If everyone in the world were equally intelligent, we probably would not even have a word for it. What makes intelligence such an interesting and important topic is that some individuals—and some groups of individuals—have more of it than others.

The average IQ is 100, and the vast majority of us—about 70 percent in fact—have IQs between 85 and 115 (see FIGURE 10.9). The people who score well above this large middle range are said to be intellectually gifted, and the people who score well below it are said to be intellectually disabled. The people who live at opposite ends of this continuum have one thing in common: They are more likely to be male than female. Although males and females have the same average IQ, the distribution of males’ IQ scores is more variable than the distribution of females’ IQ scores, which means that there are more males than females at both the very top and the very bottom of the IQ range (Hedges & Nowell, 1995; Lakin, 2013; Wai, Putallaz, & Makel, 2012). Some of this difference is surely due to the different ways in which boys and girls are socialized. Whether some of this difference is also due to innate biological differences between males and females remains a hotly debated issue in psychology (Ceci, Williams, & Barnett, 2009; Nisbett et al., 2012; Spelke, 2005).

Those of us who occupy the large middle of the intelligence spectrum often embrace a number of myths about those who live at the extremes. For example, movies typically portray the “tortured genius” as a person (usually a male person) who is brilliant, creative, misunderstood, despondent, and more than a little bit weird. Although some psychologists do think there is a link between creative genius and certain forms of psychopathology (Gale et al., 2012; Jamison, 1993; cf. Schlesinger, 2012), for the most part Hollywood has the relationship between intelligence and mental illness backwards: People with very high intelligence are less prone to mental illness than are people with very low intelligence (Dekker & Koot, 2003; Didden et al., 2012; Walker et al., 2002). Indeed, a 15 point decrease in IQ at age 20 is associated with a 50 percent increase in the risk of later hospitalization for schizophrenia, mood disorder, and alcohol-related disorders (Gale et al., 2010) as well as for personality disorders (Moran et al., 2009). Just as intelligence seems to buffer people against physical illness, it seems to buffer people against mental illness as well.

Another staple of Hollywood’s vivid imagination is the “maladjusted child braniac.” In fact, research suggests that very high IQ children are about as well-adjusted as their peers, and any special social or emotional problems they have seem to stem largely from a lack of appropriate scholastic opportunities (Garland & Zigler, 1999; Neihart, 1999). Being intellectually gifted does not necessarily make a child’s life worse, but it also does not necessarily make it better. For instance, profoundly gifted children are no more likely than moderately intelligent children to become major contributors to the fields in which they work (Richert, 1997; Terman & Oden, 1959). No one knows why the gifts of childhood do not always ripen into the fruits of adulthood: Perhaps there is a natural limit on how much intelligence can improve one’s outcomes, or perhaps the educational system fails to help gifted children make the best use of their talents (Robinson & Clinkenbeard, 1998; Winner, 2000).

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It is worth noting that gifted children are rarely gifted in all departments, but instead have gifts in a single domain such as mathematics, language, or music. More than 95 percent of gifted children show a sharp disparity between their mathematical and verbal abilities (Achter, Lubinski, & Benbow, 1996). Because gifted children tend to be “single-gifted,” they also tend to be single-minded, displaying a “rage to master” the domain in which they excel. As one expert noted, “one cannot tear these children away from activities in their area of giftedness, whether they involve an instrument, a computer, a sketch pad, or a math book. These children…can focus so intently on work in this domain that they lose sense of the outside world” (Winner, 2000, p. 162). Indeed, some research suggests that the one thing that most clearly distinguishes gifted children from their less gifted peers is the sheer amount of time they spend engaged in their domain of excellence (Ericsson & Charness, 1999). A large part of nature’s gift may be the capacity for passionate devotion to a single activity (Mayer et al., 1989; cf. Hambrick et al., 2013).

On the other end of the intelligence spectrum are people with intellectual disabilities, which can range from mild (50 < IQ < 69) to moderate (35 < IQ < 49) to severe (20 < IQ < 34) to profound (IQ < 20). About 70 percent of people with IQs in this range are male. Two of the most common causes of intellectual disability are Down syndrome (caused by the presence of a third copy of chromosome 21) and fetal alcohol syndrome (caused by a mother’s alcohol use during pregnancy). These intellectual disabilities are quite general, and people who have them typically show impaired performance on most or all cognitive tasks. Myths about people with intellectual disabilities abound. For example, many people think the intellectually disabled are mentally ill, but in fact, their rate of mental illness is similar to that found in the general population (Deb, Thomas, & Bright, 2001). Another myth about the intellectually disabled is that they are unhappy. A recent survey of people with Down syndrome (Skotko, Levine, & Goldstein, 2011) revealed that more than 96 percent are happy with their lives, like who they are, and like how they look. People with intellectual disabilities face many challenges, and being misunderstood is one of the most difficult.

