HERITABILITY AND INTELLIGENCE

One need only think of one's fellow men and women to realize that individual intellectual makeups vary in numerous ways. Psychology's most longstanding indicator of individual differences in intellectual functioning is the intelligence quotient (IQ). Although IQ was originally invented to differentiate children into groups of those whose level of intellectual functioning corresponds to that of peers of similar age and those whose level of mental functioning does not, it has since evolved into the most frequently used indicator of individual differences (or variability) in intellectual functioning. The point of this chapter, however, is not to analyze individual differences in IQ per se, but, rather, the sources of these differences. In other words, what forces make people similar (or dissimilar) in IQ?

The sources of variability in IQ have been the subject of a field known as the psychology of individual differences. This subfield is one of the oldest in psychology, and, like any mature field, it has witnessed successive theoretical transformations. In this case, different theories have been employed to explain the etiology of the traits in which people differ (e.g., Kimble, 1993). The landmark of research on the etiology of individual differences in IQ is the concept of heritability, a statistic describing the proportion of phenotypic (observed) differences among individuals in a population that can be attributed to genetic differences between them. It has been said that there are very few subfields in psychology as controversial and yet sterile as that of heritability-based studies of IQ (Danzinger, 1997). The debates concerning these studies are remarkable in number and pervasiveness.