Conservation refers to the knowledge that the quantitative properties of objects are not changed by a mere change in perceptual appearance. As adults, we take such knowledge for granted. We do not believe that the amount of juice changes if it is poured into a different-shaped container, nor are we concerned that we have less to eat if a cookie arrives in three small pieces rather than one large one.

Children, it turns out, think differently. One of the most influential discoveries in the history of child psychology was the demonstration by the Swiss psychologist Jean Piaget that young children do not at first understand principles of conservation. Across a dozen or so books and scores of experiments, Piaget and his coworkers explored just about every form of conservation imaginable: conservation of number, mass, and quantity; of length, weight, and area; of time, speed, and movement. The general approach was similar across tasks: presentation of two stimuli equal on some quantitative dimension, followed by a transformation so that the stimuli no longer looked equal. Whatever the content area, children younger than 5 or 6 consistently judged that the quantity had changed when the appearance changed. Thus, young children really seem to believe that spreading out a row of candies increases the number or that rolling a clay ball into a snake changes its weight.

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In Piaget’s theory, such non-conservation responses are a characteristic of the preoperational period of development, the period between roughly 2 and 6 when children have not yet developed logical operations and are therefore easily fooled by misleading appearances. Conversely, the gradual mastery of conservation (and some forms, Piaget showed, are slower to develop than others) is a hallmark of the concrete operational period—the period of middle childhood during which a set of mental operations evolves that allows children to solve a wide range of logical and physical problems, including the various conservation tasks.

The conservation phenomenon was the subject of hundreds of studies in the decades of the 1960s, 1970s, and 1980s. In general outline, these studies confirmed Piaget’s claims: Young children fail conservation tasks, success emerges only gradually across the childhood years, and some forms of conservation are more difficult than others. At a more specific level, however, the later work revealed a number of discrepancies from the original Piagetian account. Piaget’s assessment procedures were often unduly complicated; when the procedures were simplified in various ways, success was often evident at younger ages. Furthermore, the variability in response across different forms of conservation proved to be even greater than that reported by Piaget. Children might succeed on some conservation tasks by age 4, whereas other tasks would continue to baffle them well into adolescence. Findings such as these were a major contributor to the declining popularity of general-stage theories of development, most notably Piaget’s theory of concrete operations.

Conservation is no longer the subject of concentrated research effort, a fate that has befallen many once popular topics in the history of psychology. In large measure, however, the decline in interest reflects the success of the earlier wave of studies. Despite some differences in detail and some unresolved questions, Piaget’s general conclusions regarding conservation—including the existence of an apparently universal initial phase of nonconservation—rank among the most solidly established, and important, findings in child psychology. In addition, the general Piagetian emphases that are evident in the study of conservation—the focus on basic forms of knowledge, the interest in qualitative and not merely quantitative changes with development, the demonstration of surprising gaps in young children’s understanding— live on in contemporary work in cognitive development, most notably the burgeoning research area known as theory of mind.


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