Perhaps more than any single theme, the description of psychological growth as a tendency toward differentiation has been a unifying principle between accounts of perceptual and cognitive development. The principle has been invoked to describe development at almost all levels of cognitive functioning. Thus, Gibson (1969) makes differentiation the cornerstone of her influential theory of perceptual development, and the principle has been invoked also to describe trends in the development of conceptual knowledge, classification, problem solving, and personality (Werner, 1948, 1957). Flavell (1977) identifies differentiation as a principle operative “in many or all cases where cognitive growth occurs [p. 240].” Werner (1957) suggests that it is a defining property of all natural developmental processes.