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KEY THEME

Psychologists do not agree about the basic nature of intelligence, including whether it is a single, general ability and whether it includes skills and talents as well as mental aptitude.

KEY QUESTIONS

The Wechsler Adult Intelligence Scale and the Stanford–Binet Intelligence Scale are standardized, reliable, and valid. But do they adequately measure intelligence? The question is not as simple as it sounds. There is considerable disagreement among psychologists about the nature of intelligence, including how intelligence should best be defined and measured (Gardner, 2011; Neisser & others, 1996).

Take another look at the chapter Prologue about Tom and his family. In terms of the type of intelligence that is measured by IQ and other standardized tests, Tom is highly intelligent. But despite his high IQ, Tom can find it extremely difficult to carry out many activities that those with a more “normal” or average intelligence can perform almost effortlessly. Applying for an after-school job, joining a conversation that is already in progress, or even knowing why—or whether—a particular joke is funny are very difficult, if not impossible, tasks for Tom. As you’ll see in the next section, psychologists have struggled with the challenge of how to define intelligence for over a century.

Much of the controversy over the definition of intelligence centers on two key issues. First, is intelligence a single, general ability, or is it better described as a cluster of different mental abilities? And second, should the definition of intelligence be restricted to the mental abilities measured by IQ and other intelligence tests? Or should intelligence be defined more broadly?

Although these issues have been debated for more than a century, they are far from being resolved. In this section, we’ll describe the views of four influential psychologists on both issues.

CHARLES SPEARMAN

INTELLIGENCE IS A GENERAL ABILITY

Some psychologists believe that a common factor, or general mental capacity, is at the core of different mental abilities. This approach originated with British psychologist Charles Spearman. Although Spearman agreed that an individual’s scores could vary on tests of different mental abilities, he found that the scores on different tests tended to be similar. That is, people who did well or poorly on a test of one mental ability, such as verbal ability, tended also to do well or poorly on the other tests.

Spearman recognized that particular individuals might excel in specific areas. However, Spearman (1904) believed that a factor he called general intelligence, or the g factor, was responsible for their overall performance on tests of mental ability. Psychologists who follow this approach today think that intelligence can be described as a single measure of general cognitive ability, or g factor (Gottfredson, 1998). Thus, general mental ability could accurately be expressed by a single number, such as the IQ score. Lewis Terman’s approach to measuring and defining intelligence as a single, overall IQ score was in the tradition of Charles Spearman. In terms of Spearman’s model, Tom would undoubtedly score very highly on any test that measured g factor or general intelligence. For example, when he took the SAT as a high school junior, he scored in the top 3 percent in math and received a perfect score on the writing section.

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LOUIS L. THURSTONE

NTELLIGENCE IS A CLUSTER OF ABILITIES

Psychologist Louis L. Thurstone disagreed with Spearman’s notion that intelligence is a single, general mental capacity. Instead, Thurstone believed that there were seven different “primary mental abilities,” each a relatively independent element of intelligence. Verbal comprehension, numerical ability, reasoning, and perceptual speed are examples Thurstone gave of independent “primary mental abilities.”

To Thurstone, the so-called g factor was simply an overall average score of such independent abilities and consequently was less important than an individual’s specific pattern of mental abilities (Thurstone, 1937). David Wechsler’s approach to measuring and defining intelligence as a pattern of different abilities was very similar to Thurstone’s approach.

HOWARD GARDNER

“MULTIPLE INTELLIGENCES”

More recently, Howard Gardner has expanded Thurstone’s basic notion of intelligence as different mental abilities that operate independently. However, Gardner has stretched the definition of intelligence (Gardner & Taub, 1999). Rather than analyzing intelligence test results, Gardner (1985, 1993) looked at the kinds of skills and products that are valued in different cultures. He also studied individuals with brain damage, noting that some mental abilities are spared when others are lost. To Gardner, this phenomenon implies that different mental abilities are biologically distinct and controlled by different parts of the brain.

