Effects of Heredity and Environment on Intelligence
It is often difficult to separate the relative influences of heredity and environment on human characteristics. People who have similar genetic makeup (e.g., brothers and sisters, parents and their children) typically live in similar environments as well. So when we see similarities in IQ among members of the same family, it is hard to know whether those similarities are due to the genes or to the environments that family members share. Nevertheless, a significant body of research tells us that both heredity and environment affect 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.
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:
|Correlations of Twins’ IQs:|
|Identical twins raised in the same home||.86|
|Identical twins raised in different homes||.72|
|Fraternal twins raised in the same home||.60|
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
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.
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