Intelligence, term usually referring to a general mental capability to reason, solve problems, think abstractly, learn and understand new material, and profit from past experience. Intelligence can be measured by many different kinds of tasks. Likewise, this ability is expressed in many aspects of a person’s life. Intelligence draws on a variety of mental processes, including memory, learning, perception, decision-making, thinking, and reasoning.
Most people have an intuitive notion of what intelligence is, and many words in the English language distinguish between different levels of intellectual skill: bright, dull, smart, stupid, clever, slow, and so on
. Yet no universally accepted definition of intelligence exists, and people continue to debate what, exactly, it is. Fundamental questions remain: Is intelligence one general ability or several independent systems of abilities? Is intelligence a property of the brain, a characteristic of behavior, or a set of knowledge and skills?
The simplest definition proposed is that intelligence is whatever intelligence tests measure. But this definition does not characterize the ability well, and it has several problems. First, it is circular: The tests are assumed to verify the existence of intelligence, which in turn is measurable by the tests. Second, many different intelligence tests exist, and they do not all measure the same thing. In fact, the makers of the first intelligence tests did not begin with a precise idea of what they wanted to measure. Finally, the definition says very little about the specific nature of intelligence.
Whenever scientists are asked to define intelligence in terms of what causes it or what it actually is, almost every scientist comes up with a different definition. For example, in 1921 an academic journal asked 14 prominent psychologists and educators to define intelligence. The journal received 14 different definitions, although many experts emphasized the ability to learn from experience and the ability to adapt to one’s environment. In 1986 researchers repeated the experiment by asking 25 experts for their definition of intelligence. The researchers received many different definitions: general adaptability to new problems in life; ability to engage in abstract thinking; adjustment to the environment; capacity for knowledge and knowledge possessed; general capacity for independence, originality, and productiveness in thinking; capacity to acquire capacity; apprehension of relevant relationships; ability to judge, to understand, and to reason; deduction of relationships; and innate, general cognitive ability.
People in the general population have somewhat different conceptions of intelligence than do most experts. Laypersons and the popular press tend to emphasize cleverness, common sense, practical problem solving ability, verbal ability, and interest in learning. In addition, many people think social competence is an important component of intelligence.
Most intelligence researchers define intelligence as what is measured by intelligence tests, but some scholars argue that this definition is inadequate and that intelligence is whatever abilities are valued by one’s culture. According to this perspective, conceptions of intelligence vary from culture to culture. For example, North Americans often associate verbal and mathematical skills with intelligence, but some seafaring cultures in the islands of the South Pacific view spatial memory and navigational skills as markers of intelligence. Those who believe intelligence is culturally relative dispute the idea that any one test could fairly measure intelligence across different cultures. Others, however, view intelligence as a basic cognitive ability independent of culture.
In recent years, a number of theorists have argued that standard intelligence tests measure only a portion of the human abilities that could be considered aspects of intelligence. Other scholars believe that such tests accurately measure intelligence and that the lack of agreement on a definition of intelligence does not invalidate its measurement. In their view, intelligence is much like many scientific concepts that are accurately measured well before scientists understand what the measurement actually means. Gravity, temperature, and radiation are all examples of concepts that were measured before they were understood.
The first intelligence tests were short-answer exams designed to predict which students might need special attention to succeed in school. Because intelligence tests were used to make important decisions about people’s lives, it was almost inevitable that they would become controversial. Today, intelligence tests are widely used in education, business, government, and the military. However, psychologists continue to debate what the tests actually measure and how test results should be used.
A
Early Tests
Interest in measuring individual differences in mental ability began in the late 19th century. Sir Frances Galton, a British scientist, was among the first to investigate these differences. In his book Hereditary Genius (1869), he compared the accomplishments of people from different generations of prominent English families. No formal measures of intelligence existed at the time, so Galton evaluated each of his subjects on their fame as judged by encyclopedia entries, honors, awards, and similar indicators. He concluded that eminence of the kind he measured ran in families and so had a hereditary component. Believing that some human abilities derived from hereditary factors, Galton founded the eugenics movement, which sought to improve the human species through selective breeding of gifted individuals.
