Behavioral-genetics research has transformed our understanding of many aspects of behavior that were once explained solely in psychological terms. Some findings, such as the discovery that certain mental illnesses have a genetic component, have been accepted readily. Other findings, however, have inflamed political passions and upset people. No topic has aroused more controversy than the origins of human intelligence.

In heritability studies, the usual measure of intellectual functioning is an intelligence quotient, or IQ score. Scores on an IQ test reflect how a child has performed compared with other children of the same age, or how an adult has performed compared with other adults. The average score for each age group is arbitrarily set at 100. The distribution of scores in the population approximates a normal (bell-shaped) curve, with scores near the average (mean) being most common, and very high or very low scores being rare. Two-thirds of all test takers score between 85 and 115.

Most psychologists believe that IQ tests measure a general quality that affects most aspects of mental ability, but the tests also have many critics. Some contend that intelligence comes in many varieties, more than a single score can capture. Others maintain that IQ tests are culturally biased, tapping mostly those abilities that depend on experiences in a middle-class environment and favoring white people over people of other ethnicities. Most heritability estimates apply only to those mental skills that affect IQ test scores, and that these estimates are likely to be more valid for some groups than for others.

Despite these important qualifications, it is clear that the kind of intelligence that produces high IQ scores is partly heritable, at least in the middle-class samples usually studied. For children and adolescents, heritability estimates average around .40 or .50; that is, genetic differences explain about half of the variance in IQ scores (Chipuer, Rovine, & Plomin, 1990; Devlin, Daniels, & Roeder, 1997). For adults, most estimates are even higher—in the .60 to .80 range (Bouchard, 1995; McClearn et al., 1997; McGue et al., 1993). These estimates show that the genetic contribution becomes relatively larger and the environmental one relatively smaller with age.

In studies of twins, the scores of identical twins are always much more highly correlated than those of fraternal twins, a difference that reflects the influence of genes. In fact, the scores of identical twins reared apart are more highly correlated than those of fraternal twins reared together, as you can see in Figure3.7. In adoption studies, the scores of adopted children are more highly correlated with those of their birth parents than with those of their biologically unrelated adoptive parents; the higher the birth parents' scores, the higher the child's score is likely to be. As adopted children grow into adolescence, the correlation between their IQ scores and those of their biologically unrelated family members diminishes, and in adulthood, the correlation falls to zero (Bouchard, 1997b; Scarr, 1993; Scarr & Weinberg, 1994). This does not mean that adoption has no positive effects; adopted children score higher on IQ tests than do birth siblings who were not adopted, probably because adoptees grow up in a more enriched environment (van IJzendoorn et al., 2005).

If genes influence individual differences in intelligence, do they also help account for differences between groups, as many people assume? Unfortunately, the history of this issue has been marred by ethnic and class prejudice. Because this question has enormous political and social importance, we are going to examine it closely.

Most of the focus has been on black–white differences in IQ because African American children score lower, on average, than do white children. (The exact numbers are a matter of debate, but in any event, we are talking about averages; the distributions of scores for black children and white children overlap considerably.) A few researchers have proposed a genetic explanation of this difference and conclude that there is little point in spending money on programs that try to raise the IQs of low-scoring children, of whatever race (Murray, 2008; Rushton & Jensen, 2005).

Genetic explanations, however, have a fatal flaw: They use heritability estimates based mainly on white samples to estimate the role of heredity in group differences, a procedure that is not valid. This problem sounds pretty technical, but it is really not too difficult to understand, so stay with us.

Consider, first, not people but tomatoes. (Figure3.8 will help you visualize the following “thought experiment.”) Suppose you have a bag of tomato seeds that vary genetically; all things being equal, some will produce tomatoes that are puny and tasteless, and some will produce tomatoes that are plump and delicious. Now you take a bunch of these seeds in your left hand and another bunch from the same bag in your right hand. Although one seed differs genetically from another, there is no average difference between the seeds in your left hand and those in your right. You plant the left hand's seeds in pot A, with some enriched soil that you have doctored with nitrogen and other nutrients, and you plant the right hand's seeds in pot B, with soil from which you have extracted nutrients. You sing to pot A and put it in the sun; you ignore pot B and leave it in a dark corner.

