The Piaget stages of development is a blueprint that describes the stages of normal intellectual development, from infancy through adulthood. This includes thought, judgment, and knowledge. The stages were named after psychologist and developmental biologist Jean Piaget, who recorded the intellectual development and abilities of infants, children, and teens.
Piaget's four stages of intellectual (or cognitive) development are:
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Preoperational. Toddlerhood (18-24 months) through early childhood (age 7).
Concrete operational. Ages 7 to 12.
Formal operational. Adolescence through adulthood.
Piaget acknowledged that some children may pass through the stages at different ages than the averages noted above and that some children may show characteristics of more than one stage at a given time. But he insisted that cognitive development always follows this sequence, that stages cannot be skipped, and that each stage is marked by new intellectual abilities and a more complex understanding of the world.
During the early stages, infants are only aware of what is immediately in front of them. They focus on what they see, what they are doing, and physical interactions with their immediate environment.
Because they don't yet know how things react, they're constantly experimenting with activities such as shaking or throwing things, putting things in their mouths, and learning about the world through trial and error. The later stages include goal-oriented behavior which brings about a desired result.
At about age 7 to 9 months, infants begin to realize that an object exists even if it can no longer be seen. This important milestone -- known as object permanence -- is a sign that memory is developing.
After infants start crawling, standing, and walking, their increased physical mobility leads to increased cognitive development. Near the end of the sensorimotor stage, infants reach another important milestone -- early language development, a sign that they are developing some symbolic abilities.
During this stage, young children are able to think about things symbolically. Their language use becomes more mature. They also develop memory and imagination, which allows them to understand the difference between past and future, and engage in make-believe.
But their thinking is based on intuition and still not completely logical. They cannot yet grasp more complex concepts such as cause and effect, time, and comparison.
Concrete Operational Stage
At this time, elementary-age and preadolescent children demonstrate logical, concrete reasoning.
Children's thinking becomes less egocentric and they are increasingly aware of external events. They begin to realize that one's own thoughts and feelings are unique and may not be shared by others or may not even be part of reality. Children also develop operational thinking -- the ability to perform reversible mental actions.
During this stage, however, most children still can't tackle a problem with several variables in a systematic way.
Formal Operational Stage
Adolescents who reach this fourth stage of intellectual development are able to logically use symbols related to abstract concepts, such as algebra and science. They can think about multiple variables in systematic ways, formulate hypotheses, and consider possibilities. They also can ponder abstract relationships and concepts such as justice.
Although Piaget believed in lifelong intellectual development, he insisted that the formal operational stage is the final stage of cognitive development, and that continued intellectual development in adults depends on the accumulation of knowledge.
Wood, K. "Piaget's Stages of Cognitive Development," in M. Orey Emerging perspectives on learning, teaching, and technology, 2001.
PBS.org: "Piaget describes stages of cognitive development 1923-1952."
Huitt, W. "Piaget's Theory of Cognitive Development. Educational Psychology Interactive," 2003.