Jean Piaget is perhaps best known for his constructivist model of childhood cognitive development, which he based on longitudinal case studies and simple experiments involving his own children. In trying to develop an objective and logical structure for childhood cognitive development, Piaget also established the rudiments of epistemology, upon which all other subsequent developmental psychologists would build.
Piaget's model identifies four major stages of cognitive development, labelled by the specific sensory component through which the child learns during that stage. Each new stage is dependent upon successful cognitive acquisition of the lessons of the previous stage.
From birth until approximately two years of age, generally bounded by the child's reliable ability to walk, is the sensorimotor stage. During this stage, the child learns by physically interacting with its environment. This stage divides into six phases:
* reflex schema, lasting roughly until six weeks of age, during which reflexes gradually become voluntary actions;
* primary circular reaction, from six weeks to four months of age, during which repetitive action establishes motor schema, through classical and operant conditioning, which can thereafter be used for a purpose;
* secondary circular reactions, from four to nine months of age, which begin to establish hand-eye coordination and thus an objectified sense of logic as accidental behaviour gradually becomes purposeful behaviour with known consequences;
* co-ordination of secondary circular reactions, from nine to twelve months of age, which allow a child to learn that the solid objects in its universe have an objective existence independent of its ability to perceive them, and thus begins to develop the concept of planning;
* tertiary circular reactions, from twelve to eighteen months of age, during which the child tests new means to attain desired goals;
* and finally the invention of new means through mental combinations stage, where the child begins to internalise future testing, indicating the beginnings of insight.
Between the approximate ages of two and seven years is the pre-operational stage, during which the child learns to refine motor functions and uses its newly-attained intuition to apply them in new ways. During this stage, the child tries to generalise learned responses into wider patterns, occasionally with very strange but consistent results. For example, as the child develops the capacity to think symbolically, the symbols used may not match the standard ones, but may nevertheless be used in consistent and internally appropriate ways. Other temporary failures of generalisation are centration, where the child seems able to pay attention to only one quality of an object at a time, which leads to failure of conservation (eg. liquid poured from a tall, narrow container into a short, shallow one may be perceived to have decreased, because the child is only paying attention to the height of the container); egocentrism, where the child consistently and solely applies its own perspective to others; and finally animism, where the child also applies its own living perspective to an inanimate object such as a doll.
Following the pre-operational stage is the concrete operational stage, which lasts roughly until the age of 11 years. It is during this stage that the child begins to categorise its environment by way of learning to understand concrete and abstract similarities and differences: which in turn also allows it to begin to acquire non-egocentric perspective. Multiple aspects of objects can now be considered simultaneously as the child starts to understand the concept of reversability (eg. what came out of a container could also return to it without change). Empirical abstraction builds on the earlier discovery of insight to permit the child to gain new knowledge and develop new ways of interacting with its environment, which in turn are used to gain further new knowledge and develop new techniques.
Finally, the formal operational stage, which continues into adulthood, is when the child learns to think abstractly, without physical reference. It is also the stage when Piaget believed the child began to be aware of itself as a sexual being.
Later research has brought into question Piaget's clear developmental structure of stages, especially in the context of the mentally handicapped; and some research even suggests that trying to shoehorn children's mass education into a strictly Piaget-linear structure may do more harm than good. Nevertheless, the attempt to model child development has opened doors for future enquiry into an entirely new field of psychology.