only one out of 20 scores differs from the average IQ (100) by more than 30 points.

It has been common to associate certain levels of IQ with labels. For example, at the upper end, the label "gifted" is sometimes assigned to people with IQs over a certain point, such as 130. And at the lower end, mental retardation has been classified into different degrees depending upon IQ, so that, for example, IQs of 70-84 have been classified as borderline retarded, IQs of 55-69 as mildly retarded, IQs of 40-54 as moderately retarded, IQs of 25-39 as severely retarded, and IQs below 25 as profoundly retarded. Labeling schemes like these, however, have pitfalls and are in some ways dangerous.

First, the labels assume that conventional intelligence tests provide sufficient information to classify someone as either gifted, on the one hand, or mentally retarded, on the other. But most authorities would agree that this assumption is almost certainly false. Conventional intelligence tests are useful in providing information about some people some of the time, but the information they provide is about a fairly narrow range of abilities. To label someone as mentally retarded solely on the basis of a single test score is to risk doing a potentially great disservice and injustice to that person. Most psychologists and other authorities recognize that social as well as strictly intellectual skills are important in classifying a person as retarded. If a person adapts well to the environment, then it seems inappropriate to refer to that person as mentally retarded, a term with inescapably pejorative connotations.

Second, giftedness is generally recognized as more than just a degree of intelligence, even broadly defined. Most psychologists who have studied gifted persons agree that a variety of aspects make up giftedness. Howard E. Gruber, the Swiss psychologist, and the American psychologist Mihaly Csikszentmihalyi are among a number of researchers who are skeptical that the giftedness observed in children has much to do with the giftedness observed in adults. Gruber believes that giftedness unfolds over the course of a lifetime and involves achievement at least as much as intelligence. Gifted people, he contends, have life plans that they seek to realize, and these plans develop over the course of many years. To measure giftedness merely in terms of a single test score would be, for Gruber, a trivialization of the concept.

Third, a given test score can mean different things to different people. An IQ score for a person who has grown up in a ghetto home and gone to an inadequate school does not have the same meaning as the same IQ score for someone who has grown up in an upper-middle-class suburban environment and gone to a well-endowed school. An IQ score also does not mean the same thing for a person whose first language is not English but who takes a test in English, as it does for a native English-speaker. Another factor is that some people are "test-anxious" and may do poorly on almost any standardized test. Based on these and similar drawbacks, it has come to be believed generally that scores have to be interpreted carefully on an individual basis.

Psychologists now believe that IQ represents only a part of intelligence, and intelligence is only one factor in both retardation and giftedness. Earlier rigid concepts in the field of intelligence measurement, which led to labeling, have had undesirable effects. The growth of a more recent concept, the malleability of intelligence, has also served to discredit labeling.

The malleability of intelligence

Intelligence has historically been conceptualized as a more or less fixed trait. This view perceives intelligence as something people are born with, and the function of development is to allow this genetic endowment to express itself. A number of investigators have taken the approach that intelligence is highly heritable, transmitted through the genes. Other investigators believe that intelligence is minimally heritable, if at all. Most authorities take an intermediate position.

Various methods are used to assess the heritability of intelligence. Notable among these is the study of identical twins reared apart. For a variety of reasons, identical twins are occasionally separated at or near birth. If the twins are raised apart, and if it is assumed that when twins are separated they are randomly distributed across environments (often a dubious assumption), then the twins would have in common all of their genes but none of their environment, except for chance environmental overlap. As a result, the correlation between their performance on intelligence tests can provide an estimate of the proportion of variation in test scores due to heredity. Another method of computing the hereditary effect on intelligence involves comparing the relationship between intelligence test scores of identical twins and those of fraternal twins.

Considering the large number of studies that have investigated the heritability of intelligence, it is surprising that so much disagreement exists among researchers. It has been estimated that roughly half the variation in intelligence test scores is caused by hereditary influences. But it is significant that estimates of heritability can differ among ethnic and racial groups, as well as across time within a single group. Moreover, the estimates are computed, for the most part, on the basis of intelligence test scores, so that the estimates are only for that part of intelligence measured by the tests.

Whatever the heritability factor of IQ, a separate issue is whether intelligence can be increased. Work by a New Zealand researcher, James Flynn, has shown that, in the late 20th century, scores on intelligence tests have been rising rather steadily throughout the world. Although the reason for the increase has not been satisfactorily explained, there is little doubt that this is a developing phenomenon requiring careful investigation.

Despite the general increase in scores, average IQs continue to vary both across countries and across different socioeconomic groups. For example, many researchers have found a positive correlation between socioeconomic status and IQ, although they disagree over the reason for the relationship. Most investigators agree that differences in educational opportunities play an important role, and some investigators believe that there is a hereditary basis for the difference as well. But there is simply no broad consensus on the issue of why the differences exist, and, again, it should be noted that the differences are based on IQ, not broadly defined intelligence.

It is important to understand that no matter how heritable intelligence is, some aspects of it are still malleable. Heritability of a trait is a separate issue from its malleability. A person's height, for example, is 90 percent heritable; the best predictor of height is the height of a person's parents. Yet, because of better nutrition and health care, average heights in the United States have climbed during the 20th century. Thus, with intervention, even a highly heritable trait can be modified. There is a growing body of evidence that aspects of intelligence, too, can be modified. Intelligence, in the view of many authorities, is not a fixed trait, with its level a foregone conclusion the day a person is born. A program of training in intellectual skills can increase some aspects of a person's level of intelligence. No training program--no environmental condition of any sort--can make a genius of a person with low measured intelligence. But some gains are possible, and programs have been developed for increasing intellectual skills. A main trend for psychologists in the intelligence field has been to combine testing and training functions in order to enable people to optimize their intelligence.

Contents of this article:

Cognitive-contextual theories and biologic theories
Hemispheric studies
Development of intelligence
Post-Piaget theories and the environmental viewpoint
Measuring intelligence
The distribution of IQ scores and the malleability of intelligence