Piaget's transformation of society's conception of childhood thought and intelligence is described in 4 periods in the history of his research program, which spanned from the 1920s to the 1980s. The account stresses the enduring contribution to developmental psychology of Piaget's constructivism, his description of developmental mechanisms, his cognitivism, his explication of structural and functional analysis, and his address of epistemological issues and nontraditional methodologies. No one affected developmental psychology more than Jean Piaget (1896-1980).' From his earliest publications in the 1920s to the time of his death, the influence he exercised was extraordinary. His theory, which has no rival in developmental psychology in scope and depth, underwent change from beginning to end. With one posthumous publication appearing after another, it is still undergoing change. Nevertheless, the theory has maintained continuity in most of its core assumptions, despite one or another of its features being transformed by additions, deletions, or changes in emphasis and interpretation. In the end, it is more than a theory: It is a research program on a vast scale (Beilin, 1985). The number of experiments conducted by Piaget and his colleagues has never been tabulated, but it is unrivaled in the history of developmental psychology. At the same time, it is difficult to identify a theory that has been more debated and attacked than Piaget's. The curve of the theory's popularity looks more like a business cycle with peaks and troughs than a classical growth curve, with eventual decline. Although the theory appears to have passed its peak in popularity, it is difficult to imagine its disappearance. After the Second World War, when Piaget's theory appeared in a new guise to the English-speaking community, it created a near sensation with its striking counterintuitive experimental data and bold theoretical claims that struck at the heart of the then-dominant neobehaviorism. The reaction at first was to attempt to replicate his findings or to attack the research on methodological or theoretical grounds. Investigators, for the most part, took the path of least resistance and focused on the most counterintuitive findings (e.g, the conservations) and on those phenomena that seemed to embody the clearest theoretical claims (e.g., the counterempiricist training claims). The strategy was to strike at Achilles's heel, and Achilles (Piaget's theory) would fall flat on his face. Thus, there were countless (ad nauseam) studies of conservation, the object concept, formal operational reasoning, training, and more, some of which continues. The negative consequence of this strategy has been a generally distorted picture of Piaget's theory that has hindered a full appreciation of the theory's potential contribution.
