Considered by many to be the founder of modern developmental psychology, the Swiss psychologist Jean Piaget devoted his researches to children’s distinctive ways of knowing and to the process of developmental change leading toward adult thought. He charted a sequence of stages in children’s intellectual development whose manifestations encompass domains ranging from logical reasoning to emotional development. Trained in biology and philosophy, Piaget extrapolated from his studies of mol-lusks in their natural habitat to a conception of the development of intelligence in children as a progressive adaptation, tending toward ever greater equilibrium, with its reciprocal aspects of the "assimilation" of new information to existing concepts and the "accommodation" (i.e., modification) of the concepts to the new information. He viewed the development of intelligence, in turn, as the foundation of a "genetic epistemology," a theory of knowledge that conceived the development of ideas as part of their essence. On the basis of his observations of children’s cognitive development and his claim that children’s own action catalyzes that development, Piaget altered the face of psychology and education.
BASIC CONCEPTS: DEVELOPMENTAL STAGES
Piaget became convinced that children exhibit a distinctive type of thinking, as opposed to simply flawed adult thought, when, as a young associate working on intelligence testing in the laboratory of Alfred Binet (1857-1911), he noticed that when children answered items incorrectly, they tended to give the same wrong answer. He inferred that the children must have approached the problems methodically, only the method differed from that of adults. After altering the testing methods to include the exploration of children’s answers and devising many probes of his own, he set about determining the intellectual organization of what he eventually conceived as four broad stages of development, each embodying a progression toward increasingly flexible, systematic, and complex thought: sensorimotor, preopera-tional, concrete-operational, and formal operational.
During the sensorimotor period, extending from birth to approximately eighteen months, intelligence manifests in action. Action develops within the period from reflexive movement to means-ends behavior that comes, by the last of six substages, to incorporate tool use, foresight, and detours ( 1952). The advent of representational intelligence in the sixth substage manifests in additional ways, including deduction based upon remembered events, the imitation of events witnessed previously, symbolic play, and language. Thought now progresses to some degree independently of what is seen or otherwise directly experienced. For example, an eighteen-month-old child who sees a toy hidden in a box, then sees the box moved under a cloth, and then finds the box empty, will spontaneously search under the cloth for the missing toy, evidently having inferred that the toy left the box when the box was under the cloth (an instance of deduction). Children at this age might also imitate a funny face someone made the day before (an instance of deferred imitation) or slide a leaf along the countertop as though the leaf were a car (an instance of symbolic play). Although any of these representational activities might occur in the absence of language, language, which also depends upon conceptual connections, normally begins at around the same time.
These symptoms of nascent representational thought mark the beginning of Piaget’s second broad stage, of preoperational or intuitive intelligence, which extends roughly from two to seven years of age. Piaget found that, despite the advances it embodied, thought during this interval exhibited limitations, in particular with respect to the property of what he termed reversibility. The limitation is apparent in Piaget’s best-known experiment with school-age children, his probe of what he called the "conservation" of quantity. When water, for example, is poured from a wide flask into a thin one, although adults or older children know the water level must rise, children under the age of (approximately) seven years sometimes anticipate that the water level will remain the same. Alternatively, when confronted with the raised water level after the water is poured, they may assert that the new (thin) flask contains more water than did the original (wide) flask ( 1965).
By comparison, when presented with the same problem, children between seven and twelve years, normally the stage of concrete-operational thought, say the amount of liquid must remain the same because, for example, if one returned the water to the original flask, the water would rise to the level it reached initially. Or, they might say that although the level is higher in the second flask than it was in the first, the second flask is also narrower, or that nothing was added or taken away. Piaget perceived in these arguments the imagined reversal of an observed state of affairs and the construction of compensations between different variables of the problem.
