General cognitive ability, or general intelligence, may be the area in which we know the most from a behavior-genetic perspective. Bouchard and McGue (1981) noted over 140 studies of this domain, which yielded the largely consistent result that genetic differences account for approximately 50% of the observed variability in general cognitive ability. However, it was not until recently that we began to learn more about cognitive ability than this simple, albeit important, finding.
The conceptualization of general cognitive ability as variance shared by a number of tests of various specific abilities dates back to Spearman (1904). The development of general cognitive ability, conceptualized in this manner, from infancy through middle childhood, is the focus of this chapter. Longitudinal twin and sibling data can be used to address two aspects of the developmental process. The first aspect concerns the sources of observed variation in individual differences in general cognitive ability at each age of assessment. By comparison of correlations from identical (monozygotic) and fraternal (dizygotic) twins, as well as adoptive and biological siblings, behavior-genetic methodology allows us to partition the observed variation into variation due to differences between individuals in genetic makeup and environmental differences between individuals. These environmental differences can be further subdivided into those shared by members of the family and those nonshared environmental influences that are unique to individuals. We can then examine whether heritable and environmental contributions change across the developmental period in question.