ABSTRACT
Twin studies are usually thought of as providing information about genetic influences. They can, with equal validity, be used to examine environmental influences on cognitive abilities in adulthood. Clearly, in partitioning variance into genetic and environmental components, twin studies demonstrate not only heritability; environmental influences are also documented and further partitioned into shared and nonshared components. Behavioral genetic research that examined environmental influences on behavior has shown that shared environmental effects are important for specific cognitive abilities, especially during childhood (Plomin & Daniels, 1987). Although studies of adolescents (Scarr & Weinberg, 1978) and adults (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990) failed to find significant evidence for shared environmental influences on cognitive abilities, results from studies of older adult twins affirm their importance (Pedersen, Plomin, Nesselroade, & McClearn, 1992; Tanabs, Sundet, & Magnus, 1984). Evidence from both cross-sectional and longitudinal studies converges on the conclusion that heritability for general cognitive ability increases from infancy through childhood and adolescence, plateaus in adulthood, and decreases late in life (Pedersen & Lichtenstein, 1997). As Fig. 8.1 illustrates, the proportion of variance in cognitive abilities explained by environmental influences, both shared and nonshared, is largest during childhood and late adulthood. The aim of this chapter is to focus on the environmental portion of variance in cognitive abilities during adulthood within the context of genetically informative designs.