Figure 3.8

The Tomato Plant Experiment

When the tomato plants grow, they will vary within each pot in terms of height, the number of tomatoes produced, and the size of the tomatoes, purely because of genetic differences. But there will also be an average difference between the plants in pot A and those in pot B: The plants in pot A will be healthier and bear more tomatoes. This difference between pots is due entirely to the different soils and the care that has been given to them, even though the heritability of the variation among the plants within each pot is 100 percent (Lewontin, 1970, 2001).

The principle is the same for people as it is for tomatoes. Although intellectual differences within groups are partly genetic in origin, that does not mean differences between groups are genetic. Blacks and whites do not grow up, on the average, in the same “pots” (environments). In fact, some scientists argue that in the United States, the long history of sexual relations between blacks and whites has virtually eliminated genetic differences between them, meaning that any remaining differences are almost exclusively the product of environment (Nisbett, 2009). Because of the legacy of racial discrimination and de facto segregation, black children, as well as Latino and other minority children, often receive far fewer nutrients—literally, in terms of food, and figuratively, in terms of education, encouragement by society, and intellectual opportunities. Ethnic groups also differ in countless cultural ways that affect their performance on IQ tests. And negative stereotypes about ethnic groups may cause members of these groups to doubt their own abilities, become anxious and self-conscious, and perform more poorly than they otherwise would on tests.

Conducting good research on the origins of group differences in IQ is extremely difficult in many countries, including the United States, where racism has affected the lives of even many affluent, successful African Americans. However, the few studies that have overcome past methodological problems fail to reveal any genetic differences between blacks and whites in whatever it is that IQ tests measure. One study found that children fathered by black or white American soldiers in Germany after World War II and reared in similar German communities by similar families did not differ significantly in IQ (Eyferth, 1961). Another showed that contrary to what a genetic theory would predict, degree of African ancestry (which can be roughly estimated from skin color, blood analysis, and genealogy) was not related to measured intelligence (Scarr et al., 1977).

Even among groups popularly thought to be high achievers, purely genetic explanations are unsatisfactory. For instance, although descendants of the Ashkenazi Jews of Europe tend to have higher IQ scores than their non-Jewish white counterparts, their accomplishments exceed what would be expected on the basis of their IQ scores alone (Nisbett et al., 2012). An intelligent reading of the research on intelligence, therefore, does not direct us to conclude that differences among cultural, ethnic, or national groups are permanent, genetically determined, or signs of any group's innate superiority. On the contrary, the research suggests that we should make sure that all children grow up in the best possible soil, with room for the smartest and the slowest to find a place in the sun.

By now you may be wondering what kinds of environmental “nutrients” hinder intellectual development and what kinds of environmental “nutrients” promote it. Here are some environmental influences associated with reduced mental ability:

• Poor prenatal care. If a pregnant woman is malnourished, contracts infections, takes certain drugs, smokes, is exposed to secondhand smoke, or drinks alcohol regularly, her child is at risk of having learning disabilities and a lower IQ.

• Malnutrition. The average IQ gap between severely malnourished and well-nourished children can be as high as 20 points (Stoch et al., 1982; Winick, Meyer, & Harris, 1975).

• Exposure to toxins. Many children, especially poor and minority children, are exposed to dangerous levels of lead from dust, contaminated soil, lead paint, and old lead pipes, and lead can damage the brain and nervous system. Even children exposed to fairly low levels that are often believed to be “safe” develop attention problems, have lower IQ scores, and do worse in school (Hornung, Lanphear, & Dietrich, 2009; Koller et al., 2004). And children exposed in utero to high levels of pesticides (still legal for spraying on farm fields) later have an IQ that is 7 points lower than in children with the least exposure (Raloff, 2011).

• Stressful family circumstances. Factors that predict reduced intellectual competence include, among others, having a father who does not live with the family, a mother with a history of mental illness, parents with limited work skills, and a history of stressful events, such as domestic violence, early in life (Jeon, Buettner, & Hur, 2014; Sameroff et al., 1987). On average, each risk factor reduces a child's IQ score by 4 points. Children with seven risk factors score more than 30 points lower than those with no risk factors. And when children live in severely disadvantaged neighborhoods, their verbal IQs decline over time, even after they have moved to better areas; the drop is comparable to that seen when a child misses a year of school (Sampson, Sharkey, & Raudenbush, 2008).

