nature is deeply affected by these environments and, in turn, shapes how children respond to their experiences.

In simple terms, children affect their environments at the same time that their environments are affecting them. Moreover, no two children share the same environment, and no environment is experienced in exactly the same way by two different children. Two youngsters living in the same home influence each other and are affected by the other members of the family in unique ways. If one child is active and aggressive and the other is passive and subdued, each will elicit different responses from the parents—and each will be influenced differently by the behavior of the other.

These concepts reflect what Sameroff and Chandler (1975) characterized as the transactional nature of the developmental process and what Bronfenbrenner (1979) described as the ecology of human development (also see Horowitz, 1999). This transactional-ecological model of development provides a useful framework that moves far beyond the misleading and tired old nature-nurture debate. It helps people think in more sophisticated ways about the complex determinants of successful adaptation and health as well as those of maladaptation and disorder. It offers insights into how the same behavioral disposition may be adaptive in one context and not in another. It also fits well with what scientists are learning about the dynamic nature of the development of the brain.

Children vary in their behavioral style. Some are high-strung and some are laid-back; some are agile and some are clumsy. Children are raised in a wide variety of social circumstances and cultural contexts. Some conditions are secure and others are unstable; some encourage competition and others promote cooperation. Behaviors that are highly adaptive in one society (e.g., competitiveness among preschoolers in the United States) may not be so in another (e.g., individual assertiveness among preschoolers in Japan). Different childrearing environments promote distinctive patterns of skill development in some children and not in others (e.g., some may reinforce active, physical performance while others encourage quiet, artistic expression).

At every level of analysis, from neurons to neighborhoods, genetic and environmental effects operate in both directions (Gottlieb, 1992). On one hand, the gene-environment interactions of the earliest years set an important initial course for all of the adaptive variations that follow. On the other hand, this early trajectory is by no means chiseled in stone. The considerable degree of developmental plasticity that characterizes an immature organism is embodied in the capacity of its cells to adapt in very specific ways, in both the short and the long run, to changing demands. Neurons grow new axons, sprout new dendrites, form new synapses, and modify the strength of some established connections while eliminating others selectively over time. The impacts of varied experiences are also reflected in