Between 1884 and 1890 Galton operated a laboratory at the South Kensington Museum in London (now the Victoria and Albert Museum) where, for a small fee, people could have themselves measured on a number of physical and psychological attributes. He tried to relate intellectual ability to skills such as reaction time, sensitivity to physical stimuli, and body proportions. For example, he measured the highest and lowest pitch a person could hear and how well a person could detect minute differences between weights, colors, smells, and other physical stimuli. Despite the crude nature of his measurements, Galton was a pioneer in the study of individual differences. His work helped develop statistical concepts and techniques still in use today. He also was the first to advance the idea that intelligence can be quantitatively measured.
In the 1890s American psychologist James McKeen Cattell, who worked with Galton in England, developed a battery of 50 tests that attempted to measure basic mental ability. Like Galton, Cattell focused on measurements of sensory discrimination and reaction times. Cattell’s work—and by association, Galton’s—was unsupported in 1901, when a study showed that the measurements had no correlation with academic achievement in college. Later researchers, however, pointed out that Cattell’s test subjects were limited to Columbia University students, whose high academic performance was not representative of the general population. Better-designed tests given to broader samples have shown that reaction time and processing speed on perceptual tasks do correlate with academic achievement.
B
The Binet-Simon Test
Alfred Binet, a prominent French psychologist, was the first to develop an intelligence test that accurately predicted academic success. In the late 19th century, the French government began compulsory education for all children. Prior to this time, most schoolchildren came from upper-class families. With the onset of mass education, French teachers had to educate a much more diverse group of children, some of whom appeared mentally retarded or incapable of benefiting from education. Teachers had no way of knowing which of the “slow” students had true learning problems and which simply had behavioral problems or poor prior education. In 1904 the French Ministry of Public Instruction asked Binet and others to develop a method to objectively identify children who would have difficulty with formal education. Objectivity was important so that conclusions about a child’s potential for learning would not be influenced by any biases of the examiner. The government hoped that identifying children with learning problems would allow them to be placed in special remedial classes in which they could profit from schooling. Binet and colleague Théodore Simon took on the job of developing a test to assess each child’s intelligence.
As Binet and Simon developed their test, they found that tests of practical knowledge, memory, reasoning, vocabulary, and problem solving worked better at predicting school success than the kind of simple sensory tests that Galton and Cattell had used. Children were asked, among other tasks, to perform simple commands and gestures, repeat spoken digits, name objects in pictures, define common words, tell how two objects are different, and define abstract terms. Similar items are used in today’s intelligence tests. Binet and Simon published their first test in 1905. Revisions to this test followed in 1908 and 1911.
Binet and Simon assumed that all children follow the same course of intellectual development but develop at different rates. In developing their test, they noted which items were successfully completed by half of seven-year-olds, which items by half of eight-year-olds, and so on. Through these observations they created the concept of mental age. If a 10-year-old child succeeded on the items appropriate for 10-year-olds but could not pass the questions appropriate for 11-year-olds, that child was said to have a mental age of 10. Mental age did not necessarily correspond with chronological age. For example, if a 6-year-old child succeeded on the items intended for 9-year-olds, then that child was said to have a mental age of 9.
To judge how effectively the test predicted academic achievement, Binet asked teachers to rate their students from best to worst. The results showed that students who had been rated higher by their teachers also scored higher on the test. Thus, Binet’s test successfully predicted how students would perform in school.
C
The IQ Test
Binet’s test was never widely used in France. Henry Goddard, director of a New Jersey school for children with mental retardation, brought it to the United States. Goddard translated the test into English and began using it to test people for mental retardation. Another American psychologist, Lewis Terman, revised the test by adapting some of Binet’s questions, adding questions appropriate for adults, and establishing new standards for average performance at each age. Terman’s first adaptation, published in 1916, was called the Stanford-Binet Intelligence Scale. The name of the test derived from Terman’s affiliation with Stanford University.
