Genetics and Individual IQ
As we first saw, there are several ways to evaluate how strongly a given trait (eye color, height, career choice) is influenced by genetics. One of our main methods, though, begins with an examination of relatives, asking in particular whether people who resemble each other genetically also resemble each other in terms of the target trait. For measures of intelligence, it turns out that the correlation between the IQs of children and the IQs of their biological parents is about !.40; the correlation between the IQs of biological siblings is roughly the same. These correlations indicate a relatively strong resemblance, but these correlations, on their own, are ambiguous. On the one side, bio-logically related family members resemble each other genetically, and this might be the source of the resemblance in IQ scores. But on the other side, the members of a family usually also resemble each other in their experiences: They live in similar social and financial circumstances; they all receive similar levels of health care and are likely to receive similar levels of education. It’s plausible, then, that the resemblance in their IQs might be due to this shared environment rather than their overlapping sets of genes.
Clearly, then, we need better evidence to help us untangle the hereditary and envi-ronmental contributions to intelligence—and some of that evidence comes from the study of twins. As we’ve mentioned, there are two types of twins: Identical, or monozygotic(MZ), twins originate from a single fertilized egg. Early indevelopment, that egg splits into two exact replicas which develop into two genetically identical individuals. In contrast, fraternal, or dizygotic (DZ),twins arise from two dif-ferent eggs, each fertilized by a different sperm cell. As a result, fraternal twins share only 50% of their genetic material, just as ordinary (nontwin) siblings do.
Identical twins, therefore, resemble each other genetically more than fraternal twins do; and this fact makes it striking that identical twins resemble each other in their IQs more than fraternal twins do. In an early summary of the data, the correlation for iden-tical twins was .86; the correlation for fraternal twins was strongly positive but consid-erably lower, around .60 (Bouchard & McGue, 1981). Other, more recent data confirm this pattern (Figure 11.14). This certainly suggests a strong genetic component in the determination of IQ , with greater genetic similarity (in identical twins) leading to greater IQ similarity.
The impact of genetic factors is even clearer when we consider results obtained for identical twins who were separated soon after birth, adopted by different families, and reared in different households. The data show a correlation for these twins of about .75, which is not substantially less than the .86 correlation for identical twins reared together (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990; McGue, Bouchard,
Iacono, & Lykken, 1993; Plomin & Spinath, 2004). It appears, then, that identical genotypes lead to highly similar IQs even when the individuals grow up in different environments
Similar conclusions derive from a study that drew its data from the Colorado Adoption Project (CAP). The CAP has been tracking 245 adopted children for roughly 20 years, testing them periodi- cally on several different measures (Plomin, Fulker, Corley, &DeFries, 1997). Thus, we have intelligence scores for the children themselves at various ages; we also have scores for the children’s biological parents, who each share 50% of their genetic material with the children but who are not the adults who raised the chil- dren. Third, we have scores for the adoptive parents—the adults who did raise the children and shared (and largely created) the environment in which the children grew up.
These scores allow us to compute the resemblance between the children and their biological parents, as an indicator of how much shared genes matter. The scores also allow us to compute the resemblance between the children and their adoptive parents, as an indicator of how much a shared environment matters. The data ndicate a much greater resemblance in the first comparison—children and their bio- ogical parents—even though we’re comparing individuals who (though biologically related) have never even met. This indicates a powerful role for genetic factors in shap- ing intellectual ability (Figure 11.15).
What’s especially striking about the CAP data, though, is that the resemblance between children and their biological parents increases as the years go by. When thechildren are 4 years old, for example, there’s roughly a .10 correlation between the chil-dren’s intelligence scores and their biological parents’ scores. By the time the children are 12, this correlation is almost .20. By the time the children are 16 years old, this cor- relation is almost .40—despite the fact that, by that point, it has been more than a dozen years since the children and their biological parents have seen each other!
How should we think about this result? One possibility is that what’s inherited via the genes is a learning capacity—and so, in early childhood, a child’s potential might resemble that of her biological parents, but the potential hasn’t yet grown into skills we can measure. To detect the resemblance, we must wait until the child has had some experience in the world—and thus opportunity to use her learning capacity and to gain from the potential she inherited. Only then, when the potential has borne fruit, can we detect the full resemblance between parents and their biological off- spring (cf. Plomin & DeFries, 1985; Plomin & Spinath, 2004; Figure 11.16).