Like Thurstone, Gardner has suggested that such mental abilities are independent of each other and cannot be accurately reflected in a single measure of intelligence. Rather than one intelligence, Gardner (1993, 1998a) believes there are “multiple intelligences.” To Gardner, “an intelligence” is the ability to solve problems, or to create products, that are valued within one or more cultural settings. Thus, he believes that intelligence must be defined within the context of a particular culture (1998b, 2011). Gardner’s theory includes eight distinct, independent intelligences, which are summarized in Figure 7.7. Gardner (2011) has identified a ninth possible intelligence, called existential intelligence, but is awaiting further research before formalizing it as a ninth intelligence. Existential intelligence, or spiritual intelligence, refers to the capacity to reflect upon “ultimate issues,” such as the meaning of life.

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According to Gardner’s model of intelligence, everyone has a different pattern of strengths and weaknesses. How would Tom rate? In terms of linguistic, logical-mathematical, and spatial intelligence, Tom would measure very highly. What about an “intelligence” that wouldn’t be measured by a standard IQ test, such as musical intelligence? When I asked Tom what type of music he liked, he couldn’t tell me.

If I like a song, I like it.

If I don’t, I stop listening.

I really can’t categorize it.

It’s not within my power.

Although lacking musical intelligence probably doesn’t affect your ability to function in our culture, other “intelligences” are much more crucial. Without question, Tom’s biggest shortcoming falls in the realm of interpersonal intelligence. Understanding and relating to other people is extremely difficult for him.

In most cultures, the ability to effectively navigate social situations is crucial to at least one aspect of Wechsler’s definition of intelligence: “the ability to deal effectively with the environment.” Although people with mild symptoms of autism spectrum disorder may not be intellectually disabled, their inability to successfully interact with others leads to even mild forms of autism spectrum disorder being labeled as a disability. However, not everyone agrees with this characterization. Is autism spectrum disorder a disability—or is it a difference? We consider this topic in the In Focus box “Neurodiversity: Beyond IQ,” on page 298.

Some of the abilities emphasized by Gardner, such as logical-mathematical intelligence, might be tapped by a standard intelligence test. However, other abilities, such as bodily-kinesthetic intelligence or musical intelligence, do not seem to be reflected on standard intelligence tests. Yet, as Gardner points out, such abilities are recognized and highly valued in many different cultures, including our own.

ROBERT STERNBERG

THREE FORMS OF INTELLIGENCE

Robert Sternberg (2012, 2014b) agrees with Gardner that intelligence is a much broader quality than is reflected in the narrow range of mental abilities measured by a conventional IQ test. However, Sternberg (1988, 1995) disagrees with Gardner’s notion of multiple, independent intelligences. He believes that some of Gardner’s intelligences are more accurately described as specialized talents, whereas intelligence is a more general quality. Sternberg (1988) points out that you would be able to manage just fine if you were tone-deaf and lacked “musical intelligence” in most societies. However, if you didn’t have the ability to reason and plan ahead, you would be unable to function in any culture.

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Sternberg’s triarchic theory of intelligence emphasizes both the universal aspects of intelligent behavior and the importance of adapting to a particular social and cultural environment. More specifically, Sternberg (1997, 2012) has proposed a different conception of intelligence, which he calls successful intelligence. Successful intelligence involves three distinct types of mental abilities: analytic, creative, and practical.

Analytic intelligence refers to the mental processes used in learning how to solve problems, such as picking a problem-solving strategy and applying it. Although conventional intelligence tests measure mental abilities, they do not evaluate the strategies used to solve problems, which Sternberg considers important in determining analytic intelligence. In the Prologue, Tom’s ability to solve complex mathematical equations reflects analytical intelligence.

Creative intelligence is the ability to deal with novel situations by drawing on existing skills and knowledge. The intelligent person effectively draws on past experiences to cope with new situations, which often involves finding an unusual way to relate old information to new. We’ll explore the topic of creativity in more detail in the Psych for Your Life section at the end of the chapter.

Practical intelligence involves the ability to adapt to the environment and often reflects what is commonly called “street smarts.” Sternberg notes that what is required to adapt successfully in one particular situation or culture may be very different from what is needed in another situation or culture. He stresses that the behaviors that reflect practical intelligence can vary depending on the particular situation, environment, or culture.

How would Tom fare by Sternberg’s criteria? Although Tom ranks high in most measures of analytic intelligence, Tom would probably not rank very high in creative or practical intelligence. Successful adaptation involves the flexibility to choose the best problem-solving strategy or to know when to change strategies.