Instead of giving a person’s performance on the Stanford-Binet as a mental age, Terman converted performance into a single score, which he called the intelligence quotient, or IQ. A quotient is the number that results from dividing one number by another. The idea of an intelligence quotient was first suggested by German psychologist William Stern in 1912. To compute IQ, Stern divided mental age by the actual, chronological age of the person taking the test and then multiplied by 100 to get rid of the decimal point. For example, if a 6-year-old girl scored a mental age of 9, she would be assigned an IQ of 150 (9/6 × 100). If a 12-year-old boy scored a mental age of 6, he would be given an IQ of 50 (6/12 × 100). The IQ score, as originally computed, expressed a person’s mental age relative to his or her chronological age. Although this formula works adequately for comparing children, it does not work well for adults because intelligence levels off during adulthood. For example, a 40-year-old person who scored the same as the average 20-year-old would have an IQ of only 50.
Modern intelligence tests—including the current Stanford-Binet test—no longer compute scores using the IQ formula. Instead, intelligence tests give a score that reflects how far the person’s performance deviates from the average performance of others who are the same age. Most modern tests arbitrarily define the average score as 100. By convention, many people still use the term IQ to refer to a score on an intelligence test.
D
Creation of Group Tests
During World War I (1914-1918) a group of American psychologists led by Robert M. Yerkes offered to help the United States Army screen recruits using intelligence tests. Yerkes and his colleagues developed two intelligence tests: the Army Alpha exam for literate recruits, and the Army Beta exam for non-English speakers and illiterate recruits. Unlike previous intelligence tests, which required an examiner to test and interact with each person individually, the Army Alpha and Beta exams were administered to large groups of recruits at the same time. The items on the tests consisted of practical, short-answer problems. The Alpha exam included arithmetic problems, tests of practical judgment, tests of general knowledge, synonym-antonym comparisons, number series problems, analogies, and other problems. The Beta exam required recruits to complete mazes, complete pictures with missing elements, recognize patterns in a series, and solve other puzzles. The army assigned letter grades of A through D- based on how many problems the recruit answered correctly. The army considered the highest-scoring recruits as candidates for officer training and rejected the lowest-scoring recruits from military service. By the end of World War I, psychologists had given intelligence tests to approximately 1.7 million recruits. Modern critics have pointed out that the army tests were often improperly administered. For example, different test administrators used different standards to determine which recruits were illiterate and should be assigned to take the nonverbal Beta exam. Thus, some recruits mistakenly assigned to the Alpha exam may have scored poorly because of their limited English skills, not because of low intelligence.
The use of intelligence tests by the United States military enhanced the credibility and visibility of group mental tests. Following World War I these tests grew in popularity. Most were short-answer tests modeled on the army tests or the Stanford-Binet. For example, Yerkes and Terman developed the National Intelligence Test, a group test for schoolchildren, around 1920. The Scholastic Aptitude Test, or SAT, was introduced in 1926 as a multiple-choice exam to aid colleges and universities in their selection of prospective students.
E
Modern Intelligence Tests
The most widely used modern tests of intelligence are the Stanford-Binet, the Wechsler Intelligence Scale for Children (WISC), the Wechsler Adult Intelligence Scale (WAIS), and the Kaufman Assessment Battery for Children (Kaufman-ABC). Each of the tests consists of a series of 10 or more subtests. Subtests are sections of the main test in which all of the items are similar. Examples of subtests include vocabulary (“Define happy”), similarities (“In what way are an apple and pear alike?”), digit span (repeating digit strings of increasing length from memory), information (“Who was the first president of the United States?”), object assembly (putting together puzzles), mazes (tracing a path through a maze), and simple arithmetic problems. Each item has scoring criteria so the examiner can determine if the answer given is correct.
Items on each subtest are given in order of difficulty until the person being tested misses a certain number of items. Each subtest provides a score. The subtest scores are then added together to obtain a total raw score, which is then converted into an IQ score. Some tests, such as the Wechsler tests, give separate verbal and performance (nonverbal) scores as well as an overall score.
