Intelligence - review

Intelligence refers to individual differences in cognitive abilities, such as the capacity to reason, to solve problems, to think abstractly, to understand new material, and to learn.

Intelligence has been defined in many different ways. In this section we use the word to refer to individual differences in cognitive abilities, such as the capacity to reason, to solve problems, to think abstractly, to understand new material, and to learn from past experiences. By this definition, we are not including wisdom, creativity, or acceptance of the moral standards of one’s society. The term intelligence quotient (IQ) refers to a way of comparing one individual’s or group’s intelligence with the average on specific tests of cognitive intelligence. Historically, there has been a furious debate over whether intelligence is a single thing or reflects a cluster of specialized abilities. After a century of controversy on this topic, the answer is becoming clearer. Evaluations of cognitive ability virtually always have a positive correlation, meaning that an individual’s scores on a variety of cognitive tests usually cluster around the same measurement. This and other evidence suggest that there is an underlying dimension of mental competence, a “general intelligence,” that is reflected in a wide number of cognitive activities. There are two important qualifications to this statement. First, visual-spatial reasoning, which is the ability to solve problems that involve manipulating visual representations and understanding the space around one, varies somewhat separately from verbal and logical problem solving. Second, the degree to which cognitive performance reflects separate abilities or a single general intelligence depends on the level of the performance. With some exceptions, low levels of mental performance are almost always pervasive: people who have very low scores on a verbal test are likely to have similarly low scores on a nonverbal one. High levels of performance tend to be much more differentiated; very high performance on a verbal test is not necessarily indicative of a similarly high level of performance on a nonverbal test, although high scores in one area virtually never indicate low scores in another. Testing Intelligence The tests for cognitive competence range from brief evaluations of reading samples to the solving of abstract puzzles. Examples are the Wechsler Adult Intelligence Scale, which is the most widely used individually administered intelligence test, the Raven Progressive Matrices Tests, which are commonly used to evaluate intelligence without relying on language, and personnel screening examinations, such as the Scholastic Aptitude Test (SAT), which is required by many colleges and universities, and the Armed Services Vocational Aptitude Battery (ASVAB). Most testing is done in schools, especially to predict success in higher education, but businesses, the government, and the military also use tests of cognitive competence on occasion. Generally, people’s results on these tests correlate with a number of measures of success in life, such as academic or business achievement and the resulting improved salary and rank. In fact, in what we might consider the ultimate test, studies have found that children with high intelligence-test scores grow up to have lower mortality rates than those with low test scores and that the advantage extends into late adulthood. Of course, not every individual fits into this general pattern. Mental test scores are not as useful in determining a person’s ability to do a specific task, because intelligence is not the same as competence in solving problems within a specialized occupation or situation. Experience is a major factor in establishing competence, and test scores are likely to underestimate how easily an individual can work in a familiar setting. Studies of expertise have shown that within-field competence is very much affected by practice. Thus there are strong trade-offs between initial competence and amount of training. On the other hand, mental test scores do predict the time it will take to achieve a given level of expertise. This has been demonstrated in studies ranging from university course work (the correlation between grades and test scores, and the correlation between test scores and graduation rates) to studies of on-the-job performance by military enlisted personnel. Test scores are affected by age. After reaching a peak in early adulthood (age 20 to 35), test scores show a gradual decline until the mid-60s, and sharp declines thereafter. Again, there are very important qualifications to this statement. The first is that evaluations of mental competence that permit a person to use his or her knowledge show much less change than tests that require the solution of new or unusual problems. Second, verbal measures are more resistant to age-related decline than nonverbal measures. Finally, there are very large individual differences in the agerelated decline. The top scores of people in their 60s and 70s are only slightly below the top scores of people in their 20s. The bottom scores show major declines with age. It is not clear what contributes to the increase in variance with age, although changes in health and personal lifestyle do make a contribution. With one exception, there are absolutely no differences between men and women on intelligence tests. The exception is visual-spatial problems that require reasoning about imagined or visual motion. In this case men will, on the average, score higher than women. The Biology of Intelligence Francis Galton, who did important work on intelligence in the nineteenth century, believed that differences in general mental competence were due to individual differences in the efficiency of the nervous system as a signaling device. According to modern studies, Galton was right, in principle. We know that measures of cognitive competence are related to a person’s ability to keep track of several things at once and to focus on the aspects of a situation relevant to the problem that needs solving. These abilities are generally subsumed under the title “working memory.” Other work has related intelligence to the speed with which a person can access information in his or her long-term memory. These findings suggest that indicators of intelligence may in part reflect individual differences