So far, we have considered how our species thinks and communicates. But we differ from one another in these abilities. School boards, courts, and scientists debate the use and fairness of tests that assess people’s mental abilities and assign them a score. In psychology, no controversy has been more heated than the question of whether there exists in each person a general intellectual capacity that can be measured and quantified as a number.

In this section, we consider some findings from a century of research, as psychologists have searched for answers to these questions and more:

9-13 How do psychologists define intelligence, and what are the arguments for g?

In many studies, intelligence has been defined as whatever intelligence tests measure, which has tended to be school smarts. But intelligence is not a quality like height or weight, which has the same meaning to everyone worldwide. People assign the term intelligence to the qualities that enable success in their own time and culture (Sternberg & Kaufman, 1998). In Cameroon’s equatorial forest, intelligence may reflect understanding the medicinal qualities of local plants. In a North American high school, it may reflect mastering difficult concepts in tough courses. In both places, intelligence is the mental potential to learn from experience, solve problems, and use knowledge to adapt to new situations.

You probably know some people with talents in science, others who excel in the humanities, and still others gifted in athletics, art, music, or dance. You may also know a talented artist who is stumped by the simplest math problem, or a brilliant math student who struggles when discussing literature. Are all these people intelligent? Could you rate their intelligence on a single scale? Or would you need several different scales? Simply put, is intelligence a single overall ability or several specific abilities?

Spearman’s General Intelligence Factor

Charles Spearman (1863-1945) believed we have one general intelligence (often shortened to g) that is at the heart of all of our intelligent behavior, from navigating the sea to excelling in school. He granted that people often have special, outstanding abilities. But he noted that those who score high in one area, such as verbal intelligence, typically score higher than average in other areas, such as spatial or reasoning ability.

Spearman’s belief stemmed in part from his work with factor analysis, a statistical procedure that identifies clusters of related items. In this view, mental abilities are much like physical abilities: The ability to run fast is distinct from the eye-hand coordination required to throw a ball on target. Yet there remains some tendency for good things to come packaged together—for running speed and throwing accuracy to correlate. In both athleticism and intelligence, several distinct abilities tend to cluster together and to correlate enough to define a general underlying factor. Distinct brain networks enable distinct abilities, with g explained by their coordinated activity (Hampshire et al., 2012).

Theories of Multiple Intelligences

9-14 How do Gardner’s and Sternberg’s theories of multiple intelligences differ, and what criticisms have they faced?

Other psychologists, particularly since the mid-1980s, have sought to extend the definition of intelligence beyond the idea of academic smarts.

GARDNER’S MULTIPLE INTELLIGENCES Howard Gardner has identified eight relatively independent intelligences, including the verbal and mathematical aptitudes assessed by standardized tests (FIGURE 9.13 below). Thus, the computer programmer, the poet, the street-smart adolescent, and the basketball team’s play-making point guard exhibit different kinds of intelligence (Gardner, 1998). Gardner (1999) has also proposed a ninth possible intelligence—existential intelligence—the ability “to ponder large questions about life, death, existence.”

Gardner (1983, 2006, 2011; Davis et al., 2011) views these intelligence domains as multiple abilities that come in different packages. Brain damage, for example, may destroy one ability but leave others intact. And consider people with savant syndrome. Despite their island of brilliance, these people often score low on intelligence tests and may have limited or no language ability (Treffert & Wallace, 2002). Some can compute complicated calculations quickly and accurately, or identify almost instantly the day of the week corresponding to any historical date, or render incredible works of art or musical performance (Miller, 1999).

About four in five people with savant syndrome are males. Many also have autism spectrum disorder (ASD), a developmental disorder. The late memory whiz Kim Peek (who did not have ASD) inspired the movie Rain Man. In 8 to 10 seconds, he could read and remember a page. During his lifetime, he memorized 9000 books, including Shakespeare’s works and the Bible. He could provide GPS-like travel directions within any major U.S. city. Yet he could not button his clothes, and he had little capacity for abstract concepts. Asked by his father at a restaurant to lower his voice, he slid down in his chair to lower his voice box. Asked for Lincoln’s Gettysburg Address, he responded, “227 North West Front Street. But he only stayed there one night—he gave the speech the next day” (Treffert & Christensen, 2005).

STERNBERG’S THREE INTELLIGENCES Robert Sternberg (1985, 2011) agrees with Gardner that there is more to success than traditional intelligence and that we have multiple intelligences. But his triarchic theory proposes three, not eight or nine, intelligences:

Gardner and Sternberg differ in some areas, but they agree on two important points: Multiple abilities can contribute to life success, and varieties of giftedness bring spice to life and challenges for education. As a result of this research, many teachers have been trained to appreciate such variety and to apply multiple intelligence theories in their classrooms.

