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The Roots of Intelligence: Environment and Individual IQ


Environment and Individual IQ


Undeniably, genetic influences play a powerful role in shaping someone’s intellectual capacities. Indeed, researchers have begun to explore exactly how these genetic influ-ences unfold—including an effort to specify which genes, on which chromosomes, are the ones that shape intelligence. (For glimpses of the modest progress so far, see Posthuma & deGeus, 2006; Zimmer, 2008.)


As we’ve repeatedly noted, though, genetic effects always unfold within an environ-mental context. So—inevitably—environmental factors also shape the development of our intellect. Evidence for this point comes from many sources; and thus, as we’ll see, the IQ score someone ends up with depends on both her genes and the surroundings in which she grew up.



The effect of the environment on IQ scores is evident in many facts. For example, one Norwegian study examined a huge data set that included intelligence scores for 334,000 pairs of brothers. The researchers found that the correlation between the brothers’ intelligence scores was smaller for brothers who were more widely separated in age (Sundet, Eriksen, & Tambs, 2008). This result is difficult to explain genetically, because the genetic resemblance is the same for two brothers born, say, one year apart as it is for two brothers born five years apart. In both cases, the brothers share 50% of their genetic material. However, this result makes sense on environmental grounds. The greater the age difference between the brothers, the more likely it is that the family cir-cumstances have changed between the years of one brother’s childhood and the years of the other’s. Thus, a greater age difference would increase the probability that the brothers grew up in different environments, and to the degree that these environments shape intelligence, we would expect the more widely spaced brothers to resemble each other less than the closely spaced siblings.


We’ve also known for many years that impoverished environments can impede intel-lectual development. For example, researchers studied children who worked on canal boats in England during the 1920s and rarely attended school; they also studied chil-dren who lived in rural mountainous Kentucky, where little or no schooling was avail-able. These certainly seem like poor conditions for the development of intellectual skills, and it seems likely that exposure to these conditions would have a cumulative effect: The longer the child remains in such an environment, the lower his IQ should be (Figure 11.17). This is precisely what the data show—a negative correlation between IQ and age. That is, the older the child (the longer she had been in the impoverished envi-ronment), the lower her IQ (Asher, 1935; H. Gordon, 1923; also see Heckman, 2006). Related results come from communities where schools have closed. These closings typically lead to a decline in intelligence-test scores—in one study, a drop of about 6 points for every year of school missed (R. L. Green, Hoffman, Morse, Hayes, & Morgan, 1964; see also Ceci & Williams, 1997; Neisser et al., 1996).


More optimistically, we also know that improving the environment can to some extent increase IQ. For example, in a study in France, researchers focused on cases in which thegovernment had removed children from their biological parents because of abuse or neg-lect (Duyme, Dumaret, & Tomkiewicz, 1999) The researchers were thus able to compare the children’s “pre-adoption IQ” (i.e., when the children were still living in a high-risk environment) with their IQ in adolescence—after years of living with their adoptive fam-ilies. The data (Figure 11.18) showed substantial improvements in the children’s scores, thanks to this environmental change.


A similar conclusion flows from the effects of explicit training. The Venezuelan “Project Intelligence,” for example, gave underprivileged ado-lescents in Venezuela extensive training in various thinking skills (Herrnstein, Nickerson, de Sanchez, & Swets, 1986). Assessments after training showed substantial benefits on a wide range of tests. A similar benefit was observed for American preschool children in the Carolina Abecedarian Project (F. A. Campbell & Ramey, 1994). These programs leave no doubt that suitable enrichment and education can provide substantial improvement in intelligence-test scores. (For still other evidence that schooling lifts intelligence scores, see Ceci & Williams, 1997; Grotzer & Perkins, 2000; M. Martinez, 2000; Perkins & Grotzer, 1997.)


We should note in passing that there’s no conflict between these results and the results we mentioned earlier when docu-menting the reliability of the IQ test. There we noted that IQ scores are usually quite stable across the life span, so that (for example) if we know someone’s IQ at, say, age 10, we can accurately predict what her IQ will be a decade or more later. This stabil-ity in scores is easily observed if a person lives in a consistent environment. As we now see, though, changes in the environment can produce substantial shifts in IQ—by a dozen or more points. Thus the IQ test is reliable, but this doesn’t mean that IQ scores can’t change.