In the early 1900s, Stanford professor Lewis Terman improved on Binet and Simon’s work and produced the intelligence test now known as the Stanford–Binet Intelligence Scale. Among the things his test revealed was that whites performed better than non-whites. “Are the inferior races really inferior, or are they merely unfortunate in their lack of opportunity to learn?” he asked, and then answered unequivocally: “Their dullness seems to be racial, or at least inherent in the family stocks from which they come.” He went on to suggest that “children of this group should be segregated into separate classes…[because] they cannot master abstractions but they can often be made into efficient workers” (Terman, 1916, pp. 91–92).

A century later, these sentences make most of us cringe. Terman appeared to be making three claims: First, that intelligence is influenced by genes; second, that members of some racial groups score better than others on intelligence tests; and third, that the difference in scores is due to a difference in genes. Virtually all modern scientists agree that Terman’s first two claims are true: Intelligence is influenced by genes and some groups do perform better than others on intelligence tests. However, Terman’s third claim—that differences in genes are the reason why some groups outperform others—is not a fact. Indeed, it is a provocative conjecture that has been the subject of both passionate and acrimonious debate. What does science have to tell us about it?

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Before answering that question we should be clear about one thing: Between-group differences in intelligence are not inherently troubling. No one is troubled by the possibility that Nobel laureates are on average more intelligent than shoe salesmen, and that includes the shoe salesmen. On the other hand, most of us are troubled by the possibility that people of one gender, race, or nationality may be more intelligent than people of another because intelligence is a valuable commodity and it does not seem fair for a few groups to corner the market by accidents of birth or geography.

But fair or not, they do. Asians routinely outscore Caucasians, who routinely outscore Latinos, who routinely outscore blacks (Neisser et al., 1996; Rushton, 1995). Women routinely outscore men on tests that require rapid access to and use of semantic information, production and comprehension of complex prose, fine motor skills, and perceptual speed of verbal intelligence, and men routinely outscore women on tests that require transformations in visual or spatial memory, certain motor skills, spatiotemporal responding, and fluid reasoning in abstract mathematical and scientific domains (Halpern, 1997; Halpern et al., 2007). Indeed, group differences in performance on intelligence tests “are among the most thoroughly documented findings in psychology” (Suzuki & Valencia, 1997, p. 1104). Although the average difference between groups is considerably less than the average difference within groups, so that there is a huge amount of overlap between the groups, Terman was right when he noted that some groups perform better than others on intelligence tests. The question is why?

One possibility is that there is something wrong with the tests. In fact, there is now little doubt that the earliest intelligence tests asked questions whose answers were more likely to be known by members of one group (usually white Western Europeans) than by members of another. For example, when Binet and Simon asked students, “When anyone has offended you and asks you to excuse him, what ought you to do?,” they were looking for answers such as “accept the apology graciously.” Answers such as “demand three goats” would have been counted as wrong. But intelligence tests have come a long way in a century, and one would have to look hard to find questions on a modern intelligence test that have the same blatant cultural bias that Binet and Simon’s test did (Suzuki & Valencia, 1997). Moreover, group differences emerge even on those portions of intelligence tests that measure nonverbal skills, such as Raven’s Progressive Matrices Test (see Figure 10.4). It would be difficult to argue that the large differences between the average scores of different groups is due entirely—or even largely—to a cultural bias in IQ tests.

Of course, even when test questions are unbiased, testing situations may not be. For example, studies show that African American students perform more poorly on tests if they are asked to report their race at the top of the answer sheet, because doing so causes them to feel anxious about confirming racial stereotypes (Steele & Aronson, 1995) and anxiety naturally interferes with test performance (Reeve, Heggestad, & Lievens, 2009). European American students do not show the same effect when asked to report their race. When Asian American women are reminded of their gender, they perform unusually poorly on tests of mathematical skill, presumably because they are aware of stereotypes suggesting that women cannot do mathematics. But when the same women are instead reminded of their ethnicity, they perform unusually well on such tests, presumably because they are aware of stereotypes suggesting that Asians are especially good at mathematics (Shih, Pittinsky, & Ambady, 1999). Indeed, when women read an essay suggesting that mathematical ability is strongly influenced by genes, they perform more poorly on subsequent mathematics tests (Dar-Nimrod & Heine, 2006). Findings such as these remind us that the situation in which intelligence tests are administered can affect members of different groups differently and may cause group differences in performance that do not reflect group differences in actual intelligence.

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Biases in the testing situation may explain some of the between-group differences in intelligence test scores, but probably not all. If we assume that some of these differences reflect real differences in the abilities that intelligence tests are meant to measure, then what accounts for these ability differences?

There is broad agreement among scientists that environment plays a major role. For example, African American children have lower birth weights, poorer diets, higher rates of chronic illness, poorer medical care, attend worse schools, and are three times more likely than European American children to live in single-parent households (Acevedo-Garcia et al., 2007; National Center for Health Statistics, 2004). Given the vast differences between the SES of European Americans and African Americans, it is not very surprising that African Americans score on average 10 points lower on IQ tests than do European Americans. Do genes play any role in this difference? So far, scientists have not found a single fact that requires such a conclusion, but they have found several facts that make such a conclusion difficult to accept. For example, the average African American has about 20 percent European genes, but those who have more are no smarter than those who have fewer, which is not what we would expect if European genes made people smart (Loehlin, 1973; Scarr et al., 1977). Similarly, African American children and mixed-race children have different amounts of European genes, and yet, when they are adopted into middle-class families, their IQs do not differ (Moore, 1986). These facts do not rule out the possibility that between-group differences in intelligence are caused by genetic differences, but they do make that possibility unlikely.

