Jean Piaget's theory of cognitive development is the most comprehensive and influential account of how children's thinking changes from birth to adolescence. Piaget proposed that children are active constructors of knowledge who progress through four invariant stages: sensorimotor (0-2 years), preoperational (2-7), concrete operational (7-11), and formal operational (11+). Each stage represents a qualitatively different way of understanding the world.
The Four Stages
In the sensorimotor stage, infants learn through physical interaction with the environment, developing object permanence and early symbolic thought. In the preoperational stage, children develop symbolic representation (language, pretend play) but are limited by egocentrism (difficulty taking others' perspectives) and lack of conservation (failing to recognize that quantity is preserved despite changes in appearance). In the concrete operational stage, children master conservation, classification, and seriation but reason only about concrete objects and events. In the formal operational stage, adolescents develop abstract, hypothetical, and systematic reasoning.
Core Mechanisms
Piaget proposed that development is driven by adaptation through two complementary processes: assimilation (incorporating new experiences into existing schemas) and accommodation (modifying schemas to fit new experiences). When existing schemas cannot handle new information, disequilibrium occurs, motivating accommodation and producing cognitive growth. This constructivist view — children actively building understanding through interaction with the environment — remains Piaget's most enduring contribution.
Research has shown that Piaget underestimated children's abilities (infants show some object permanence earlier than he claimed) and overestimated stage-like transitions (development is more gradual and domain-specific than his theory suggests). Nevertheless, his identification of qualitative changes in children's thinking, his constructivist epistemology, and his clinical interview method transformed developmental psychology and continue to influence education, AI, and cognitive science.