CRITICISMS OF MULTIPLE INTELLIGENCE THEORIES Wouldn’t it be wonderful if the world were so just that a weakness in one area would be compensated by genius in another? Alas, say critics, the world is not just (Ferguson, 2009; Scarr, 1989). Research using factor analysis has confirmed that there is a general intelligence factor (Johnson et al., 2008): g matters. It predicts performance on various complex tasks and in various jobs (Gottfredson, 2002a,b, 2003a,b; see also FIGURE 9.14). Much as jumping ability is not a predictor of jumping performance when the bar is set a foot off the ground—but becomes a predictor when the bar is set higher—so extremely high cognitive ability scores predict exceptional attainments, such as doctoral degrees and publications (Kuncel & Hezlett, 2010).

Even so, “success” is not a one-ingredient recipe. High intelligence may help you get into a profession (via the schools and training programs that take you there), but it won’t make you successful once there. Success is a combination of talent and grit: Those who become highly successful tend also to be conscientious, well connected, and doggedly energetic. Researchers report a 10-year rule: A common ingredient of expert performance in chess, dancing, sports, computer programming, music, and medicine is “about 10 years of intense, daily practice” (Ericsson, 2002, 2007; Simon & Chase, 1973). Becoming a professional musician, for example, requires native ability. But it also requires practice—about 11,000 hours on average, and a minimum of 3000 hours (Campitelli & Gobet, 2011). The recipe for success is a gift of nature plus a whole lot of nurture.

Emotional Intelligence

9-15 What are the four components of emotional intelligence?

Is being in tune with yourself and others also a sign of intelligence, distinct from academic intelligence? Some researchers say Yes. They define social intelligence as the know-how involved in social situations and managing yourself successfully (Cantor & Kihlstrom, 1987). People with high social intelligence can read social situations the way a skilled soccer player reads the defense or a meteorologist reads the weather. The concept was first proposed in 1920 by psychologist Edward Thorndike, who noted, “The best mechanic in a factory may fail as a foreman for lack of social intelligence” (Goleman, 2006, p. 83).

One line of research has explored a specific aspect of social intelligence called emotional intelligence, consisting of four abilities (Mayer et al., 2002, 2011, 2012):

Emotionally intelligent people are both socially aware and self-aware. Those who score high on managing emotions enjoy higher-quality interactions with friends (Lopes et al., 2004). They avoid being hijacked by overwhelming depression, anxiety, or anger. They can read others’ emotional cues and know what to say to soothe a grieving friend, encourage a workmate, and manage a conflict. They can delay gratification in pursuit of long-range rewards, rather than being overtaken by immediate impulses. They often succeed in career, marriage, and parenting situations where academically smarter (but emotionally less intelligent) people might fail (Cherniss, 2001a,b; Ciarrochi et al., 2006). Emotionally intelligent people also perform modestly better on the job (O’Boyle et al., 2011).

Some scholars, however, are concerned that emotional intelligence stretches the intelligence concept too far (Visser et al., 2006). Howard Gardner (1999) includes interpersonal and intrapersonal intelligences as two of his multiple intelligences. But let us instead, he offers, respect emotional sensitivity, creativity, and motivation as important but different. Stretch intelligence to include everything we prize and the word will lose its meaning.

* * *

For a summary of these theories of intelligence, see TABLE 9.2.

9-16 What is an intelligence test, and what is the difference between achievement and aptitude tests?

An intelligence test assesses people’s mental abilities and compares them with others, using numerical scores. Psychologists classify such tests as either aptitude tests, intended to predict your ability to learn a new skill, or achievement tests, intended to reflect what you have already learned. How do we design such tests, and what makes them credible? Consider why psychologists created tests of mental abilities and how they have used them.

What Do Intelligence Tests Test?

9-17 When and why were intelligence tests created, and how do today’s tests differ from early intelligence tests?

Barely a century ago, psychologists began designing tests to assess people’s abilities. Some measured aptitude (ability to learn). Others assessed achievement (what people have already learned).

ALFRED BINET: PREDICTING SCHOOL ACHIEVEMENT Modern intelligence testing traces its birth to early-twentieth-century France, where a new law required all children to attend school. French officials knew that some children, including many newcomers to Paris, would struggle and need special classes. But how could the schools make fair judgments about children’s learning potential? Teachers might assess children who had little prior education as slow learners. Or they might sort children into classes on the basis of their social backgrounds. To minimize such bias, France’s minister of public education gave Alfred Binet (1857-1911) the task of solving this problem.

