Piaget identified four clearly delineated and structured stages of cognitive development:

Piaget recognized that infants are born with the basic abilities necessary for intellectual development, and he focused his research on sensory abilities and motor reflexes. Piaget believed that the nature of intelligence and learning is qualitatively different at each developmental stage and that the accomplishments of each stage build on those of the previous stages: Inadequate development at any stage affects achievement in later stages. His theory encompasses a learning dynamic that is a balance between the novel and the familiar, an exchange between individuals and their environments, and the ongoing lifelong process by which individuals organize their worlds by organizing themselves, a psychological view known as constructivism.

Early twenty-first-century theories of cognitive development are generally in agreement with Piaget's developmental milestones, which provided some of the foundation for constructivist learning theories and promoted the discovery and support of children's developing interests as primary instructional techniques. However, subsequent research modified various details of Piaget's model. In particular, Piaget described the stages of cognitive development as occurring in discrete steps or leaps, whereas developmental psychologists tend to view cognitive development as a continuum. For example, studies have found that children between the ages of five and eight, who are at the cusp of logical thinking, may use intensional logic but not extensional logic. Intensional logic defines the properties of a class. Extensional logic determines who or what can be a member of a particular class. Thus, children who understand the meaning (intensionality) of a category of objects that are red might decide not to include certain red objects for reasons that an older child or adult would find illogical, such as the object is too small. Piaget knew that preoperational children could practice intensional logic, but he believed that such incomplete logical thought was, by definition, prelogical. Piaget also suggested that before ages three or four, egocentrism prevented children from understanding that other people's thoughts and viewpoints may differ from their own, a concept known as theory of mind. It is now clear that theory of mind begins to develop at an earlier age.

Piaget's belief that biological development drives movement to the next cognitive stage has generally Page 894  |  Top of Articlebeen supported by cross-sectional studies of children in various Western cultures, at least for Piaget's first three stages. However, studies of adolescents indicate that only 30%–35% of high-school seniors have reached the formal operational stage, and many adults do not think formally. Thus, although biological maturity makes it possible to attain this stage, the transition is not automatic; rather, special environmental factors may be required.