In contrast, a healthy and stimulating environment can raise IQ scores, as several intervention studies with at-risk children have shown. Attending a good-quality preschool increases the reading and math skills of children from racial and ethnic minorities, especially if they are not getting much cognitive stimulation elsewhere (Tucker-Drob, 2012). Two important longitudinal studies—the Abecedarian Project and the Child–Parent Center Education Program in Chicago—found that inner-city children who got lots of mental enrichment at home and in preschool showed significant IQ gains and had much better school achievement than did children in a control group (Campbell & Ramey, 1995; Reynolds et al., 2011). In another important study of abandoned children living in Romanian orphanages that provided little stimulation, researchers randomly assigned some children to remain in the orphanages and others to move to good foster homes. By age 4, the fostered children scored dramatically higher on IQ tests than did those left behind. Children who moved before age 2 showed the largest gains, almost 15 points on average. A comparison group of children reared in their biological homes did even better, with average test scores 10 to 20 points higher than those of the foster children (Nelson et al., 2007). (Since this study was done, Romania has stopped institutionalizing abandoned children younger than 2 unless they are seriously disabled.)

Although no single activity is going to turn anyone into a genius, certain experiences do appear to contribute to overall intelligence. In general, children's mental abilities improve when their parents talk to them about many topics and describe things accurately and fully, encourage them to think things through, read to them, and expect them to do well. A child's abilities also improve when the child's peers value and strive for intellectual achievement (Harris, 2009). Some kinds of enrichment classes may also help. When Canadian researchers randomly assigned first graders to weekly piano, singing, or drama lessons during the school year, or to a control group that received no extracurricular lessons, those children who learned to play the piano or sing showed an average IQ increase of 7 points by the end of the school year, compared to 4.3 points in the other groups. This difference was not large, but it was statistically significant (Schellenberg, 2004). The music lessons may have helped the children pay attention, use their memories, and hone their fine-motor skills, thus contributing to development of brain areas involved in intelligence.

Perhaps the best evidence for the importance of environmental influences on intelligence is the fact that in developed countries, IQ scores (as well as scores on related tests) have been climbing steadily for several generations (Flynn, 1987, 2013); genes cannot possibly have changed enough to account for these findings (see Figure3.9). What, then, has caused this rise? One possibility is improvements in the education, health care, diets, and job opportunities of the poorest, lowest-scoring people, which increases the overall mean. If that is so, we would expect to see large and rapid increases in developing countries, and so we do: In Kenya, IQ scores of rural 6- to 8-year-old children jumped about 11 points between 1984 and 1998, the fastest rise in a group's average IQ scores ever reported (Daley et al., 2003). But even in a highly developed country such as the United States, the scores of children in the top 5 percent of achievement tests are still climbing (Wai & Putallaz, 2011).

We see, then, that although heredity may provide the range of a child's intellectual potential—a Homer Simpson can never become an Albert Einstein—many other factors affect where in that range the child will fall.

Throughout this chapter, we have seen that heredity and environment always interact to produce the qualities typical of human beings in general and the mix of qualities that makes each of us unique. Genetically influenced traits can affect how we respond to a specific environment and which environments we find most rewarding or compatible; the environment in turn influences the genome, through its effects on mutations and epigenetic changes. Human behavior is responsive to a constantly changing network of interlinked influences, both biological and environmental, plus the additional, unpredictable spice of chance and luck. The development of a human being (or other animal) is the result of a constant dialogue between the genome and its environment (Plomin et al., 2013; Zhang & Meaney, 2010).

The key lesson to be learned from this dynamic interaction is that we can no more speak of genes or the environment “causing” personality, intelligence, or behavior in a straightforward way than we can speak of butter, sugar, or flour individually causing the taste of a cake (Lewontin, Rose, & Kamin, 1984). Many people do speak that way, however, out of a desire to make things clearer than they actually are, and sometimes to justify prejudices about ethnicity, gender, or class.

An unstated assumption in many debates about nature and nurture is that the world would be a better place if certain kinds of genes prevailed. This assumption overlooks the fact that nature loves genetic diversity, not similarity. The ability of any species to survive depends on such diversity. If every penguin, porpoise, or person had exactly the same genetic strengths and weaknesses, these species could not survive major changes in the environment; a new virus or a change in climate would wipe out the entire group. With diversity, at least some penguins, porpoises, or people have a chance of making it.

Psychological diversity is adaptive, too. Each of us has something valuable to contribute, whether it is artistic talent, academic ability, creativity, social skill, athletic prowess, a sense of humor, mechanical aptitude, practical wisdom, a social conscience, or the energy to get things done. Our complicated, fast-moving world requires all of these qualities if we are to survive as communities and thrive as individuals.