Intelligence

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Distribution of IQ Scores

IQ scores, like many other biological and psychological characteristics, are distributed according to a normal distribution, which forms a normal curve, or bell curve, when plotted on a graph. In a normal distribution, most values fall near the average, and few values fall far above or far below the average. Although raw scores are not exactly normally distributed, test makers derive IQ scores using a formula that forces the scores to conform to the normal distribution. The normal distribution is defined by its mean (average score) and its standard deviation (a measure of how scores are dispersed relative to the mean). Usually the mean of an IQ test is arbitrarily set at 100 with a standard deviation of 15. Other tests use different values. For example, the SAT originally used a mean of 500 and a standard deviation of 100, although these are now recomputed annually.

Because IQs are distributed along a normal curve, a fixed percentage of scores fall between the mean and any standard deviation value. For example, 34 percent of IQ scores fall between the mean and one standard deviation. For a standard IQ distribution with a mean of 100 and a standard deviation of 15, 34 percent of the cases would fall between 100 and 115. Since the normal curve is symmetrical about the mean, 34 percent of the scores would also fall between 85 and 100, which represents one standard deviation below the mean. To interpret the score of any test, it is important to know the mean and standard deviation of the test. Along with knowledge of the standard deviation and the normative sample used for the test, one can then interpret the score in terms of the percentage of the population scoring higher or lower. If a person obtains a score of 115 on an IQ test, approximately 16 percent of the population will score higher and 84 percent will score lower.

When an IQ test is revised, it is restandardized with a new normative sample. The distribution of raw scores in the sample population determines the IQ that will be assigned to the raw scores of others who take the test. By analyzing the performance over the years of different normative samples on the same tests, researchers have concluded that performance on intelligence tests has risen significantly over time. This phenomenon, observed in industrialized countries around the world, is known as the Flynn effect, named after the researcher who discovered it, New Zealand philosopher James Flynn. Scores on some tests have increased dramatically. For example, scores on the Raven’s Progressive Matrices, a widely used intelligence test, increased 15 points in 50 years when scored by the same norms. In other words, a representative sample of the population that took the test in 1992 scored an average of 15 points higher on the test than a representative sample that took the test in 1942.

It appears that people are getting smarter. However, only some tests show these changes. Tests of visual-spatial reasoning, like the Raven’s test, show the largest changes, while vocabulary and verbal tests show almost no change. Some psychologists believe that people are not really getting smarter but are only becoming better test takers. Others believe the score gains reflect real increases in intelligence and speculate they may be due to improved nutrition, better schooling, or even the effects of television and video games on visual-spatial reasoning.

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Uses of Intelligence Tests

Intelligence tests and similar tests are widely used in schools, business, government, the military, and medicine. In many cases, intelligence tests are used to avoid the biases more arbitrary methods of selection introduce. For example, it was once common for colleges to admit students whose parents had attended the college or who came from socially prominent families. By using tests, colleges could select students based on their ability instead of their social position.

Intelligence tests were originally designed for use in schools. In elementary and secondary schools, educators use tests to assess how well a student can be expected to perform and to determine if special educational programs are necessary. Intelligence tests can help to identify students with mental retardation and to determine an appropriate educational program for these students (see Education of Students with Mental Retardation). Intelligence tests may also be required for admission into programs for the gifted or talented (see Education of Gifted Students). Institutions of higher education use achievement or aptitude tests, which are very similar to intelligence tests, for the selection and placement of students.

In business, employers frequently use intelligence and aptitude tests to select job applicants. Since World War I, the United States military has had one of the most comprehensive testing programs for selection and job assignment. Anyone entering the military takes a comprehensive battery of tests, including an intelligence test. For specialized and highly skilled jobs in the military, such as jet pilot, the testing is even more rigorous. Intelligence tests are helpful in the selection of individuals for complex jobs requiring advanced skills. The major reason intelligence tests work in job selection is that they predict who will learn new information required for the job. To a lesser extent, they predict who will make “smart” decisions on the job.

In medicine, physicians use intelligence tests to assess the cognitive functioning of patients, such as those with brain damage or degenerative diseases of the nervous system. Psychiatrists and psychologists may use intelligence tests to diagnose the mental capacities of their clients.

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Criticisms of Intelligence Tests

Properly used, intelligence tests can provide valuable diagnostic information and insights about intellectual ability that might otherwise be overlooked or ignored. In many circumstances, however, intelligence testing has become extremely controversial, largely because of misunderstandings about how to interpret IQ scores.

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Validity

One criticism of intelligence tests is that they do not really measure intelligence but only a narrow set of mental capabilities. For example, intelligence tests do not measure wisdom, creativity, common sense, social skills, and practical knowledge—abilities that allow people to adapt well to their surroundings and solve daily problems. The merit of this criticism depends on how one defines intelligence. Some theorists consider wisdom, creativity, and social competence aspects of intelligence, but others do not. Psychologists know little about how to objectively measure these other abilities. Another criticism of IQ tests is that some people may not perform well because they become anxious when taking any timed, standardized test. Their poor performance may reflect their anxiety rather than their true abilities. However, test anxiety is probably not a major cause of incorrect scores.

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Misinterpretation and Misuse

Critics of intelligence testing argue that IQ tests tend to be misinterpreted and misused. Because IQ tests reduce intelligence to a single number, many people mistakenly regard IQ as if it were a fixed, real trait such as height or weight, rather than an abstract concept that was originally designed to predict performance in school. Furthermore, some people view IQ as a measurement of a person’s intrinsic worth or potential, even though many factors other than those measured by IQ tests contribute to life success.

Critics also note that intelligence testing on a large scale can have dangerous social consequences when the results are misused. For example, during the 1920s IQ tests were used to identify “feeble-minded” persons. These persons were then subject to forced sterilization. In the 1927 case Buck v. Bell, the United States Supreme Court upheld the right of states to sterilize individuals judged to be feeble-minded.

In judging the uses of intelligence tests, one must compare how decisions would be made without using the tests. When tests are used to make a decision, there should be evidence that the decision made using the test is better with the test than without it. For example, if schools did not use intelligence or aptitude tests to determine which students need remedial education, teachers would be forced to rely on more subjective and unreliable criteria, such as their personal opinions.

In some cases, institutions use tests when they do not need to. Some colleges and universities require students to take admission tests but then admit 80 percent or more of applicants. Tests are of little use in selection decisions when there is little or no selection. Another criticism of intelligence tests is that they sometimes lead to inflexible cutoff rules. In some states, for example, a person with mental retardation must have an IQ of 50 or below before being allowed to work in a special facility known as a sheltered workshop. Although intelligence is important in determining performance, it is not the only determinant. People with an IQ of 50 vary widely in their skills and abilities. Using an arbitrary cutoff of 50 can make it difficult for people whose IQ is 51 to get essential services.