Effects of Heredity and Environment on Intelligence

Evidence for Hereditary Influences

Earlier we mentioned that measures of information processing speed correlate with IQ scores. Speed of processing depends on neurological efficiency and maturation, which are genetically controlled. From this standpoint, then, we have some support for a hereditary basis for intelligence (Perkins, 1995). The fact that children with certain genetic defects (e.g., Down syndrome) have, on average, significantly lower IQ scores than their nondisabled peers (Keogh & MacMillan, 1996) provides further evidence of heredity’s influence. But perhaps the most convincing evidence comes from twin studies and adoption studies.

Twin studies

Numerous studies have used monozygotic (identical) twins and dizygotic (fraternal) twins to get a sense of how strongly heredity affects IQ. Because monozygotic twins begin as a single fertilized egg which then separates, they are genetically equivalent human beings. In contrast, dizygotic twins are conceived as two separate fertilized eggs. They share about 50 percent of their genetic makeup, with the other 50 percent being unique to each twin. If identical twins have more similar IQ scores than fraternal twins, we can reasonably conclude that heredity influences intelligence.

Most twins are raised together by the same parent(s) and in the same home, and so they share similar environments as well as similar genes. Yet even when twins are raised separately (perhaps because they have been adopted and raised by different parents), they typically have similar IQ scores (Bouchard & McGue, 1981; N. Brody, 1992; Mackintosh, 1998; Plomin & Petrill, 1997). In a review of many twin studies, Bouchard and McGue (1981) found these average (median) correlations:

The correlation of .72 indicates that identical twins raised in different environments tend to have very similar IQ scores. In fact, these twins are more similar to each other than are fraternal twins raised in the same home.⁴

Adoption studies

Another way to separate the effects of heredity and environment is to compare adopted children with both their biological and adoptive parents. Adopted children tend to be similar to their biological parents in genetic makeup. Their environment, of course, more closely matches that of their adoptive parents. Researchers have found that adopted children’s IQ scores are more highly correlated with their biological parents’ IQs than with their adoptive parents’ IQs. In other words, in a group of people who place their infants up for adoption, those with the highest IQs tend to have offspring who, despite being raised by other people, also have the highest IQs. Furthermore, the IQ correlations between adopted children and their biological parents become stronger, and those between the children and their adoptive parents become weaker, as the children grow older, especially during late adolescence (Bouchard, 1997; McGue, Bouchard, Iacono, & Lykken, 1993; Plomin, Fulker, Corley, & DeFries, 1997; Plomin & Petrill, 1997). (If you find this last research result puzzling, we’ll offer an explanation shortly.)

Keep in mind that twin studies and adoption studies do not completely separate the effects of heredity and environment (W. A. Collins, Maccoby, Steinberg, Hetherington, & Bornstein, 2000; Wahlsten & Gottlieb, 1997). For example, adopted children have shared a common environment for at least 9 months—the 9 months of prenatal development—with their biological mothers. Likewise, monozygotic twins who are raised in separate homes have shared a common prenatal environment and often have similar, if not identical, postnatal environments as well. Furthermore, twin studies and adoption studies do not allow researchers to examine the ways in which heredity and environment might interact in their effects on measured intelligence. Any interactive effects are often added to the “heredity” side of the scoreboard (A. Collins et al., 2000; Turkheimer, 2000). Despite such glitches, twin and adoption studies point convincingly to a genetic component in intelligence (Bouchard, 1997; N. Brody, 1992; E. Hunt, 1997; Neisser, 1998a; Petrill & Wilkerson, 2000).

This is not to say that children are predestined to have an intelligence level similar to that of their biological parents. In fact, most children with high intelligence are conceived by parents of average intelligence rather than by parents with high IQ scores (Plomin & Petrill, 1997). Children’s genetic ancestry, then, is hardly a surefire predictor of what their own potential is likely to be. Environment also makes an appreciable difference, as we shall now see.

Evidence for Environmental Influences

Numerous sources of evidence converge to indicate that environment has a significant impact on IQ scores. We find some of this evidence in twin studies and adoption studies. Studies of the effects of nutrition, toxic substances, home environment, early intervention, and formal schooling provide additional support for the influence of environment. Also, a steady increase in performance on intelligence tests over the past several decades—known as the Flynn effect—is almost certainly attributable to environmental factors.

Twin studies and adoption studies revisited

Let’s look once again at the IQ correlations for identical twins raised in the same home versus in different homes. The median correlation for twins raised in different homes is .72, whereas that for twins raised in the same home is .86. In other words, twins raised in different homes have less similar IQs than twins raised in the same home. The distinct environments that different families provide do have some influence on intellectual development.

Adoption studies, too, indicate that intelligence is not determined entirely by heredity (Capron & Duyme, 1989; Devlin, Fienberg, Resnick, & Roeder, 1995; Waldman, Weinberg, & Scarr, 1994). For instance, in one study (Scarr & Weinberg, 1976), some children of poor parents (with unknown IQs) were adopted by middle-class parents with IQs averaging 118–121. Other children remained with their biological parents. IQ averages of the children in the two groups were as follows:

Although the adopted children’s IQ scores were, on average, lower than those of their adoptive parents, they were about 15 points higher than the scores for the control group children, who were raised by their biological parents.

