Environmental experience is a function of diversity and access—itself limited or controlled by a variety of social, cultural and physical factors. These factors coact with each other and the child's personality in a complex ecological process of growth and development. Urie Bronfenbrenner has outlined a unified, multilevel, ecological approach to human development that allows all the social forces impinging on the quality of a child's life to be reckoned with, in research and policy formulation. In Children's Outdoor Environment, Pia Bjorklid discusses child development from the "interactional" perspective of environmental and ecological psychology, which assumes that "the environment itself develops, and can be modified, changed and moulded by the individual." In Children's Experience of Place, Roger Hart articulates the developmental significance of the physical environment. Most child development research has been conducted at preschool level within the fixed and geographically limited boundaries of adult-managed, domestic and institutional settings, with the children's attention focused on small toys and educational materials.