What about practical intelligence? As shown in the Prologue, dealing with the social and academic environment of high school is not easy for Tom. However, many people with autism spectrum disorder find a compatible niche in workplaces in which their ability to focus their attention on a particular technical problem, their preoccupation with details, and other unusual abilities are highly valued, such as in computer programming or engineering firms. In such an environment, their lack of social skills might actually be advantageous (Mayor, 2008). Some companies have figured this out. For example, the German-based international company SAP has committed to hiring 650 people with autism—1 percent of all of their employees—by 2020 (Wang, 2014). They are clear that this is not a charitable initiative; rather, they have found that some people with autism are able to work very methodically and carefully, without zoning out like most of us do. This makes them excel at certain jobs, like debugging computer software. As Sternberg’s theory would predict, behaviors that are deemed “intelligent” in one environment—such as a software development firm like SAP—might well be maladaptive in another environment, like a high school cafeteria (Sternberg, 2008).

The exact nature of intelligence will no doubt be debated for some time. However, the intensity of this debate pales in comparison with the next issue we consider: the origins of intelligence.

KEY THEME

Both genes and environment contribute to intelligence, but the relationship is complex.

KEY QUESTIONS

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Given that psychologists do not agree on the definition or nature of intelligence, it probably won’t surprise you to learn that psychologists also do not agree on the origin of intelligence. On the surface, the debate comes down to this: Do we essentially inherit our intellectual potential from our parents, grandparents, and great-grandparents? Or is our intellectual potential primarily determined by our environment and upbringing?

Virtually all psychologists agree that both heredity and environment are important in determining intelligence level. Where psychologists disagree is in identifying how much of intelligence is determined by heredity and how much by environment. The implications of this debate have provoked some of the most heated arguments in the history of psychology.

Let’s start with some basic points about the relationship between genes and the environment. It was once believed that genes provided an unchanging, permanent blueprint for human development. As we’ll discuss in Chapter 9, the “genes as blueprint” metaphor has today been replaced by a “genes as data bank” metaphor (Marcus, 2004). It’s now known that environmental factors influence which of the many genes we inherit are “expressed,” meaning actually switched on, or activated. As psychologist Bernard Brown (1999) writes, “Genes are not destiny. There are many places along the gene–behavior pathway where genetic expression can be regulated.”

Take the example of height. You inherit a potential range for height, rather than an absolute number of inches. Environmental factors influence how close you come to realizing that genetic potential. If you are healthy and well-nourished, you may reach the maximum of your genetic height potential. But if you are poorly nourished and not healthy, you probably won’t.

To underscore the interplay between heredity and environment, consider the fact that the average height of Americans has increased by several inches in the past half-century. The explanation for this increase is that nutritional and health standards have steadily improved, not that the genetic heritage of Americans has fundamentally changed. However, heredity does play a role in establishing limits on height. If you’re born with “short” genes, you’re unlikely to reach six foot four, no matter how good your nutrition. (For more on genetics, see Chapter 9.)

The roles of heredity and environment in determining intelligence and personality factors are much more complex than the simple examples of height or eye color (Plomin, 2003; Johnson, 2010). However narrowly intelligence is defined, the genetic range of intellectual potential is influenced by many genes, not by one single gene (Plomin & Spinath, 2004; Nisbett & others, 2012). No one knows how many genes might be involved. Given the complexity of genetic and environmental influences, how do scientists estimate how much of intelligence is due to genetics and how much to environment?

TWIN STUDIES

SORTING OUT THE INFLUENCE OF GENETICS VERSUS ENVIRONMENT

One way this issue has been explored is by comparing the IQ scores of individuals who are genetically related to different degrees. Identical twins share exactly the same genes because they developed from a single fertilized egg that split into two. Hence, any dissimilarities between them must be due to environmental factors rather than hereditary differences. Fraternal twins are like any other pair of siblings because they develop from two different fertilized eggs.

As you can see in Figure 7.8 on the next page, comparing IQ scores in this way shows the effects of both heredity and environment. Identical twins raised together have very similar IQ scores, whereas fraternal twins raised together have IQs that are less similar (Plomin & Spinath, 2004).