Effects of early nutrition

Severe malnutrition, either before birth or during the early years of life, can limit neurological development and have a long-term impact on cognitive development and intelligence (Ricciuti, 1993; S. A. Rose, 1994; Sigman & Whaley, 1998). Attention, memory, abstract reasoning, and general school achievement are all likely to suffer from inadequate nutrition. Children sometimes recover from short periods of poor nourishment (due, perhaps, to war or illness), but the adverse effects of long-term deprivation are more enduring (Sigman & Whaley, 1998).

Some research studies have examined the effects of providing medically approved food supplements and vitamins to infants and young children who would not otherwise have adequate nutrition. Such interventions are most likely to enhance children’s development of motor skills, but in some instances cognitive development is enhanced as well (Pollitt & Oh, 1994; Sigman & Whaley, 1998).

Effects of toxic substances

A variety of toxic substances, or teratogens, in children’s prenatal or early postnatal environments—for instance, alcohol, drugs, radiation, lead-based paint dust—affect neurological development and thus also affect children’s later IQ scores (e.g., Michel, 1989; Neisser et al., 1996; Streissguth, Barr, Sampson, & Bookstein, 1994; Vogel, 1997; Vorhees & Mollnow, 1987). An example of such effects is fetal alcohol syndrome, in which children whose mothers consumed large amounts of alcohol during pregnancy show poor motor coordination, delayed language, and mental retardation (Dorris, 1989).

Effects of home environment

One likely explanation for the beneficial effects of adoption is that adoptive parents, who typically have adequate financial resources and high levels of education, can provide a more stimulating home environment than the biological parents might have been able to offer. Correlational studies indicate that stimulating home environments—those in which parents interact frequently with their children, make numerous learning and reading materials available, encourage the development of new skills, use complex sentence structures in conversation, and so on—are associated with higher IQ scores in children (Bradley & Caldwell, 1984; Brooks-Gunn et al., 1996; McGowan & Johnson, 1984). Furthermore, when two biologically unrelated children of the same age are raised by the same parents (typically because one or both children are adopted), the children’s IQs tend to be more similar than we would expect by chance alone—a relationship that can be attributed only to the influence of a common home environment (N. L. Segal, 2000).

We find especially compelling evidence for the beneficial effects of stimulating home environments in an ongoing project in Romania (C. A. Nelson, 2005). As a result of previous government policies, most Romanian orphans were at one time placed in large institutions to be raised. After a change in government and the intervention of a team of developmental psychologists, some institutionalized infants (randomly selected) were placed with adults willing to serve as foster parents. (Sadly, the intervention team could not find foster families for all of the infants.) As researchers periodically assessed the children’s physical and cognitive development, they found dramatic differences between the two groups. Despite adequate nutrition, children remaining in an institution throughout infancy and the preschool years had smaller head circumferences and less brain activity than the foster children. When intelligence was assessed, the institutionalized children had an average IQ of 64 (indicating mental retardation), whereas the foster children, on average, had IQs in the normal range.

Effects of early intervention

Unfortunately, not all children live in homes that provide ongoing stimulation and nurturance. When children live in impoverished or neglectful home environments, enriching preschool programs and other forms of early intervention can make an appreciable difference. For instance, high-quality child care and preschool programs (e.g., Head Start) frequently lead to short-term IQ gains and other cognitive and academic benefits (Bronfenbrenner, 1999; NICHD Early Child Care Research Network, 2002b; Zigler, 2003). The effects of such programs don’t continue indefinitely, however. Without follow-up interventions during the elementary school years, cognitive advantages (e.g., higher IQ scores and academic achievement) often diminish over time and in some cases disappear altogether (Brooks-Gunn, 2003; Farran, 2001).

We must not be disheartened by such results. Publicly funded preschool programs such as Head Start often enroll the most economically disadvantaged children in the community. To study the long-term effects of these programs, researchers sometimes have difficulty finding an appropriate control group. For instance, they may compare children who attended the programs with children who, though not attending preschool, grew up in more advantaged circumstances (Schnur, Brooks-Gunn, & Shipman, 1992). Furthermore, early intervention often leads to long-term improvements in areas not reflected in IQ test scores. For instance, children who attend intensive, developmentally appropriate academic preschool programs are, later on, more likely to have high achievement motivation and self-esteem, less likely to exhibit serious behavior problems or need special education services, and more likely to graduate from high school (F. A. Campbell & Ramey, 1995; McCall & Plemons, 2001; Spencer et al., 2001; Washington & Bailey, 1995).

Early intervention is most effective in fostering intellectual development when it is tailored to children’s existing abilities and interests. But bombarding infants and small children with constant or intense stimulation is not effective. Children seem to have a natural desire to learn about their environment, and most eagerly explore their surroundings. But they can handle only so much information—and certainly only so much new information—at any one time. Furthermore, pushing young children into exceptionally challenging (perhaps age-inappropriate) activities can cause stress, depression and, in some cases, physical harm (Elkind, 1987). And ultimately, a secure, supportive relationship with one or more caregivers or teachers is just as important as age-appropriate toys and activities (S. Loeb, Fuller, Kagan, & Carrol, 2004; NICHD Early Child Care Research Network, 2002b).

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.

Notes

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.