in brain functioning. Behavioral genetics studies provide further evidence that this is so. A great many studies, using a variety of designs and tests, have shown that IQ scores, especially for measures of general intelligence, have a hereditary component. In fact, genetic inheritance is a larger influence on intelligence, at the population level, than any known social or physical environmental influence. More recent quantitative genetics studies indicate that intelligence is partly a multigenetic trait. Some individual genes have been implicated in the development of intelligence, but a definitive list of the genes for intelligence has not yet been obtained. Given this evidence, it is natural to ask just what structures in the brain might be related to intelligence. Over the years a number of investigators have asked whether or not intelligence was related to overall brain size. This resulted in considerable debate, caused in part by our inability to measure the size of the live brain. There are major problems with measurement and interpretation of data based on pathological or even archaeological analysis, or on such measures as skull size (which Galton used in his studies). Modern medical imaging techniques have resolved some of these questions. There is a reliable positive correlation between brain size and test score, but the closeness of the correlation is in question because brain imaging technology is too expensive for studies of large groups. A more useful approach than correlating test scores and brain size is to examine the role that specific parts of the brain play in establishing intelligence. Here imaging techniques are extremely helpful, and we have learned a good deal by analyzing the behavior of brain-damaged patients. These studies have implicated three areas of the cortex. The dorsolateral frontal cortex is selectively activated when people attempt tasks that are good indicators of general intelligence, such as the Raven matrices tests mentioned earlier. Visual reasoning tasks activate many of the areas of the brain associated with visual perception. Finally, the hippocampus is involved in committing information to long-term memory, a vital building block for intelligence, and is also involved in the ability to reason about geographic routes. Influences on Intelligence Many disorders can produce losses in intelligence. A variety of genetic disorders, as well as perinatal disorders, which occur primarily five months before and one month after birth, produce profound loss of mental competence. These include genetic abnormalities, such as Down’s syndrome, Turner’s syndrome (which is one of the few syndromes that produces a specific effect, in this case loss of spatial-visual reasoning), and Williams syndrome. Following birth, virtually any injury to the brain may produce a drop in intelligence. Situations that produce prolonged coma, such as severe closed head injury, are particularly dangerous. Profound losses of intelligence are associated with some diseases that involve neural degeneration. Alzheimer’s disease is one of the most common and best publicized, but a similar loss of reasoning ability is often associated with the latter stages of other diseases, such as Pick’s disease, dementia with Lewy bodies, and frontotemporal dementia, that affect the central nervous system. Certain specific environmental and lifestyle influences can produce smaller effects on intelligence. By far the most common of these is alcoholism. In adults, high alcohol consumption is generally associated with lowered intelligence-test scores, but whether this is a cause-and-effect relationship is difficult to discern. However, alcoholism carried to the point of unconsciousness is clearly not a good idea, both for its effect on intelligence and for many other reasons. Heavy alcohol consumption by pregnant women may produce fetal alcohol syndrome in the child. Environmental toxins, especially atmospheric lead, have also been shown to have deleterious effects on intelligence, but these effects are so small that they only appear in fairly large studies. There is some evidence that social withdrawal and general lack of intellectual engagement can produce lowered test scores in adults. For instance, people who read widely appear to be considerably better problem solvers than people who do not. But while it would be nice to think that maintaining an active intellectual life protects one from the general decline associated with advanced age, the cause-and-effect relationships are very hard to untangle. Dynamic Intelligence Because intelligence appears to have a substantial genetic component, there has been a popular tendency to regard intelligence as fixed, but this does not follow, either logically or in fact. Test scores of mental performance that approximate samples of everyday cognitive behavior, such as the Scholastic Aptitude Test, virtually all respond to education. In fact, test scores in industrialized countries rose steadily from 1925 through 2000. The reason is not clear, but it is hard to believe that near-universal public education was not partly responsible. In addition, studies have found that individuals in the bottom 20 percent of the population, the below-normal to educable-retarded group, can often be productive citizens if they live in a wellstructured environment with orchestrated social support. It is extremely unlikely that an “intelligence pill” will be discovered in the near future. On the other hand, it is conceivable that researchers will discover therapies for some of the major causes of abnormally low intelligence, including the neural degeneration associated with Alzheimer’s disease and changes in brain structure or function that are associated with the genetic mental diseases. If you wish to maintain your intelligence until great old age, there are really only two pieces of advice: ■ Maintain an active intellectual life. ■ Avoid things that can injure the brain. In particular, do not drink large doses of alcohol, abuse drugs, or expose yourself to situations that are likely to result in brain injury, ranging from reckless driving to unnecessary exposure to environmental toxins. We cannot do much about our genes, or our vulnerability to other people’s actions, but we can increase our chances of enjoying our intelligence for a long time by using that intelligence to avoid needless risks.