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However, notice that identical twins raised in separate homes have IQs that are slightly less similar, indicating the effect of different environments. And, although fraternal twins raised together have less similar IQ scores than do identical twins, they show more similarity in IQs than do nontwin siblings. Recall that the degree of genetic relatedness between fraternal twins and nontwin siblings is essentially the same. But because fraternal twins are the same age, their environmental experiences are likely to be more similar than are those of siblings who are of different ages.

Thus, both genetic and environmental influences are important. Genetic influence is shown by the fact that the closer the genetic relationship, the more similar the IQ scores. Environmental influences are demonstrated by two findings: First, two people who are genetically identical but are raised in different homes have different IQ scores. And second, two people who are genetically unrelated but are raised in the same home have IQs that are much more similar than are those of two unrelated people from randomly selected homes.

Using studies based on degree of genetic relatedness and using sophisticated statistical techniques to analyze the data, researchers have scientifically estimated heritability—the percentage of variation within a given population that is due to heredity. The currently accepted heritability estimate is about 50 percent for the general population (see Plomin, 2003; Plomin & Spinath, 2004).

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In other words, approximately 50 percent of the difference in IQ scores within a given population is due to genetic factors. But there is disagreement even over this figure, depending on the statistical techniques, data sources used, and age and social class of the population (Deary & others, 2009; Nisbett & others, 2012).

It is important to stress that the 50 percent figure does not apply to a single individual’s IQ score. If Mike’s IQ is 120, it does not mean that 60 IQ points are due to Mike’s environment and 60 points are genetically inherited. Instead, the 50 percent heritability estimate means that approximately 50 percent of the difference in IQ scores within a specific group of people is due to differences in their genetic makeup. More on this key point shortly.

DIFFERENCES WITHIN GROUPS VERSUS DIFFERENCES BETWEEN GROUPS

Some group differences in average IQ scores do exist (Ceci & Williams, 2009). In many societies, including the United States, the average IQ scores of minority groups tend to be lower than the average IQ scores of the dominant or majority groups. There also are differences among countries when national estimates of intelligence are determined (Hunt, 2012). And countries with higher overall intelligence tend to have better economic and health outcomes. The question is how to explain such differences. Heritability cannot be used to explain group differences. Although it is possible to estimate the degree of difference within a specific group that is due to genetics, it makes no sense to apply this estimate to the differences between groups (Rose, 2009). Why? A classic analogy provided by geneticist Richard Lewontin (1970) may help you understand this important point (see Ceci & Williams, 2009).

Suppose you have a 50-pound bag of corn seeds and two pots. A handful of seeds is scooped out and planted in pot A, which has rich, well-fertilized soil. A second handful is scooped out and planted in pot B, which has poor soil with few nutrients (see Figure 7.9).

Because the seeds are not genetically identical, the plants within group A will vary in height. So will the plants within group B. Given that the environment (the soil) is the same for all the plants in one particular pot, this variation within each group of seeds is completely due to heredity—nothing differs but the plants’ genes.

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However, when we compare the average height of the corn plants in the two pots, pot A’s plants have a higher average height than pot B’s. Can the difference in these average heights be explained in terms of overall genetic differences between the seeds in each pot? No. The overall differences can be attributed to the two different environments, the good soil and the poor soil. In fact, because the environments are so different, it is impossible to estimate what the overall genetic differences are between the two groups of seeds.

Note also that even though, on the average, the plants in pot A are taller than the plants in pot B, some of the plants in pot B are taller than some of the plants in pot A. In other words, the average differences within a group of plants tell us nothing about whether an individual member of that group is likely to be tall or short.

The same point can be extended to the issue of average IQ differences between racial groups. Unless the environmental conditions of two racial groups are virtually identical, it is impossible to estimate the overall genetic differences between the two groups. Even if intelligence were primarily determined by heredity, which is not the case, IQ differences between groups could still be due entirely to the environment.

Other evidence for the importance of the environment in determining IQ scores derives from the improvement in average IQ scores that has occurred in several cultures and countries during the past few generations, a phenomenon called the Flynn Effect (Flynn, 2009; Nisbett & others, 2012).

