Proposes that cognitive developmental stages can be accounted for in terms of information processing factors which limit the highest level of cognitive system which children can attain at any given age. Delineates four progressively more complex levels of cognitive mediation of the environment. Two experiments which support the developmental model…

Explores two hypotheses about the effect of symbolic systems on psychological functions: (1) culturally elaborated systems of signs affect psychological functioning in a general way; and (2) systems of signs, when used to support skilled action, affect the organization of human activity. Reviews research that supports the second hypothesis. (LB)