The state of affairs had to be concrete and observable, however, whereas from roughly twelve years onward, during the formal operational stage, individuals can reason about hypothetical states and solve both abstract and concrete problems systematically, by taking account of, and if necessary manipulating, all variables pertinent to a problem. Thus, as a scientist would do, a child at this stage confronted with the problem of determining which of a series of chemicals was responsible for altering the color of a solution, for instance, would systematically vary each possible combination of liquids to isolate the necessary reaction (Inhelder and Piaget  1958).
Development through the stage of concrete operations is believed to be universal, whereas only a fraction of even well-educated teenagers from the United States exhibit formal operational reasoning when they are given the problems originally employed by Piaget. As suggested by Jean Retschitzki (1989) in a study of expert players of a popular game in the Ivory Coast, however, evidence exists that formal-operational reasoning may be employed even by members of undeveloped countries with low literacy when people participate in culturally indigenous activities in which they are expert.
Piaget characterized these tiers of development in a second way, as embodying three "Copernican revolutions" ( 1968), in which children, initially "egocentric" at the level in question, gradually progressed toward the ability to take alternative perspectives into account. Although Piaget eventually retracted the term egocentric in response to its apparent misconstrual by other psychologists, he retained the substantive theses underlying it.
During the sensorimotor period of infancy, as conceived within this second scheme, children come gradually to acknowledge a self, a world, and their separation. They come to recognize, for example, that their caregivers come and go on their own and that the external world generally operates independently of them ( 1954).
During the preoperational period of early childhood, children progress from an obliviousness to the perspective of others to the awareness that others may perceive things differently from what they perceive, and they anticipate the alternative perspective. Thus, for instance, whereas when an interlocutor’s back is turned, a four-year-old might point to the location of an object for which the interlocutor is searching, an older child would describe the location ( 1955).
Corresponding to the advent of the capacity for formal operations is an interest in abstract ideals and, according to Piaget, a progression during adolescence from the view that the world should submit to one’s schemes to the understanding that one does not know everything ( 1968).
Piaget believed his stage sequences extended to many areas of cognition, including logical thought and children’s conceptions of objects, space, time, causality, number, chance, and probability, as well as aspects of perception and memory. He perceived their manifestations also in areas of social life, especially morality. Corresponding to the progression he observed toward concrete operations, for example, he documented the emergence of an understanding of and interest in social rules and an appreciation of their purpose in regulating people’s relations and protecting their welfare. He also documented a trend from the negative moral valuation of people whose actions produce bad outcomes to the negative valuation of those whose intentions are bad ( 1965).
In delineating the myriad developments he did, Piaget ascribed greater importance to the sequence of developments he described than to the uniformity of development across areas of activity. He believed, however, that the capacity for more primitive types of reasoning remained in all areas throughout life and, especially in the case of morality, were more evident in some individuals and groups than in others.
BASIC CONCEPTS: THE PROCESS OF DEVELOPMENT
All of the developments Piaget described resulted, he believed, from natural, spontaneous development. With the possible exception of moral judgment, whose early stages he suggested reflected the influence of adult disciplinary tactics, the developments were not taught or otherwise prompted by the environment, which at most afforded (or limited) opportunities for growth. He contended, moreover, that the advances he charted arose through feedback from children’s own action, as opposed to from the maturation of innately given abilities.
Piaget was sufficiently convinced of the potential of the mind to construct itself that he believed the entire series of developments he described evolved from a starting point of only reflexive action and the "invariant functions" of assimilation, accommodation, and the progressive storage of their results, which Piaget called organization. Merely by sucking, grasping, listening, and looking—initially reflexively—and repeating these behaviors, as followed from the inborn tendency to repeat experiences (the most primitive manifestation of assimilation), the resulting chance effects would eventually bring about change in the actions. Further changes would follow from there, until individuals reached the final equilibrium of adult forms of thought ( 1952).
CRITICISM OF THE THEORY
Piaget’s studies of children have been replicated worldwide, in both developed and undeveloped countries. Many have also extended his sequences to domains he did not investigate, such as attachment relations in infancy and friendships during childhood.
