In studying child development, the traditional assumption has been that a line can be drawn between the influences of nature and those of nurture. Nature contributes the structure of the brain and its schedule of maturation. Nurture provides for the psychological growth of the child and the flexible processes of learning. For those interested in cognitive and emotional development, nurture has seemed to be the relevant domain. Given that a child has an intact brain, a knowledge of neural mechanisms has not seemed necessary for a theory of human psychological development. Similarly, for studying brain development, it has not seemed important to consider whether psychological experience can alter the course of neuroanatomical differentiation.