If everyone in the world were equally intelligent, we probably wouldn’t 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% in fact—have IQs between 85 and 115 (see FIGURE 9.16). 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 weird. But 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) and very high IQ children are about as well-adjusted as their peers (Garland & Zigler, 1999; Neihart, 1999). Another myth about geniuses is that they are brilliant at everything. In fact, gifted children are rarely gifted in all departments but instead have gifts in a single domain such as math, language, or music. More than 95% 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. Indeed, some research suggests that the 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).

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On the other end of the intelligence spectrum are people with intellectual disabilities, which is defined by an IQ of less than 70. About 70% 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). Unlike intellectual gifts, intellectual disabilities tend to be quite general, and people who have them typically show impaired performance on most or all cognitive tasks. Perhaps the greatest 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% are happy with their lives, like who they are, and like how they look. People with intellectual disabilities face many challenges, and being mistaken for miserable people is clearly one of them.

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 he answered unequivocally: “Their dullness seems to be racial, or at least inherent in the family stocks from which they come” (Terman, 1916, pp. 91–92).

Was Terman right or wrong? Terman made three claims: First, he claimed that intelligence is influenced by genes; second, he claimed that members of some racial groups score better than others on intelligence tests; and third, he claimed that the difference in scores is due to a difference in genes. Virtually all modern scientists agree that Terman’s first two claims are indeed 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 by any means a scientific fact. Indeed, it is a provocative conjecture that has been the subject of both passionate and acrimonious debate. What does science have to say about it?

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 most 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 doesn’t seem fair for a few groups to corner the market by accidents of birth or geography.

But fair or not, they do. Whites 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, Terman was right when he noted that some groups perform better than others on intelligence tests. The question is why?

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Tests and Test Takers

One possibility is that there is something wrong with the tests. In fact, the earliest intelligence tests asked questions whose answers were more likely to be known by members of one group (usually White 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). 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.

And yet, 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 can’t do math. 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 math (Shih, Pittinsky, & Ambady, 1999). Findings such as these remind us that the situation in which intelligence tests are administered may cause group differences in performance that do not reflect group differences in actual intelligence.

Environments and Genes

Biases in the testing situation may explain some of the between-group differences in intelligence test scores, but probably not all. There is broad agreement among scientists that environment also plays a major role. For example, African American children have on average lower birth weights, poorer diets, higher rates of chronic illness, and poorer medical care; and they 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 isn’t very surprising that African Americans score on average 10 points lower on IQ tests than European Americans do.

Do genes play any role in this difference? So far, there is no compelling evidence that they do. For example, the average African American has about 20% European genes, and yet those who have more are no smarter than those who have fewer, which is not what we’d 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 don’t differ (Moore, 1986). These facts do not definitively rule out the possibility that between-group differences in intelligence are caused by genetic differences, of course, but they do make that possibility less likely. Indeed, some experts, such as psychologist Richard Nisbett, believe the debate on this topic 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 can’t just snap their fingers and become wealthier, and education takes time. Is there anything that parents can do to raise their child’s IQ? Research suggests that four things reliably raise a child’s intelligence (Protzko, Aronson, & Blair, 2013). 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 so-called 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 behavior. For example, stimulants such as Ritalin (methylphenidate) and Adderall (mixed amphetamine salts) can enhance cognitive performance by improving people’s ability to focus attention, manipulate information in working memory, and 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).

Some scientists believe that cognitive enhancement will eventually be achieved by altering the brain’s 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 intelligence to know how to handle it.

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