In 1905, Binet and his student, Théodore Simon, first presented their work under the archaic title, “New Methods for Diagnosing the Idiot, the Imbecile, and the Moron” (Nicolas & Levine, 2012). They began by assuming that all children follow the same course of intellectual development, but that some develop more rapidly. A “dull” child should score much like a typical younger child, and a “bright” child like a typical older child. Binet and Simon now had a clear goal: They would measure each child’s mental age, the level of performance typically associated with a certain chronological age. The average 8-year-old, for example, has a mental age of 8. An 8-year-old with a below-average mental age (perhaps performing at the level of a typical 6-year-old) would struggle with schoolwork considered normal for 8-year-olds.

Binet and Simon tested a variety of reasoning and problem-solving questions on Binet’s two daughters, and then on “bright” and “backward” Parisian schoolchildren. Items that the successful students more often answered correctly could then be used to predict how well other French children would handle their schoolwork. Binet hoped his test would be used to improve children’s education, but he also feared it would be used to label children and limit their opportunities (Gould, 1981).

LEWIS TERMAN: THE INNATE IQ Binet’s fears were realized soon after his death in 1911, when others adapted his tests for use as a numerical measure of inherited intelligence. Stanford University professor Lewis Terman (1877-1956) found that the Paris-developed questions and age norms worked poorly with California schoolchildren. He adapted some items, added others, and established new standards for various ages. He also extended the upper end of the test’s range from teenagers to “superior adults” and gave his revision the name it retains today—the Stanford-Binet. Terman assumed that certain ethnic groups were naturally more intelligent, and he supported the controversial eugenics movement, which aimed to protect and improve human genetic quality through selective sterilization and breeding.

From such tests, German psychologist William Stern derived the famous intelligence quotient, or IQ. The IQ was simply a person’s mental age divided by chronological age and multiplied by 100 to get rid of the decimal point. Thus, an average child, whose mental age (8) and chronological age (8) are the same, has an IQ of 100. But an 8-year-old who answers questions at the level of a typical 10-year-old has an IQ of 125:

The original IQ formula worked fairly well for children but not for adults. (Should a 40-year-old who does as well on the test as an average 20-year-old be assigned an IQ of only 50?) Most current intelligence tests, including the Stanford-Binet, no longer compute an IQ in this manner (though the term IQ still lingers in everyday vocabulary as shorthand for “intelligence test score”). Instead, they represent the test-taker’s performance relative to the average performance of others the same age. This average performance is arbitrarily assigned a score of 100, and about two-thirds of all test-takers fall between 85 and 115.

DAVID WECHSLER: SEPARATE SCORES FOR SEPARATE SKILLS Psychologist David Wechsler created what is now the most widely used individual intelligence test, the Wechsler Adult Intelligence Scale (WAIS). There is a version for school-age children (the Wechsler Intelligence Scale for Children [WISC]), and another for preschool children (Evers et al., 2012). The WAIS (2008) edition consists of 15 subtests, including:

The WAIS yields both an overall intelligence score and individual scores for verbal comprehension, perceptual organization, working memory, and processing speed. Striking differences among these individual scores can provide clues to cognitive strengths or weaknesses. For example, a low verbal comprehension score combined with high scores on other subtests could indicate a reading or language disability. Other comparisons can help a therapist establish a rehabilitation plan for a stroke patient. In such ways, these tests help realize Binet’s aim: to identify opportunities for improvement and strengths that teachers and others can build upon.

Three Tests of a “Good” Test

9-18 What is a normal curve, and what does it mean to say that a test has been standardized and is reliable and valid?

To be widely accepted, a psychological test must be standardized, reliable, and valid. The Stanford-Binet and Wechsler tests meet these requirements.

WAS THE TEST STANDARDIZED? The number of questions you answer correctly on an intelligence test would reveal almost nothing. To know how well you performed, you would need some basis for comparison. That’s why test-makers give new tests to a representative sample of people. The scores from this pretested group become the basis for future comparisons. If you later take the test following the same procedures, your score will be meaningful when compared with others. This process is called standardization.