Other intelligence tests, like the Peabody Picture Vocabulary Test or Raven’s Progressive Matrices, consist of only one item type. In the Peabody Picture Vocabulary Test, the test taker must define a word by deciding which picture out of four pictures best represents the meaning of the word said by the examiner. In Raven’s Progressive Matrices, a person is shown a matrix of patterns with one pattern missing. The person must figure out the rules governing the patterns and then use these rules to pick the item that best fills in the missing pattern. The Raven’s test was designed to minimize the influence of culture by relying on nonverbal problems that require abstract reasoning and do not require knowledge of a particular culture.
All of the tests mentioned so far can be individually administered. An examiner tests one person at a time for a specific amount of time, ranging from 20 to 90 minutes. There are also group-administered tests. The Army Alpha test described above was one of the earliest group-administered tests. This test developed into what is now known as the Armed Services Vocational Achievement Battery (ASVAB), which is used to select and classify military recruits. Group tests usually are not as reliable as individually administered tests. They are often shorter and have less variety in item types because of restrictions inherent in group administration. Furthermore, the administrator of an individual test can more fully supervise the test taker’s performance. For example, the administrator can make sure the test taker is motivated and provide additional information when necessary. But group tests are efficient because they can be given to large numbers of people in a short time and at a relatively low cost.
Achievement tests and aptitude tests are very similar to intelligence tests. An achievement test is designed to assess what a person has already learned, whereas an aptitude test is designed to predict future performance or assess potential for learning. Usually the items on achievement tests and aptitude tests relate to a specific area of knowledge, such as mathematics or vocabulary. Because intelligence tests frequently include these same areas of knowledge, many experts believe that it is impossible to distinguish between intelligence tests, achievement tests, and aptitude tests. Often, test makers call their tests achievement tests or aptitude tests to avoid the word intelligence, which can be frightening to some test takers. Examples of achievement and aptitude tests that are widely used include the SAT, the Graduate Record Exam (GRE), the California Achievement Test, the Law School Admissions Test (LSAT), and the Medical College Admission Test (MCAT).
F
Standardization, Reliability, and Validity of Tests
An intelligence test, like any other psychological test, must meet certain criteria in order to be accepted as scientific and accurate. A test must be standardized, reliable, and valid.
Standardization refers to the process of defining norms of performance to which all test takers are compared. Before an intelligence test can be used to make meaningful comparisons, the test makers first give the test to a sample of the population representative of the individuals for whom the test is designed. This sample of people is called a normative sample, because it is used to establish norms (standards) of performance on the test. Normative samples usually consist of thousands of people from all areas of the country and all strata of society. Test scores of people in the sample are statistically analyzed to compile the test norms. When the test is made available for general use, these norms are used to determine a score for each person who takes the test. The IQ score or overall score reflects how well the person did compared to people of the same age in the normative sample.
Reliability refers to the consistency of test scores. A reliable test yields the same or close to the same score for a person each time it is administered. In addition, alternate forms of the test should produce similar results. By these criteria, modern intelligence tests are highly reliable. In fact, intelligence tests are the most reliable of all psychological tests.
Validity is the extent to which a test predicts what it is designed to predict. Intelligence tests were designed to predict school achievement, and they do that better than they do anything else. For example, IQ scores of elementary school students correlate moderately with their class grades and highly with achievement test scores. IQ tests also predict well the number of years of education that a person completes. The SAT is somewhat less predictive of academic performance in college. Educators note that success in school depends on many other factors besides intelligence, including encouragement from parents and peers, interest, and motivation.
Intelligence tests also correlate with measures of accomplishment other than academic success, such as occupational status, income, job performance, and other measures of vocational success. However, IQ scores do not predict occupational success as well as they predict academic success. Twenty-five percent or less of the individual differences in occupational success are due to IQ. Therefore, a substantial portion of the variability in occupational success—75 percent or more—is due to factors other than intelligence.
Validity also refers to the degree to which a test measures what it is supposed to measure. A valid intelligence test should measure intelligence and not some other capability. However, making a valid intelligence test is not a straightforward task because there is little consensus on a precise definition of intelligence. Lacking such a consensus, test makers usually evaluate validity by determining whether test performance correlates with performance on some other measure assumed to require intelligence, such as achievement in school.
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