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Surveying intelligence test scores around the world, James R. Flynn found that 14 nations showed significant gains in average IQ scores in just one generation (Flynn, 1994, 1999). Subsequent research by Flynn and other researchers documented similar gains in IQ scores in over 30 different countries worldwide (Flynn, 2009). The average IQ score in the United States has also steadily increased over the past century (Flynn, 2007a, 2007b; Weiss, 2010). Such changes in a population can be accounted for only by environmental changes because the amount of time involved is far too short for genetically influenced changes to have occurred.

The effect of social discrimination on intelligence test scores has been shown in numerous cross-cultural studies (see Ogbu, 1986, 2008). In many different societies, average IQ is lower for members of a discriminated-against minority group, even when that group is not racially different from the dominant group. The Culture and Human Behavior box “Performing with a Threat in the Air” explains how belonging to a stigmatized group can affect performance on tests of many different abilities.

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Take the case of the Burakumin people of Japan. Americans typically think of Japan as relatively homogeneous, and indeed the Burakumin are not racially different from other Japanese. They look the same and speak the same language. However, the Burakumin are the descendants of an outcast group that for generations worked as tanners and butchers. Because they handled dead bodies and killed animals, the Burakumin were long considered unclean and unfit for social contact. For centuries, they were forced to live in isolated enclaves, apart from the rest of Japanese society (DeVos, 1992; DeVos & Wagatsuma, 1967).

Today, there are about 3 million Burakumin in Japan. Although the Burakumin were legally emancipated from their outcast status many years ago, substantial social discrimination against them persists (Ikeda, 2001; Payton, 1992). Because there is no way to tell if a Japanese citizen is of Burakumin descent, there are dozens of private detective agencies in Tokyo and other Japanese cities that openly specialize in tracking Burakumin who are trying to “pass” and hide their background from employers or prospective marriage partners.

The Burakumin are the poorest people in Japan. They are only half as likely as other Japanese to graduate from high school or attend college. Although there are no racial differences between the Burakumin and the other Japanese, the average IQ scores of the Burakumin are well below those of other Japanese. As shown in Table 7.2, their average IQ scores are about 10 to 15 points below those of mainstream Japanese. But, when Burakumin families immigrate to the United States, they are treated like any other Japanese. The children do just as well in school—and on IQ tests—as any other Japanese Americans (Ogbu, 1986).

Of course, Japan is not the only society that discriminates against a particular social group. Many societies discriminate against specific minority groups, such as the Harijans in India (formerly called the untouchables), West Indians in Great Britain, Maoris in New Zealand, and Jews of non-European descent in Israel.

Children belonging to these minority groups score 10 to 15 points lower on intelligence tests than do children belonging to the dominant group in their societies. Children of the minority groups are often one or two years behind dominant-group children in basic reading skills and mathematical skills. Minority-group children are overrepresented in remedial programs and in school dropout rates. They are also underrepresented in higher education. The impact of discrimination on group differences in IQ remains even when the minority-group and dominant-group members are of similar socioeconomic backgrounds (Ogbu, 1986, 2008). In many ways, the educational experiences of these minority groups seem to parallel those of minority groups around the world, providing a cross-cultural perspective on the consistent effects of discrimination in many different societies.

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ARE IQ TESTS CULTURALLY BIASED?

Another approach to explaining group differences in IQ scores has been to look at cultural bias in the tests themselves. If standardized intelligence tests reflect white, middle-class cultural knowledge and values, minority-group members might do poorly on the tests not because of lower intelligence, but because of unfamiliarity with the white, middleclass culture.

Researchers have attempted to create tests that are “culture-fair” or “culture-free.” However, it is now generally recognized that it is virtually impossible to design a test that is completely culture-free (Greenfield, 2003). As cross-cultural psychologist Patricia Greenfield (1997) argues, ability tests “reflect the values, knowledge, and communication strategies of their culture of origin.” Within that culture, the intelligence test may be a valid measure. Thus, a test will tend to favor the people from the culture in which it was developed.

Cultural differences may also be involved in test-taking behavior (Sternberg, 1995, 2012). People from different cultural backgrounds may use strategies in solving problems or organizing information that are different from those required on standard intelligence tests (Miller-Jones, 1989). In addition, such cultural factors as motivation, attitudes toward test taking, and previous experiences with tests can affect performance and scores on tests.