environments with greater diversity can have beneficial effects. However, it would be misleading to locate the complexity of the animal environments near the enriched part of the continuum from deprivation to enrichment. In fact, the environments in these experiments were probably less complex than the rats' natural wild habitats. Nevertheless, these studies do point to the existence of a multidimensional continuum of environments and indicate that development (and recovery) is improved as one moves toward the anchor point of enrichment. Moving from these animal studies to research on the neurological aspects of human cognitive, linguistic, and social-emotional development is a big leap, but one that warrants a major investment of time and resources. The need for research that can illuminate how environments that exceed some minimal threshold of adequacy affect human brain development is especially needed, in light of the fact that most of the research on how experience affects the developing brain explores the detrimental consequences of harmful experiences.

Research on early biological insults provides fundamental insights into the vulnerability and resilience of the developing central nervous system. This area of research also offers a compelling illustration that plasticity cuts both ways, leaving the developing fetus and young child simultaneously vulnerable to harm and receptive to positive influences. It also suggests that the current emphasis on the years from birth to age 3 may have unwittingly bypassed an important stage of development: the prenatal period is when damaging environmental conditions may have some of the most devastating effects on development and, consequently, is when preventive efforts may have the greatest benefits.

Environmental factors that play a significant role in modulating prenatal and early postnatal brain development include substances and circumstances that are necessary for normal brain development, as well as exposures to chemicals, diseases, and stressors that are toxic or disruptive. As with psychosocial risks, such as poverty and family violence, their effects on development are probabilistic; they increase, but do not seal, the odds of impaired development. Table 8-1 lists some of the environmental factors that are beneficial and some that are detrimental. The factors listed, by no means exhaustive, are examples selected on the basis of clinical importance, availability of basic research on brain effects, and existence of relevant clinical studies of human infants. We consider below a few of these detrimental conditions and substances in more detail: an infectious disease (rubella); a developmental neurotoxin (alcohol), and a nutrient deficiency (lack of iron).