The impact of environmental factors on IQ scores is also undeniable in another fact. Around the globe, scores on intelligence tests have been  gradually  increasing  over  the  last  few  decades,  at  a  rate  of approximately  3  points  per  decade.  This  pattern  is  known  as  the Flynn effect, after James R. Flynn (1984, 1987, 1999, 2009; see also Daley,  Whaley,  Sigman,  Espinosa,  &  Neumann,  2003;  Kanaya, Scullin, & Ceci, 2003), one of the first researchers to document this effect  systematically.  This  improvement  has  been  documented  in many countries, including many developed (and relatively affluent) nations and also relatively impoverished third world nations (Figure 11.19). (There’s also some suggestion that the improvement has now leveled off in some countries—Britain, for example—and may even be reversing; but it’s too soon to make a judgment on this point; Flynn, 2009.)

Could it be that people in the modern world are simply accumulating more and more information? If so, then the Flynn effect would be most visible in measures of crystal-lized intelligence. However, that’s not what the evidence shows. Instead, the effect is stronger in measures of fluid intelligence—such as the Raven’s Matrices—so it seems to be a genuine change in how quickly and flexibly people can think.

Some  scholars  suggest  that  this  broad  increase  in  scores  is  attributable  to  wide- spread improvement in nutrition and health care, and these factors surely do contribute to the Flynn effect in some parts of the world (for a study in Kenya, for example, see Daley et al., 2003). But we need some other explanation for why the effect is also evi-dent in middle-class populations in relatively wealthy countries (Flynn, 2009). One proposal is that this worldwide improvement is the result of the increasing complexity and sophistication of our shared culture: Each of us is exposed to more information and a wider set of perspectives than were our grandparents, and this exposure may lead to a sharpening of skills that show up in our data as an improvement in IQ (for a broad discussion, see Dickens & Flynn, 2001; Greenfield, 2009; Neisser, 1997, 1998).


Whatever the explanation, though, one point is clear: The Flynn effect cannot be explained genetically. While the human genome does change (a prerequisite, of course, for human evolution), it doesn’t change at a pace commensurate with this effect. Therefore, this worldwide improvement becomes part of the larger package of evidence documenting that intelligence can indeed be improved by suitable environ-mental conditions.




Let’s return, though, to the effects of poverty on IQ , because these effects are informa-tive in two ways. First, these effects help us understand exactly how the environment shapes intelligence. Second, these effects also illuminate the interaction between envi-ronmental and genetic factors in shaping IQ.

The overall effects of poverty on IQ are easily documented, and in fact there’s a correlation of .40 between a child’s intelligence scores and the socioeconomic status of the family in which the child is raised (Lubinski, 2004). Looking beyond these broad effects, though, we can ask what aspects of intelligence are especially affected as well as how poverty shapes intelligence. We know, for example, that the impact of poverty isespecially salient in tests of language skills and also in tasks hinging on executive control (Hackman & Farah, 2009). In addition, children who live in poverty in their preschool years seem more at risk than children who live in poverty in middle or late childhood (G. Duncan, Yeung, Brooks-Gunn, & Smith, 1998; Farah et al., 2006). Apparently, then, many of the harmful effects of poverty aren’t due to inferior educa-tion. Instead, the effects derive from a mix of other factors, including exposure to vari-ous toxins found in lower-quality housing, lack of stimulation, poor nutrition, and inferior health care—and probably also the chronic stress that goes with poverty. All of these factors can interfere with the normal development of the brain, and they have important (and deeply unfortunate) consequences for intellectual functioning. (For more on the neurocognitive effects of poverty, see Hackman & Farah, 2009.)


These various problems, all associated with poverty, have a direct effect on brain development and also interact with genetic influences on development (Turkheimer, Haley, Waldron, D’Onofrio, & Gottesman, 2003). Specifically, when researchers focus on higher-SES families, they find the pattern we’ve already described—an appreciably stronger resemblance between identical twins’ IQ scores than there is between fraternal twins’ scores. This tells us (as we’ve discussed) that genetic factors are playing an important role here, so that people who resemble each other genetically are likely to resemble each other in their test scores. Among lower-SES families, though, the pattern is different. In this group, the degree of IQ resemblance is the same for identical and fraternal twins—which tells us that in this setting, genetic factors seem to matter much less for shaping a person’s intelligence.


What’s going on here? Specifically, it may be best to think about our genes as providing our potential—a capacity to grow and develop if we’re suitably nurtured. If, therefore, a child receives good schooling, health care, and adequate nutrition, he’ll be able to develop this potential; and as the years go by, he’ll be able to make the most of the genetically defined predisposition he was born with. But if a child grows up in an impoverished environment with poor schooling, minimal health care, and inadequate nutrition, it matters much less whether he has a fine potential—because the environ-ment doesn’t allow the potential to emerge. Hence, in impoverished environments, genetic factors—the source of the potential—count for relatively little.


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