The theory has also been challenged by researchers who, after modifying the measures Piaget used to assess the abilities he investigated, have produced results the researchers believe attest to children’s grasp of mature concepts at an earlier stage than Piaget specified. Some of this work appears in reviews by Paul Harris (1989) and John Flavell and colleagues (2002). The researchers assert that the results warrant a different model of development in which, rather than gradually developing the concepts in question, as Piaget described, children begin development in possession of the concepts’ essentials and progressively "access" the essentials in increasingly reflective thought and over ever broader areas of application; Paul Rozin lays out a version of the model in "The Evolution of Intelligence and Access to the Cognitive Unconscious" (1976). According to the general view, younger children falter on Piaget’s tasks not because they lack the concepts under examination, but because they become confused by nonessential features of Piaget’s experiments. With age, the view says, as children’s working memory and atten-tional capabilities increase, children became better able to negotiate Piaget’s tasks.
Some researchers in the foregoing line have challenged Piaget’s theory based not upon new experimental procedures, which most of the aforementioned studies employ, but upon naturalistically occurring behavior, a focus more in keeping with Piaget’s original observations. For example, in 1973 Marilyn Shatz and Rochel Gelman reported that, contrary to the idea that preschoolers fail to take in others’ perspectives when addressing them, four-year-olds simplify their speech when they talk to two-year-olds. More recently, Debra van Ausdale and Joseph Feagin (2001) found evidence in preschoolers’ social interactions of racist ideas and practices that the authors believe possible only with the "operational" thought Piaget ascribed to middle childhood.
Another line of criticism questions the cogency of Piaget’s theoretical constructs independently of the data, as exemplified by works by Sophie Haroutunian (1983) and Susan Sugarman (1987a), or the formalisms he used to represent the constructs, as argued by Daniel Osherson (1974).
Few dispute Piaget’s sequences as measured by his own tests. Controversy continues about the equivalence of the concepts measured by newer tests to the concepts assessed by Piaget’s procedures. The two bodies of work differ fundamentally in method. Whereas the work that challenges Piaget’s norms replaces his tests at younger ages, Piaget extrapolated to his sequences from developmental changes in children’s behavior on the same measure at all ages he tested; the strategy dates to Piaget’s days as a researcher in intelligence testing. The difference makes it difficult to render any final conclusion about the import of the apparently conflicting results (see Sugarman 1987b for discussion of this).
With respect specifically to the challenges brought by fresh observations of naturally occurring behavior, evidence can be found within Piaget’s own observations of allegedly "preoperational" thought more complex than that ascribed by his account of preoperational thinking (Sugarman 1987a). Nonetheless, the particular complexities Piaget associated with "operational" thinking, in the abstract reflective thought in which he sought them, do not appear in either Piaget’s or newer data. The claim that the racist ideas and practices of some preschoolers require operational thought presents the additional problem that racism bears precisely the hallmarks of preoperational mentality as Piaget originally defined it. These properties include a one-sided point of view, often based upon appearances, that does not take account of alternative vantage points, is impervious to contradiction, and consequently remains unaware of itself as a point of view. Especially in the moral domain, which, as Piaget discusses in The Moral Judgment ofthe Child ( 1965), draws upon incompletely comprehended adult influences, thought can become rigid and nearly mystical as a result of these tendencies. Given the persistence of these characteristics into adult morality, the telling question raised by the observation of racism in preschoolers might be not how preschoolers manage to exhibit racism, but why adults remain susceptible to it, given their apparent possession of more sophisticated forms of thought.
Despite the empirical challenges to his theory, Piaget’s developmental milestones in children’s thought dominate research in developmental psychology and retain a strong influence in education. Although conceptual critiques of the theory suggest the presence of gaps in the edifice, Piaget’s general philosophy of how development occurs, namely through the exertions of a knowing subject, remains widely embraced.