FIGURE 3-2 Idealized representation depicting displacement of subgroups with regard to main population on any variable that is normally distributed.

SOURCE: Case, Griffin, and Kelly (1999). Reprinted with permission.

of the curve back, as in the “subpopulation” for those developing in high-risk environments. This shift simultaneously increases the number of children with special needs at the lower end and decreases the number of high achievers who may be identified as gifted at the upper end. In a sense then, this chapter is about both groups, although those cases at the left tail of the distribution have been studied more because of their distinguishing characteristics than those in the right tail.

BIOLOGICAL CONTRIBUTORS TO COGNITION AND BEHAVIOR

The importance of the early years of life to development is incontrovertible (Ramey et al., 2000; Ramey and Ramey, 1999; NRC, 2000a). The unparalleled pace of brain growth and the development of fundamental cognitive, emotional, social, and motor processes make the period from conception through infancy one of exceptional opportunity and vulnerability (McLoyd and Lozoff, 2001). While the plasticity of the brain appears to extend well into adolescence, with growth in some areas of the brain as late as the third decade of life (NRC, 2000a), children who experience biological insults and stressors early in life are at greater risk for long-term developmental problems (McLoyd and Lozoff, 2001). Deprivation in the extreme can produce functional mental retardation and aberrant social and emotional behavior in animals born healthy and with good genetic endowment (Ramey and Ramey, 1999). In humans, mild mental retardation with



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