If we construct a graph of test-takers’ scores, the scores typically form a bell-shaped pattern called the normal curve. No matter what attributes we measure—height, weight, or mental aptitude—people’s scores tend to form a bell curve. The highest point is the midpoint, or the average score. On an intelligence test, we give this average score a value of 100 (FIGURE 9.15). Moving out from the average, toward either extreme, we find fewer and fewer people. For the Stanford-Binet and Wechsler tests, a person’s score indicates whether that person’s performance fell above or below the average. A performance higher than all but 2 percent of all scores earns an intelligence score of 130. A performance lower than 98 percent of all scores earns an intelligence score of 70.

IS THE TEST RELIABLE? Knowing your score in comparison to the standardization group still won’t tell you much unless the test has reliability. A reliable test gives consistent scores, no matter who takes the test or when they take it. To check a test’s reliability, researchers test people many times. They may retest people using the same test, or they may split the test in half and see whether odd-question scores and even-question scores agree. If the two sets of scores generally agree, or correlate, the test is reliable. The higher the correlation, the higher the test’s reliability. The tests we have considered so far—the Stanford-Binet, the WAIS, and the WISC—are, after early childhood, very reliable (about +.9). When retested, people’s scores generally match their first score closely.

IS THE TEST VALID? High reliability does not ensure a test’s validity—the extent to which the test actually measures or predicts what it promises. Imagine using a miscalibrated tape measure to measure people’s heights. Your results would be very reliable. No matter how many times you measured, people’s heights would be the same. But your results would not be valid, because you would not be giving the information you promised: real height.

Tests that tap the pertinent behavior, or criterion, have content validity. The road test for a driver’s license has content validity because it samples the tasks a driver routinely faces. Course exams have content validity if they assess your mastery of course material. But we expect intelligence tests to have predictive validity: They should predict future performance, and to some extent they do.

The predictive power of aptitude tests is fairly strong in the early school years, but later it weakens. Past grades, which reflect both aptitude and motivation, are better predictors of future achievements.

Aging and Intelligence

9-19 How does aging affect crystallized and fluid intelligence?

Does intelligence increase, decrease, or remain constant as we age? The answer depends on the type of intellectual performance we measure:

How do we know? Developmental psychologists use longitudinal studies (restudying the same group at different times across their life span) and cross-sectional studies (comparing members of different age groups at the same time) to study the way intelligence and other traits change with age. (See Appendix A for more information.) With age we lose and we win. We lose recall memory and processing speed, but we gain vocabulary and knowledge (FIGURE 9.16 below). Fluid intelligence may decline, but older adults’ social reasoning skills increase, as shown by an ability to take multiple perspectives, to appreciate knowledge limits, and to offer helpful wisdom in times of social conflict (Grossman et al., 2010). Decisions also become less distorted by negative emotions such as anxiety, depression, and anger (Blanchard-Fields, 2007; Carstensen & Mikels, 2005).

Age-related cognitive differences help explain why older adults are less likely to embrace new technologies (Charness & Boot, 2009; Lenhart, 2015). These cognitive differences also help explain why mathematicians and scientists produce much of their most creative work during their late twenties or early thirties, when fluid intelligence is at its peak (Jones et al., 2014). In contrast, authors, historians, and philosophers tend to produce their best work in their forties, fifties, and beyond—after accumulating more knowledge (Simonton, 1988, 1990). Poets, for example, who depend on fluid intelligence, reach their peak output earlier than prose authors, who need the deeper knowledge reservoir that accumulates with age. This finding holds in every major literary tradition, for both living and dead languages.

Stability Over the Life Span

9-20 How stable are intelligence test scores over the life span?

What about the stability of early-life intelligence scores? For most children, casual observation and intelligence tests before age 3 only modestly predict their future aptitudes (Humphreys & Davey, 1988; Tasbihsazan et al., 2003). Even Albert Einstein was once thought “slow”—as he was in learning to talk (Quasha, 1980).

By age 4, however, children’s performance on intelligence tests begins to predict their adolescent and adult scores. The consistency of scores over time increases with the age of the child. By age 11, the stability becomes impressive, as Ian Deary and his colleagues (2004, 2009, 2013) discovered. Their amazing longitudinal studies have been enabled by their country, Scotland, which did something that no nation has done before or since. On June 1, 1932, essentially every child in the country born in 1921—87,498 children around age 11—took an intelligence test. The aim was to identify working-class children who would benefit from further education. Sixty-five years later to the day, Patricia Whalley, the wife of Deary’s co-worker, Lawrence Whalley, discovered the test results on dusty storeroom shelves at the Scottish Council for Research in Education, not far from Deary’s Edinburgh University office. “This will change our lives,” Deary replied when Whalley told him the news.

