Home | | Psychology | The Roots of Intelligence: Genetics and Individual IQ

Chapter: Psychology: Intelligence

| Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail |

The Roots of Intelligence: 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.

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).


 

Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail


Copyright © 2018-2020 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.