Effects of Heredity and Environment on Intelligence (page 3)

By — Pearson Allyn Bacon Prentice Hall
Updated on Jul 20, 2010

Effects of formal schooling

The very act of attending school leads to small increases in IQ. In Western societies, children who begin their educational careers early and attend school regularly have higher IQ scores than children who do not. When children must start school later than they would otherwise for reasons beyond their families’ control, their IQs are at least 5 points lower for every year of delay. Furthermore, children’s IQ scores decline slightly (usually only temporarily) over the course of the summer months, when children are not attending school. And other things being equal, children who drop out have lower IQ scores than children who remain in school, losing an average of almost 2 IQ points for every year of high school not completed (Ceci, 2003; Ceci & Williams, 1997).

The benefits of schooling for intellectual growth are seen in a wide variety of cultures. One probable reason why school attendance affects IQ is that it encourages acquisition of more advanced cognitive processes—rehearsal, organization, metacognition, and so on (M. Cole, 2006). And as Vygotsky pointed out, school provides a systematic means through which children can acquire many concepts and perspectives that previous generations have developed to tackle day-to-day tasks and problems effectively.

The Flynn effect

The last few decades have seen a slow, steady increase in people’s average performance on IQ tests throughout the industrialized world (Flynn, 1987, 1999, 2003; Neisser, 1998b). This trend is commonly known as the Flynn effect. A similar change has been observed in children’s performance on traditional Piagetian tasks (Flieller, 1999). Such improvements are difficult to attribute to heredity because the same gene pool (albeit with an occasional mutation) is passed along from one generation to the next, and so the cause is almost certainly environmental. Theorists disagree as to the likely explanations, however. Better nutrition, smaller family sizes, higher quality home environments, better schooling (for parents as well as children), and more enriching and informative stimulation (increased access to television, reading materials, etc.) are all possibilities (Daley, Whaley, Sigman, Espinosa, & Neumann, 2003; Flynn, 2003; Neisser, 1998b).

How Nature and Nurture Interact in Their Influence on Intelligence

Clearly both nature and nurture influence intelligence. What is less clear is how much influence each of these factors has. A few theorists have tried to estimate nature’s contribution (the heritability of IQ) from the correlations obtained in twin and adoption studies (e.g., McGue et al., 1993; Plomin et al., 1997). But most psychologists now believe that it may ultimately be impossible to separate the relative effects of heredity and environment. They suggest that the two combine to influence children’s cognitive development and measured IQ in ways that we can probably never disentangle (e.g., W. A. Collins et al., 2000; Flynn, 2003; Rogoff, 2003; Turkheimer, 2000). Theorists have made the following general points about how nature and nurture interact as they affect intellectual development:

Heredity establishes a range rather than a precise figure. Heredity does not dictate that a child will have a particular IQ score. Instead, it appears to set a range of abilities within which children will eventually fall, with the actual ability level each one achieves depending on his or her specific environmental experiences (Weinberg, 1989). Heredity may also affect how susceptible or impervious a child is to particular environmental influences (Rutter, 1997). For example, high-quality instruction may be more important for some children than for others. In the opening case study, Gina learned how to read before she attended school, and with only minimal help from her mother. Yet other, equally intelligent children may learn to read only when they have systematic reading instruction tailored to their individual needs.

Genetic expression is influenced by environmental conditions. Genes are not entirely self-contained, independent “carriers” of developmental instructions. Rather, the particular instructions they transmit are influenced by the supportive or nonsupportive nature of children’s environments. In an extremely impoverished environment—one with a lack of adequate nutrition and little if any stimulation—heredity may have little to say about the extent to which children develop intellectually. In an ideal environment—one in which nutrition, parenting practices, and educational opportunities are optimal and age-appropriate—heredity can have a significant influence on children’s IQ scores (Ceci, 2003; D. C. Rowe et al., 1999; Turkheimer, Haley, Waldron, D’Onofrio, & Gottesman, 2003).

Intelligence is almost certainly the result of many genes, each contributing a small amount to measured IQ (Sattler, 2001). These genes may “kick in” at different points in development, and their expression will be influenced by particular environmental conditions at those times. Thus we do not have a single heredity-environment interaction, but rather a number of heredity-environment interactions all contributing to intellectual growth (Simonton, 2001).

Especially as they get older, children choose their environments and experiences. Children may actively seek out environmental conditions that match their inherited abilities—a phenomenon known as niche-picking (Flynn, 2003; Halpern & LaMay, 2000; Scarr & McCartney, 1983). For example, children who, genetically speaking, have exceptional quantitative reasoning ability may enroll in advanced mathematics courses, delight in tackling mathematical brainteasers, and in other ways nurture their inherited talents. Children with average quantitative ability are less likely to take on such challenges and so have fewer opportunities to develop their mathematical skills. In such circumstances the relative effects of heredity and environment are difficult to tease apart.

Earlier we mentioned that the IQ correlations between adopted children and their biological parents become stronger over time. We now have a possible explanation for this finding. Children gain increasing independence as they get older. Especially as they reach adolescence, they spend less time in their home environments, and they make more of their own decisions about the kinds of opportunities to pursue—decisions undoubtedly based, in part, on their natural talents and tendencies (McGue et al., 1993; Petrill & Wilkerson, 2000).

You might think of intelligence as being the result of four factors (Gottlieb, 1991, 1992). Genetic activity affects neural activity (i.e., the operation of neurons in the brain), which in turn affects behavior, which in turn affects the environment. But influence moves in the opposite direction as well: The environment affects behavior, and these two (through stimulation, nutritional intake, physical activity, etc.) affect neural activity and genetic expression.


4In our teaching experiences, we have found that some students erroneously interpret the higher correlations as indicating that identical twins have higher intelligence. This is, of course, not the case. The size of each correlation indicates the strength of the relationship between twins’ IQs, not the level of twins’ intelligence per se.

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