What would it take to prove that behavioural and psychological differences between groups have a genetic origin? It would take the kind of evidence that scientists often find when they study the physical differences between groups. For example, people who have hepatitis C are often given a prescription for antiviral drugs, and European Americans typically benefit more from this treatment than African Americans do. Although physicians once thought this was because European Americans were more likely to take the medicine they were given, scientists have recently discovered a gene that makes people unresponsive to these antiviral drugs—and guess what? African Americans are much more likely than European Americans to have that gene (Ge et al., 2009). This kind of clear-cut evidence of genetic differences between groups is exactly what is lacking in the debate on the causes of between-group differences in intelligence. As one researcher recently noted, “No individual genetic variants are conclusively related to intelligence or its change with age in healthy individuals” (Deary, 2012, p. 463). This may mean that intelligence is influenced by many very tiny genetic effects rather than by a few major “intelligence genes” (Davies et al., 2011). Until researchers can locate such genes, and until they can show that those genes are more prevalent in one group than another, most psychologists are unlikely to be persuaded by genetic explanations of between-group differences in intelligence. Indeed, some experts, such as psychologist Richard Nisbett, believe the debate is all but over: “Genes account for none of the difference in IQ between blacks and whites; measurable environmental factors plausibly account for all of it” (Nisbett, 2009, p. 118).

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Intelligence can be improved—by money, for example, and by education. But most people cannot just snap their fingers and become wealthier, and education takes time. Is there anything that average parents can do to raise their child’s IQ? Researchers recently analyzed the data from all the high-quality scientific studies on this question that have been performed over the last few decades (Protzko, Aronson, & Blair, 2013), and they found that four things reliably raise a child’s intelligence. First, supplementing the diets of pregnant women and neonates with long-chain polyunsaturated fatty acids (a substance found in breast milk) raises children’s IQ by about 4 points. Second, enrolling low-SES infants in early educational interventions raises their IQ by about 6 points (though surprisingly, enrolling them at a younger age seems to be no better than enrolling them at an older age). Third, reading to children in an interactive manner raises their IQ by about 6 points (and in this case, the earlier the parent starts reading the better). Fourth and finally, sending children to preschool raises their IQ by about 6 points. Clearly, there are some things that parents can do to make their kids smarter.

Perhaps all this will be simpler in the future. Cognitive enhancers are drugs that produce improvements in the psychological processes that underlie intelligent behaviour. For example, stimulants such as Ritalin (methylphenidate) and Adderall (mixed amphetamine salts) can enhance cognitive performance (Elliott et al., 1997; Halliday et al., 1994; McKetin et al., 1999), which is why there has been an increase in their use by healthy students over the past few years. Surveys suggest that almost 7 percent of students in American universities have used prescription stimulants for cognitive enhancement, and that on some campuses the number is as high as 25 percent (McCabe et al., 2005). Across Canadian university campuses, the percentages range from 1 to 33 percent (Ainsworth-Vincze, 2009). These drugs improve people’s ability to focus attention and manipulate information in working memory, and improve flexibly control responses (Sahakian & Morein-Zamir, 2007). Cognitive performance can also be enhanced by a class of drugs called ampakines (Ingvar et al., 1997). Modafinil is one such drug, and it has been shown to improve short-term memory and planning abilities in healthy, young volunteers (Turner et al., 2003).

We should of course be worried about the abuse of these drugs. Nonetheless, the distinction between enhancing cognition by taking drugs and enhancing it by other means is not cut and dried. As one distinguished group of scientists (Greely et al., 2008, p. 703) recently concluded, “Drugs may seem distinctive among enhancements in that they bring about their effects by altering brain function, but in reality so does any intervention that enhances cognition. Recent research has identified beneficial neural changes engendered by exercise, nutrition and sleep, as well as instruction and reading.” In other words, if both drugs and exercise enhance cognition by altering the way the brain functions, then what exactly is the difference between them? Other scientists believe this question will soon be moot because cognitive enhancement will be achieved not by altering the brain’s chemistry in university, but by altering its basic structure at birth. By manipulating the genes that guide hippocampal development, scientists have created a strain of “smart mice” that have extraordinary memory and learning abilities, leading the researchers to conclude that “genetic enhancement of mental and cognitive attributes such as intelligence and memory in mammals is feasible” (Tang et al., 1999, p. 64). Although no one has yet developed a safe and powerful “smart pill” or “smart gene therapy,” many experts believe that this will happen in the next few years (Farah et al., 2004; Rose, 2002; Turner & Sahakian, 2006). When it does, we will need a whole lot of wisdom to know how to handle it.

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