ABSTRACT
The goals of developmental theory and research are twofold. First, it is necessary to know what develops; second, it is necessary to know how these new developments occur. Both of these questions have long and venerable traditions and are reflected in the major developmental theories of the 20th century. For example, Piaget's (1954) theory of cognitive development described how the child's knowledge of many fundamental categories of knowledge—such as object, space, time, and causality—changed with cognitive development. This theory also attempted to explain these developmental changes in terms of a specific form of equilibration based on the processes of assimilation and accommodation (Piaget, 1952).