And so it has, with dozens of studies of the stability and the predictive capacity of these early test results. For example, when the intelligence test administered to 11-year-old Scots in 1932 was readministered to 542 survivors as turn-of-the-millennium 80-year-olds, the correlation between the two sets of scores—after nearly 70 years of varied life experiences—was striking (FIGURE 9.17). Ditto when 106 survivors were retested at age 90 (Deary et al., 2013). Another study that followed Scots born in 1936 from ages 11 to 70 confirmed the remarkable stability of intelligence, independent of life circumstance (Johnson et al., 2010).

Children and adults who are more intelligent also tend to live healthier and longer lives. Why might this be the case? Deary (2008) proposes four possible explanations:

Extremes of Intelligence

9-21 What are the traits of those at the low and high intelligence extremes?

One way to glimpse the validity and significance of any test is to compare people who score at the two extremes of the normal curve. The two groups should differ noticeably, and with intelligence testing, they do.

THE LOW EXTREME At one extreme of the intelligence test normal curve are those with unusually low scores. The American Association on Intellectual and Developmental Disabilities guidelines list two criteria for a diagnosis of intellectual disability (formerly referred to as mental retardation):

Intellectual disability is a developmental condition that is apparent before age 18, sometimes with a known physical cause. Down syndrome, for example, is a disorder of varying intellectual and physical severity caused by an extra copy of chromosome 21.

People diagnosed with a mild intellectual disability—those just below the 70 score—might be better able to live independently today than many decades ago, when they were institutionalized. The tests have been periodically restandardized. As that happened, individuals who scored near 70 on earlier tests have suddenly lost about 6 test-score points. Two people with the same ability level could thus be classified differently, depending on when they were tested (Kanaya et al., 2003; Reynolds et al., 2010). As the intellectual-disability boundary has shifted, more people have become eligible for special education and for Social Security payments. And in the United States (one of only a few industrialized countries with the death penalty), fewer people are now eligible for execution: The U.S. Supreme Court ruled in 2002 that the execution of people with an intellectual disability is “cruel and unusual punishment.” For people near that cutoff score of 70, intelligence testing can be a high-stakes competition. And so it was for Teresa Lewis, a “dependent personality” with limited intellect, who was executed by the state of Virginia in 2010. Lewis, whose reported test score was 72, allegedly agreed to a plot in which two men killed her husband and stepson in exchange for a split of a life insurance payout (Eckholm, 2010). If only she had scored 69.

THE HIGH EXTREME In one famous project begun in 1921, Lewis Terman studied more than 1500 California schoolchildren with IQ scores over 135. Terman’s high-scoring children (the “Termites”) were healthy, well adjusted, and unusually successful academically (Friedman & Martin, 2012; Koenen et al., 2009; Lubinski, 2009a). When restudied over the next seven decades, most had attained high levels of education (Austin et al., 2002; Holahan & Sears, 1995). Many were doctors, lawyers, professors, scientists, and writers, though no Nobel Prize winners. (The two future physics Nobel laureates Terman tested failed to score above his gifted-sample cutoff [Hulbert, 2005].)

Recent studies have followed the lives of precocious youths who had aced the math SAT at age 13—by scoring in the top quarter of 1 percent of their age group. By their fifties, these 1650 math whizzes had secured 681 patents (Lubinski et al., 2014). Compared with the math aces, 13-year-olds scoring high on verbal aptitude were, by age 38, more likely to have become humanities professors or written a novel (Kell et al., 2013). About 1 percent of Americans earn doctorates. But among those scoring in the top 1 in 10,000 on the SAT at age 12 or 13, 63 percent had done so.

One of psychology’s whiz kids was Jean Piaget, who by age 15 was publishing scientific articles on mollusks and who went on to become the twentieth century’s most famous developmental psychologist (Hunt, 1993). Children with extraordinary academic gifts are sometimes more isolated, shy, and in their own worlds (Winner, 2000). But most thrive.

Learning Objectives

Test Yourself by taking a moment to answer each of these Learning Objective Questions (repeated here from within the chapter). Research suggests that trying to answer these questions on your own will improve your long-term memory of the concepts (McDaniel et al., 2009).

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Terms and Concepts to Remember

Test yourself on these terms.

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Experience the Testing Effect

Test yourself repeatedly throughout your studies. This will not only help you figure out what you know and don’t know; the testing itself will help you learn and remember the information more effectively thanks to the testing effect.

Question 9.13

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Use to create your personalized study plan, which will direct you to the resources that will help you most in .