Piaget hypothesized sequences of age-related changes in ways of dealing with reality. His conclusions were based on the careful observation of a few selected cases. The voluminous research since Piaget's time overwhelmingly supports the sequence he outlined. The process almost never reverses. Once a child understands the conservation of substance, for example, his or her former conclusion that "Now there is more" seems to the child not simply wrong but absurd. Even within a stage, there is a sequence. Conservation of mass, for example, precedes conservation of volume.
Post-Piagetian research has nevertheless led to a fine-tuning of some of Piaget's conclusions and a modification of others. Piaget believed that transitions to more advanced cognitive levels awaited neurological maturation and the child's spontaneous discoveries. Several researchers have found that specific training in simplified and graded conservation and categorization tasks can lead to an early ripening of these skills. Other research has called into question Piaget's timetable. The fact that, within a few months of birth, infants show subtle differences in their reactions to familiar versus unfamiliar objects suggests that recognition memory for objects may begin earlier than Piaget's age for object permanence. If conservation tasks are simplified—if all distraction is avoided, and simple language and familiar materials are used—it can be shown that concrete operations also may begin earlier than Piaget thought. Formal operations, on the other hand, may not begin as early or be applied as universally in adult problem solving as suggested by Piaget's thesis. A significant percentage of older adolescents and adults fail tests for formal operations, particularly in new problem areas.
More basic than readjustments of his developmental scheduling is the reinterpretation of Piaget's stages. The stage concept implies not only an invariant sequence of age-related changes but also developmental discon tinuities involving global and fairly abrupt shifts in an entire pattern or structure. The prolonged development and domain-specific nature of many operational skills, however, suggest a process that is neither abrupt nor global. An alternative view is that Piaget's sequences can be understood as the results of continuous improvements in attention, concentration, and memory. Stages represent only transition points on this continuous dimension. They are more like the points of a scale on a thermometer than the stages of the metamorphosis of a caterpillar into a moth.
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